Leafhoppers (Homoptera: Cicadellidae): a major …...history of North American grasslands and their insect fauna is intimately tied to glaciation. This is particularly true of interglacial

Hamilton, K. G. A. and R. F. Whitcomb. 2010. Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats. In Arthropods of Canadian Grasslands (Volume 1): Ecology and Interactions in Grassland Habitats. Edited by J. D. Shorthouse and K. D. Floate. Biological Survey of Canada. pp. 169-197. © 2010 Biological Survey of Canada. ISBN 978-0-9689321-4-8 doi:10.3752/9780968932148.ch8

Chapter 8Leafhoppers (Homoptera: Cicadellidae):

a Major Family Adapted to Grassland Habitats

K. G. Andrew HamiltonBiodiversity Program, Research Branch

Agriculture and Agri-Food Canada K. W. Neatby Bldg., 960 Carling Ave.

Ottawa, Ontario, Canada K1A 0C6

Robert F. Whitcomb1

Plant Virology Laboratory Crops Research and Entomology Research Division

USA Department of Agriculture, Agricultural Research Service

Beltsville, Maryland 20705 USA

Abstract. Many Canadian grasslands are not, as commonly believed, merely extensions of those in the United States. Analyses of their leafhopper faunas indicate that grasslands have unique Pleistocene histories that confer equally unique features of ecology and biodiversity, with 470 species of grassland-endemic leafhoppers of which at least 223 species in 66 genera occur in Canada. Of the latter, at least 132 species are strict monophages.This rich endemic fauna appears to reflect a combination of glacial and postglacial adaptations to environmental factors, such as short summers, which in many cases permit only a single generation per year. Also remarkable is the apparent resilience of leafhopper populations to stressors, including floods and habitat fragmentation. This resilience allows relict populations to persist in grasslands isolated in agricultural and forested landscapes. However, these populations cannot readily disperse across altered landscapes and may be vulnerable to extinction under global warming. Such isolated grasslands may be either postsettlement ecosystems or postglacial-age ecosystems, a distinction that is strongly reflected in their leafhopper faunas. Postglacial-age grasslands include glacial-age refugia, post-Altithermal relicts of maximum prairie extent, and grasslands that emerged on glacially scoured landscapes such as alvars. Relict populations of native grasses and their leafhopper specialists indicate that Altithermal prairie extended as far east as Windsor, Ontario, while an older periglacial grassland reached to Lake Champlain near the Quebec–New York border. Grasses that were adapted to sandy conditions, and their insect herbivores, probably followed glacial moraines all the way to the Atlantic Ocean after each glacial retreat.

Résumé. Contrairement à la croyance générale, les prairies canadiennes ne sont pas toujours de simples prolongements des prairies américaines. L’étude de leurs populations de cicadelles montre qu’à partir du Pléistocène, elles ont acquis des caractéristiques uniques aux plans de l’écologie et de la biodiversité, avec 470 espèces de cidadelles endémiques desquelles on retrouve au moins 223 espèces réparties en 66 genres dans les prairies du Canada. De ces derniers 66 genres, il y a au moins 132 espèces qui sont des monophages stricts. Cette faune endémique riche semble refléter une combinaison d’adaptations glaciaires et postglaciaires à des facteurs environnementaux—par exemple, les étés courts qui ne permettent souvent de produire qu’une seule génération par année. Il convient également de souligner l’apparente résistance des cicadelles aux agents stressants, y compris les inondations et la fragmentation de l’habitat. Cette résistance permet à des populations reliques de persister dans des prairies isolées au milieu de territoires agricoles et d’écosystèmes forestiers. Toutefois, ces populations ne peuvent pas se disperser facilement au-delà des paysages modifiés et pourraient être exposées à l’extinction sous l’effet du réchauffement climatique. Ces prairies isolées peuvent être de deux types—écosystèmes formés après l’arrivée des Européens, ou écosystèmes remontant à l’époque postglaciaire—et cette distinction se reflète clairement dans les populations de cicadelles. Les prairies d’âge postglaciaire comprennent des refuges d’âge

1 Deceased.

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170 K. G. A. Hamilton and R. F. Whitcomb

glaciaire, des reliques de l’expansion maximale des prairies remontant à l’époque postérieure à l’altithermal, et des prairies qui se sont formées sur les paysages érodés par les glaces—par exemple, les alvars. Les populations reliques d’herbacées indigènes et de leurs espèces spécialisées de cicadelles donnent à conclure que la prairie altithermale s’étendait vers l’est jusqu’à Windsor (Ontario), tandis qu’une prairie périglaciaire plus ancienne atteignait le lac Champlain, près de la frontière du Québec et de l’État de New York. Les graminées qui étaient adaptées à des habitats sableux et les insectes qui s’en nourrissaient ont probablement suivi les moraines glaciaires jusqu’à l’Atlantique après chaque période de retrait des glaces.

Introduction

All native Canadian grasslands bear eloquent testimony to glacial activity. Vast sand and gravel moraines that stretched in huge arcs across the continent, together with erosional features such as exposed rock outcrops and deep coulees carved by immense outpourings of glacial meltwater, were all prime substrates for the development of unique grasslands. Today, although many of these features have been obliterated by erosion or concealed by encroaching vegetation, they nevertheless underlie the structure of many northern grassland ecosystems. The biodiversity and distribution of their phytophagous insects developed on such plants and substrates over the last 10,000 years of postglacial temperatures. This is reflected most noticeably in their associated suites of endemic herbivores (Fig. 1). Thus, the history of North American grasslands and their insect fauna is intimately tied to glaciation. This is particularly true of interglacial grasslands, adapted to warm summers, which had to find a refugium south of the ice cap for the preceding 40,000 years.

Regions farther south were hardly less affected by the Ice Age than were Canadian flora and fauna. Vastly expanded boreal forests during this period displaced most of the interglacial vegetation. As world climates ameliorated, the interglacial vegetation again spread northward into Canada from refugia that have been thought to be near (or on) the Gulf coast (Ross 1970). From this perspective, the faunas of Canadian grasslands have often been considered to be simply extensions of those that exist today in the south-central United States (e.g., Oman 1949). In this chapter, we examine this thesis from the perspective of plant-feeding bugs that are adapted to living in native grasslands. Our separate experiences

Fig. 1. “Whitcomb’s beauty,” Stirellus bicolor (Van Duzee), a leafhopper whose polymorphism RFW was able to demonstrate through controlled breeding experiments. This insect shows latitudinal differences in host specificity, being polyphagous in the tropics, oligophagous in the southern United States, and monophagous on the Canadian prairies, where it is restricted to little bluestem and is an indicator of tallgrass prairie. All individuals shown are females. Photographs, clockwise from spring variety, lower left, courtesy of Lynette Schimming, Graham Montgomery, Charles Schurch Lewallen, Tam Stewart, Tyler C. Christensen, and Samuel Houston.

Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats 171

in sampling the northern and southern faunas indicate that the grasslands that now occupy glaciated terrain differ in biodiversity and ecology from those of unglaciated sites, both on the Great Plains and on the adjacent desert plains of the Southwest United States. Our target organisms are the phytophagous true bugs known as leafhoppers (Homoptera: Cicadellidae). We examine these insects as tools for differentiating biotic communities and distinguishing different types of grasslands. Somewhat surprisingly, leafhoppers can also be used as indicators of the prehistoric origins and pre-settlement characteristics of native grasslands in Canada (Hamilton 2005). We also discuss possible reasons for the life history strategies of these insects, their present biogeography, and reactions to stressors.

Leafhoppers and Relatives

Leafhoppers (Fig. 1) are jumping insects belonging to Homoptera, suborder Auch- enorrhyncha. They suck vascular fluids with slender beaks that arise at the back of the head. Unlike aphids and their kin (Homoptera, suborder Sternorrhyncha), leafhoppers and their relatives have tiny, bristle-like antennae and are thus sometimes called “short-horned” bugs. These insects are both abundant and diverse in North American grasslands. There are almost 1,500 species of all such bugs known from Canada (Maw et al. 2000), of which at least 258 species are confined to grasslands (Hamilton 2004a). Of the total Canadian fauna, the great majority (1,088 known species) are leafhoppers. The remainder (Fig. 2) are mainly planthoppers (Fulgoroidea, chiefly the family Delphacidae), with smaller numbers of cicadas (Cicadidae, 20 native species in Canada), spittle bugs or froghoppers (Cercopidae and Clastopteridae, 36 species), and treehoppers (Membracidae, more than 100 species).

The majority of cicadas, spittle bugs, and treehoppers reside in forested lowlands of southern Canada and include only a few species characteristic of native grasslands. All four species of cicadas in the grasslands of Canada are confined to southern Alberta. These are not prairie endemics but are derived from intermontane western (Cordilleran) grasslands, such as those of Utah. The only common prairie spittle bug, Philaenarcys bilineata (Say), is abundant and widespread not only on the plains, but also in boreal marshes (Hamilton 1982: Map 32). Three spittle bugs characteristic of prairies in the eastern United States are

Fig. 2. Leafhopper relatives on the prairies, right to left from upper right: Okanagana synodica (Say), Cicadidae; Muirodelphax arvensis (Fitch), Delphacidae; Lepyronia gibbosa Ball, Cercopidae; Campylenchia rugosa (Fowler), Membracidae. Photographs courtesy of Dan Johnson, Tyler C. Christensen, University of Minnesota, Bill Johnson, Andy Daun, and Lynette Schimming.

CicadidaeDelphacidaeCercopidaeMembracidae

female macropternymphnymph

male brachypteradultadult


also known from sandy areas of Ontario and New England forests (Hamilton 1982, 1995a), although there are only five documented occurrences of Lepyronia gibbosa Ball, Philaenarcys killa Hamilton, and Prosapia ignipectus (Fitch) in eastern Canada. Treehoppers, as their name suggests, prefer woodlands. However, some feed on herbaceous plants in grasslands and others on trees in savannas or along prairie rivers; none feed on grasses.

Leafhoppers are one of the most diverse families of any organism found in grasslands (Ross 1970; Whitcomb et al. 1994; Hamilton 1995a). Oman (1949) made a conservative estimate of 800 species of leafhoppers, including wind-transported microleafhoppers (subfamily Typhlocybinae) in the prairies of the central states, Colorado, and Texas. An additional 100 species have since been found on the Canadian prairies (Hamilton 2004a), raising the estimate to at least 900 species. This makes the biodiversity of grassland leafhoppers roughly comparable to that of cutworm moths (Lepidoptera: Noctuidae) and ground beetles (Coleoptera: Carabidae), taxa that are frequently used as models for biodiversity, although these have few prairie endemics (Ricketts et al. 1999). By contrast, more than 470 species of leafhoppers are wholly endemic to the prairies. This is a larger number of prairie endemics than in any family of plants: 337 species of prairie-inhabiting Asteraceae, 258 of Poaceae, and 137 of Cyperaceae (Barkley 1977).

Leafhoppers are among the most abundant phytophagous insects in grasslands (Osborn and Ball 1897). Local populations on a British grassland may exceed a million individuals per hectare by mid-summer (Morris 1971). In the American Southwest, in a single collection, more than 10,000 nymphs of Balclutha neglecta DeLong and Davidson were obtained in 100 sweeps in a Bouteloua-dominated grassland (RFW, unpublished) and in Alaska, 4,000 adults of Diplocolenus evansi (Ashmead) were taken in 100 sweeps in an open meadow (H.H. Ross, unpublished).

Many species of grassland leafhoppers form intimate associations with dominant or subdominant species of grass or forbs. For this reason, leafhoppers are an excellent source of data for the assessment of grassland properties such as stand continuity and integrity (Hamilton 2004b). The biodiversity and frequently bright colours of leafhoppers are impressive; and their abundance, activity, and moderate size (usually 4–15 mm) make them easy to find with various collecting techniques. By contrast, planthoppers are less well studied. They are less diverse and, because they are tiny and relatively rare, are easily overlooked. Individuals of some delphacid species are only 2 mm long. Many species of this family look superficially alike and live near the root crowns of bunchgrasses. Some of these species can be obtained only by vacuum collecting, and exacting techniques are required to identify them. Thus, planthoppers are infrequently encountered by general collectors and are even less frequently identified.

Grassland Types

The grasslands most familiar to us are associated with activities of human settlement, such as those leading to the deliberate introduction of pasture or crop grasses or the weedy spread of inadvertently or deliberately introduced exotics. These alien species often dominate deforested land and follow transportation corridors. These species have few associated leafhoppers,,and those that are found are mainly Eurasian or widespread native species. Therefore, they are of little scientific or conservation interest and are excluded from this study.

By studying the insect faunas of native grasslands, we can begin to define grasslands by the association of similar faunal suites with characteristic floral communities. From an insect’s point of view, even very small patches of grass may be sufficient for food and


shelter. If there are enough patches to serve as “stepping stones” they can be conduits for grassland insects to cross into other ecosystems. Therefore, all such mosaics of grass, shrubs, and trees of any size are tentatively considered here as grasslands, although in many ecological classifications trees are said to ‘dominate’ such grasslands.

Ephemeral grasslands, such as those that occur as glades in extensive forests, are colonized mainly by widely dispersing bugs. In these transitory grasslands, the number of endemic insect species is seldom more than one or two. Glades within eastern forests of Canada and the northeastern United States give rise to local monocultures of native perennial grasses such as poverty grass or wire-grass, Danthonia spicata (L.) Beauv. and its specialists, Laevicephalus melsheimerii (Fitch) and Latalus personatus Beirne. Likewise, grasses commingled with sedges and rushes in most wet meadows in forested areas or on arctic tundra, where the fraction of grass species of the flora is much lower than in true grasslands, have at most only a few associated leafhoppers. Most tundra and fen formations have unique leafhopper faunas and are not really part of the grassland rubric. This is not true of superficially similar wet areas of prairies that are dominated by salt-adapted sedges, reeds and rushes, or alkaline fens that harbour prairie-adapted grasses. Many species of leafhoppers specialize on varied monocotyledonous hosts on prairies, and these wetlands are an intrinsic part of the grassland story.

Although most grassland-specific species of leafhoppers in Canada are inhabitants of the Great Plains or intermontane valleys of the west, suites of several species also occur on smaller grasslands outside the Great Plains. In the west, patches of grasslands are interspersed in pine and aspen forests. In the east, suites of sand-adapted species of grass, and their leafhopper specialists, persist on the dry, shallow horizons of sand dune regions. These regions occur not only on the Atlantic shores and around large lakes, but also on inland formations such as eskers and sand hills. Other grasslands persist on the thin soil that crusts glacier-smoothed limestone plains called “alvars,” which have not been colonized by trees. Alvars are home to the largest suites of grassland short-horned bugs east of Illinois (Hamilton 1995a). Many of these species are characteristic of prairie ecosystems (Bouchard et al. 2002), suggesting that alvars have remained treeless since at least Altithermal times.

Life History Strategies

In all grasslands that we have studied, the associated insects have two distinct strategies. One strategy belongs to the “generalists,” of which the most dispersive are termed “tramps.” Generalist insects colonize a wide variety of grasses and forbs. Generalists tend to occur on many of the plant species in a given association. The second strategy is that of the specialists. These species occur either on a single host plant species or, at most, several related plant species. Assemblages of grassland leafhoppers in a given vegetational formation often consist of a few generalists and many specialists. Most grassland leafhoppers specialize on grassland-endemic plants. Such species of bugs are considered to be themselves grassland endemics. But in other cases, particularly if their hosts are unknown, species are considered to be endemics if their populations are found in grasslands more than 95% of the time (Ricketts et al. 1999). Thus, some species of leafhoppers associated with plants not limited to native grasslands are nevertheless considered here to be prairie species if their populations are largely limited to grasslands, including prairies (e.g., the flightless Neocoelidia tumidifrons Gillette and Baker, which occurs on various goldenrods, Solidago spp., is rarely found outside prairies).


Not all stands of common native grasses have an associated suite of leafhopper specialists for reasons that are not always clear. For example, cord grasses (Spartina spp.) are widely distributed across rivers draining into the Great Lakes in Ontario (Dore and McNeill 1980: Map 187). However, the species of leafhoppers on cord grasses (Table 1) are restricted to prairie and oak savanna west and south of Lake Michigan. The eastern extent of this fauna is at the tip of southern Ontario at Ojibway Prairie in Windsor (see Chapter 9). Such distributions appear to be the result of a combination of factors, which include, in addition to historical features, reproductive potential, host specificity, phenology, and dispersal ability.

Reproductive PotentialOn a regional scale, generalists have a competitive advantage over specialists because they can locate and colonize a wide variety of plant species. Even more than typical generalists, tramps adopt strategies that maximize reproductive potential. The dispersion that these strategies encourage enables tramps to colonize grasslands over a wide geographical area. Specialized sensory adaptations for locating a specific host (if they ever had them) have been lost in the course of evolution.

The reproductive effort needed by specialists to maintain their breeding populations should be lower than that of generalists because these species are sedentary and do not suffer losses from dispersal. But dispersal is only one possible cause of population depletion. Other factors, such as wildfire and floods, may account for a mortality rate that is sufficient to favour high fecundity. Aflexia rubranura (DeLong ) is often found in prodigious numbers on its host, prairie dropseed (Sporobolus heterolepis A. Gray), on alvars that are frequently subject to spring flooding, and in oak savanna, where frequent fire is important for maintaining grassland in the face of repeated forest invasion. We believe that the high reproductive effort of some species may compensate for losses to fire. However, this compensation runs counter to the normal strategies of specialist species of insects. If such compensation can be verified, it would be a reversal in the general theory for reduction of reproductive potential expected in K-selection (MacArthur and Wilson 1967). Such a circumstance should remind us that mortality in dispersal is only one of the possible reasons for high mortality in life history strategies (Cole 1954).

Conversely, populations of leafhoppers in some tallgrass prairies with extensive plant cover are surprisingly small. Numerous reasons can be adduced. One simple explanation is recent burning, which we have often observed to decimate local insect populations. A second possible explanation is sampling bias. Sweeping is much more inefficient in dense grass thatch or in old-growth grass clumps than in short or diffuse grass stands typical of shortgrass prairie, resulting in undersampling of insects that are thatch dwellers. However, populations in many old-growth prairies that have not been burned recently seem to be low even when sampled by vacuum collecting. These observations suggest that biological factors are involved. Perhaps incidence of attack by predators and parasites in extensive grasslands severely reduces population levels of prairie bugs. One often finds large populations of leafhoppers that are almost 100% parasitized. By contrast, isolated grass patches (such as those in alvars or small prairie reserves) may have higher populations of leafhoppers because their enemies may be more sporadic.

Host SpecificityLeafhoppers occupy the full spectrum of resource exploitation. The least restricted are polyphagous, feeding equally well on woody or herbaceous plants. Some leafhoppers are facultative, preferring herbaceous hosts, but will accept woody plants when their preferred


host plants have dried up. Others are generalists on woody plants, or on herbs, or on a combination of grasses and sedges. Many more leafhoppers exhibit various degrees of specialization, feeding on species of a single plant family (oligophagy), on a few closely related genera of plants (stenophagy), or even on a single host species (monophagy) (Hamilton 1983a, 1985).

When more than one stenophagous species with a restricted host range nevertheless feeds on more than one plant genus, the plant genera may have a close phyletic relationship. Both wheat grasses (Agropyron spp.) and wild ryes (Elymus spp.) are common hosts for no fewer than five stenophagous leafhopper species (Table 1). Unsurprisingly, members of these two grass genera are sometimes combined as Leymus spp. Similarly, little bluestem (which was formerly classified as an Andropogon species before its transfer to Schizachyrium), is colonized by several species of leafhoppers that also feed indiscriminately on both big and little bluestem (Table 1).

Leafhopper specialists exhibit various patterns of host selection. Some feed on various hosts, but oviposit on only one. This specialization is a type of monophagy. A pattern of specialization on a single plant species (hyperspecialist) is sometimes accompanied by physical adaptations to the host plant. The cicada Okanagana synodica (Say) is the most striking example in the Canadian homopteran fauna. It is unusual in having a black body barred with yellow (Fig. 2, right) that resembles the dappled shade cast by the sagebrush shrubs where it sings on hot summer days. Thus, if an association between insect and plant species is of sufficient long standing, morphological characters may evolve to reinforce the specialization.

Patterns of leafhopper host specialization are most easily discernible in arboricolous faunas. Canadian leafhopper species that feed on trees are usually restricted to related plants, mostly a single host species or a genus (Hamilton 1985). Of 174 tree-feeding species of leafhoppers native to Canada, only 16 are generalists on trees. A mere six species are oligophagous, whereas 11 times as many are stenophagous. Of the latter, 34 species feed on a single genus of trees, and 26 species feed on only closely related tree species within one genus. This leaves 85 leafhopper species that are apparently monophagous because they have been found on only a single native tree species.

Although it is difficult to study host relationships in grasslands, our work spans more than 40 years, includes thousands of host records, and is therefore robust. We have found in most situations that almost pure stands, which we value highly for the purposes of ascertaining the probable host of grassland-specialist leafhoppers, are rare on typical prairie because small soil and moisture differences in microhabitats shift the balance of optimal conditions from one grass species to another. Natural pure stands are most frequently found in peripheral grasslands where grass patches are isolated or where there are few species of grass. Also, pure stands sometimes occur on the most challenging soils or in zones of lake, pond, and slough shores where moisture and salinity gradients form a cline. Natural depressions that retain standing water offer a selective force that separates grass species that otherwise may be closely associated. Buffalo grass, Büchloe dactyloides (Nutt.) Engelmann, is more flood tolerant than blue grama, Bouteloua gracilis (HBK.) Lag., its codominant in shortgrass prairie. But most pure stands of grass have been created by human activities. Plant material centres in which plots of native grasses are set out for seed production, or conservation plantings where (wisely or unwisely) grass monocultures have been planted, or highway rights-of-way often afford an opportunity to study host specificity. Although we normally have found substantial populations of leafhopper specialists on these more or less artificial pure stands, these populations were no higher than those on relatively pure stands in native grasslands.

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Table 1. Host grasses Poaceae and their 84 known or suspected specialist leafhoppers in Canada.

Scientific Name of Host Common Name Leafhopper Specialists

POOIDEAE Cool-season grasses 32 Leafhopper species

Ammophila breviligulata Fern Beach grass 1 sp.: Paluda gladiola (Ball)

Poa spp. Bluegrass 3: Auridius auratus (Gill. & Bak.), A. ordinatus (Ball), A. sandaraca Hamilton

Festuca spp. Fescue 1: Orocastus (Cabrulus) tener (Beamer & Tuthill)

Deschampsia flexuosa (L.) Trin. Hair-grass 1: Rosenus acutus (Beamer)

Koeleria macrantha (Ledeb.) Schultes June grass 8: Amblysellus acuerus DeL. & Hm., Amblysellus punctatus Osborn & Ball, A. wyomus Kramer, Athysanella obesa Ball & Beamer, A. robusta Baker, Auridius helvus (DeLong), Memnonia maia Hamilton, Rosenus cruciatus (Osborn & Ball)

Stipa comata Trin. & Rupr. Needle-and-thread grass, speargrass

2: Orocastus (Cabrulus) labeculus (DeLong), O. (s.s.) perpusillus (Ball & DeLong)

Stipa spartea Trin. Porcupine grass 1: Commellus colon (Osborn & Ball)

Oryzopsis asperifolia Michx. Rice-grass 2: Latalus latidens (Sanders & DeLong), L. remotus Hamilton

Puccinellia nuttalliana (Schultes) Hitchc. Salt-meadow grass 2: Deltocephalus serpentinus Hamilton & Ross, Laevicephalus saskatchewanensis Hamilton & Ross

Hordeum jubatum L. Wild barley 1: Psammotettix knullae Greene

Leymus spp. (Agropyron + Elymus spp.) Wheat grass and wild rye

8: Athysanella attenuata Baker, Attenuipyga (Dorycara) minor (Osborn), A. (D.) platyrhynchus (Osborn), Commellus comma (Van Duzee), C. sexvittatus (Van Duzee), Hebecephalus occidentalis Beamer & Tuthill, H. rostratus Beamer & Tuthill, H. truncatus

Agropyron smithii Rydb. Wheat grass, western 1: Mocuellus caprillus Ross & Hamilton

Agropyron trachycaulum (Link) Malte Wheat grass, slender 1: Mocuellus americanus Emeljanov

CHLORIDOIDEAE / PANICOIDEAE Warm-season grasses 52 Leafhopper specialists

Andropogon/Schizachyrium spp. Bluestems 3: Laevicephalus unicoloratus (Gillette & Baker), Hecalus flavidus (Signoret), Stirellus bicolor (Van Duzee)

L

eafhoppers (Hom

optera: Cicadellidae): a M

ajor Fam

ily Adapted to G

rassland Habitats

177

Andropogon gerardi Vitman Bluestem, big 1: Flexamia prairiana DeLong

Andropogon scoparius Michx. (=Schizachyrium scoparium)

Bluestem, little 7: Athysanella incongrua Baker, Chlorotettix spatulatus Osborn & Ball, Flexamia dakota Young & Beirne, F. delongi Ross & Cooley, F. graminea (DeLong), Paraphlepsius lobatus (Osborn), Polyamia caperata (Ball)

Bouteloua gracilis (HBK.) Lag. Blue grama 4: Athysanella bifida Ball & Beamer, A. sinuata Osborn, Flexamia abbreviata (Osborn & Ball), Flexamia flexulosa (Ball)

Buchloë dactyloides (Nutt.) Engelmann Buffalo grass 1: Athysanella texana (Osborn)

Spartina gracilis Trin., S. pectinata Link Cord grasses 5: Destria crocea (Beirne), Neohecalus lineatus (Uhler), N. magnificus Hamilton, Paraphlepsius solidaginis (Walker), Pendarus magnus (Osborn & Ball)

Sporobolus heterolepis A. Gray Dropseed, prairie 2: Aflexia rubranura (DeLong), Memnonia panzeri Hamilton

Sporobolus cryptandrus (Torr.) A. Gray Dropseed, sand 4: Athysanella occidentalis Baker, Dicyphonia ornata (Baker), Unoka dramatica Hamilton, U. gillettei Metcalf

Sorghastrum nutans (L.) Nash Indian grass 1: Flexamia reflexa (Osborn & Ball)

Eragrostis spectabilis (Pursh) Steud. Love grass, purple 1: Flexamia areolata (Ball)

Muhlenbergia richardsonis (Trin.) Rydb. Muhly, mat 6: Athysanella secunda Blocker & Wesley, Flexamia decora Beamer and Tuthill, F. serrata Beamer & Tuthill, Laevicephalus poudris Tuthill, Lonatura teretis Beamer, Memnonia anthalopus Hamilton

Muhlenbergia cuspidata (Torr.) Rydb. Muhly, prairie 2: Flexamia stylata (Ball), Lonatura megalopa (Osborn & Ball)

Distichlis stricta (Torr.) Rydb. Salt grass 4: Athysanella kadokana Knull, Lonatura melina (DeLong), L. salsura (Ball), Memnonia brunnea (Ball)

Spartina patens (Ait.) Muhl. Salt hay 2: Amplicephalus littoralis (Ball), A. simplarius (Osborn & Ball)

Calamovilfa longifolia (Hook.) Scribn. Sand reed grass 3: Athysanella. terebrans Gill. & Bak., Flexamia grammica (Ball), Laevicephalus exiguus Knull

Muhlenbergia asperifolia (Nees & Mey.) Scratch grass 1: Flexamia inflata (Osborn & Ball)

Bouteloua curtipendula (Michx.) Torr. Side-oats grama 1: Laevicephalus minimus (Osborn & Ball)

Panicum virgatum L. Switch grass 4: Chlorotettix fallax Sanders & DeLong, Flexamia atlantica (DeLong), Graminella oquaka DeLong, G. pallidula (Osborn)


Some exotic hosts, such as crested wheat grass, Agropyron cristatum (L.) Gaertn., appear to be unappealing to native insects, although the only specimen of the Aristida specialist Attenuipyga balli Oman collected in Canada was taken from a pan trap in a crested wheat grass stand. In some regions, however, introduced grasses provide an alternative host that supports generalists but also, occasionally, specialists. Tall fescue (Festuca arundinacea Schreb.) is accumulating a makeshift fauna as it is planted on roadsides from the Carolinas to Canada. The suites of specialists that colonize such grasses vary from region to region. In the southern United States, bermuda grass, Cynodon dactylon (L.) Pers., and weeping love grass, Eragrostis curvula (Nox), have “stolen” a few specialist species, whereas farther north, occasional small populations of native wheat grass specialists, such as Attenuipyga platyrhynchus Osborn, are found on quackgrass, Agropyron repens (L.) Gould. In cases where exotic grasses are planted extensively on a regional scale, they even have the potential to steal specialists from their normal but increasingly rare hosts. Exotic grasses almost always prove to have somewhat different phenologies from the dominant grass hosts of their native leafhopper specialists. The populations derived from these transfers, however, are often extirpated. For example, Elymus specialists that have colonized quackgrass may be unable to reach maturity if this host plant undergoes an unusually dry summer.

Insects that feed on widely scattered perennials must be more proficient at dispersal than colonists of dominant grasses. There is a parallel in tropical jungles. The immense diversity of plants in such ecosystems leads to a reduction in insect host specificity as compared with savannas at the same latitude (Ribeiro 2003). Thus, it should not be surprising that populations of leafhoppers of the same species become less host specific in southeastern localities of North America (Whitcomb et al. 1987a), where floral diversity increases at the expense of dominance.

Why, therefore, are there any insects that specialize on single native host species or small sets of species? Specialists have a vastly different strategy from that of generalists. They stake their existence on tracking a single plant species or a small set of related plant species. Their host species are almost always perennials because perennials assure a constant food supply. Where conspecific plant colonies are stable and adjacent, as is usually the case in Canada, phytophagous insects that feed on them may become closely attuned to the physiology and phenology of a single host species at the expense of losing their ability to breed on other species. Feeding on inappropriate hosts, although often successful in the short term, can have disastrous consequences in the long term. For example, leafhoppers that reproduce successfully on an inappropriate host during the summer may fail to enter diapause (Whitcomb and Coan 1990: 680).

The degree of specialization of most species of grassland leafhoppers falls somewhere between hyperspecialism and oligophagy. On the Canadian prairies, most of the small number of species of leafhoppers that are widespread in both grasslands and forests are generalists, whereas those adapted to grasslands (Hamilton 2004a) are mostly stenophagous or even monophagous. Specialists are often coloured like grasses or woody plants, for example, grass green as in Memnonia panzeri Hamilton, dappled white and gray as in Empoasca subgenus Hebata on sagebrush and Ceratagallia cinerea (Osborn and Ball) on winterfat, or twig brown as in Prairiana cinerea (Uhler). Only a few are hyperspecialists, some of which are more or less shaped like the seeds of the host, such as Attenuipyga minor (Osborn) on wheat grass, or are patterned like seeds, such as Flexamia areolata (Ball) on purple love grass, Eragrostis spectabilis (Pursh) Steud.

More than half of all species of leafhoppers endemic to Canadian grasslands (132 of 223) have extremely narrow feeding habits, usually specializing on a grass or woody shrub.


Most monophagous prairie leafhoppers (85 species) feed on grasses, family Poaceae (Table 1). Another 53 species attack plants in eight other families, primarily composites, sedges, and willows (Table 2). At least 10 of these are stenophagous or oligophagous in the United States, having acquired one or more alternative hosts at lower latitudes.

Some species of leafhoppers have been able to extend their ranges by transferring to closely related host species. For example, Flexamia abbreviata (Osborn and Ball), occupies most of the entire range of blue grama, its only host in Canada. However, beyond the periphery of its eastern range limit in the United States, it is represented by a biotype that is adapted to hairy grama (Bouteloua hirsuta Lag.). Also, in southwestern grasslands, F. abbreviata colonizes several grama grasses that are not utilized elsewhere as a food source (Whitcomb et al. 1987a). From these considerations, we must revise our notions of “specialist” and “monophagy” to state that these designations must be used for a particular biotype and not for the species as a whole. Often, a presumably new biotype is generated at the northern periphery of the leafhopper host range. This latitudinal shift in host range often occurs at latitude 40°N or somewhat farther south. For example, Laevicephalus exiguus Knull feeds on grama grasses on the southern Great Plains but has shifted to feeding on sand reed grass, Calamovilfa longifolia (Hooker) Hackel, at Sauble Beach on Lake Huron. Similarly, Driotura robusta Osborn and Ball is monophagous on gumweed (Table 2) throughout the Canadian plains, although it is reported on aster, erigerons, or both farther south (Whitcomb et al. 1987b).

Although such range extensions are more often latitudinal, they are sometimes longitudinal. For example, Flexamia inflata (Osborn and Ball) has two distinct biotypes, a western one from Saskatchewan and Washington State on scratch grass, Muhlenbergia asperifolia (Nees and Meyen) Parodi (Table 1), and an eastern one from Manitoba to Ontario, which is usually on rushes (Juncus spp.). Similarly, Paluda gladiola (Ball), which is a widespread feeder on a cool-season grass, Calamagrostis canadensis (Michx.) Beauv., has a separate biotype on the Atlantic coast on beach grass (Ammophila breviligulata Fernald), a warm-season perennial. In regions of the United States in which the suite of grass species contains a diversity of warm- and cool-season grasses, some species of leafhoppers shift from cool- to warm-season grasses in the summer. For example, Gillettiella fasciata Ball and Beamer breeds abundantly on cool-season grasses in the spring in northern New Mexico, whereas in the summer individuals shift to Muhlenbergia species.

Dispersal Many species of grassland leafhoppers are not restricted to the Great Plains; a few others may at first appear to be, but in fact are not. For example, the small and slender Balclutha neglecta is a characteristic and abundant insect throughout the Great Plains, but it may be found in many other sites as far northeast as Lake Nipigon in the boreal forest of northern Ontario. Apparently, this species is wind dispersed, as are many of the tiny leafhoppers of the subfamily Typhlocybinae. Still others, such as Ceratagallia uhleri (Van Duzee), suddenly appear as populations of adults in locations where breeding populations are absent, such as mountaintops in Colorado (Hamilton 1998). These transient populations are clearly migrants, leaving drought-stricken hosts in search of more favourable sites in cooler, moister areas. By contrast, eastern leafhoppers such as the aster leafhopper, Macrosteles quadrilineatus (Forbes), are long-distance dispersers, invading northern latitudes and high altitudes each summer from stable populations at low elevations in the south.

Conversely, many species of grassland leafhoppers are found in only part of the range of their host, especially in the southwestern United States, where they are adapted to regions

180 K

. G. A

. Ham

ilton and R. F. W

hitcomb

Table 2. Host plants (other than grasses) and their 53 known grassland leafhoppers in Canada.

Scientific Name of Host Common Name Leafhopper Specialists

ASTERACEAE

Artemisia cana Pursh, A. tridentata Nutt. Sagebrush 8 spp.: Ballana ortha DeLong, Carsonus aridus (Ball), Empoasca medora DeLong, E. nigroscuta Gillette & Baker, Idiocerus canae Hamilton, Norvellina columbiana (Ball), Texananus extremus (Ball)

Artemisia frigida Willd. Pasture sage 5: Acinopterus viridis Van Duzee, Frigartus frigidus (Ball), Prairiana cinerea (Uhler), Stragania rufoscutellata (Baker), S. atra (Baker)

Artemisia gnaphaloides Nutt. Prairie sage 1: Mesamia ludovicia Ball

Balsamorhiza sagittata (Pursh) Nutt. Balsamroot 1: Ballana chelata DeLong

Chrysothamnus nauseosus (Pall.) Britt. Rabbitbrush 1: Ballana remissa DeLong

Eriogonum spp. Fleabane 1: Norvellina rubida (Ball)

Grindelia perennis A. Nels. Gumweed 1: Driotura robusta Osborn & Ball

Helianthus spp. Sunflower 2: Mesamia nigridorsum (Ball), M. straminea (Osborn)

Solidago spp. Goldenrod 1: Neocoelidia tumidifrons Gillette & Baker

BETULACEAE

Betula occidentalis Hook. River birch 2: Oncopsis incidens Hamilton, O. juno Hamilton

CHENOPODIACEAE

Atriplex spp. Atriplex 2: Aplanus albidus (Ball), Norvellina clarivida (Van Duzee)

Eurotia lanata (Pursh) Moq. Winterfat 1: Ceratagallia cinerea (Osborn & Ball)

L

eafhoppers (Hom

optera: Cicadellidae): a M

ajor Fam

ily Adapted to G

rassland Habitats

181

CYPERACEAE

Carex filifolia Nutt. Prairie sedge 2: Hardya dentata (Osborn & Ball), Stenometopiellus cookei (Gillette)

Carex spp. Sedge 3: Deltocephalus lineatifrons Oman, Paraphlepsius continuus (DeLong), P. turpiculus (Ball)

Eleocharis spp. Spike-rush 5: Limotettix bisoni Knull, L. uneolus (Ball), L. urnura Hamilton, L. elegans Hamilton, Dorydiella kansana Beamer

JUNCACEAE

Juncus balticus Willd. Baltic rush 2: Cicadula longiseta (Van Duzee), Pasaremus concentricus (Van Duzee)

Scirpus validus Vahl Great bulrush 2: Limotettix uneolus (Ball), L. utahnus (Lawson)

PINACEAE

Juniperus horizontalis Moench Creeping juniper 1: Texananus marmor (Sanders & DeLong)

PRIMULACEAE

Glaux maritima L. Sea-milkwort 1: Erythroneura carbonata McAtee

SALICACEAE

Populus angustifolia James Narrow-leaf poplar 1: Empoasca angustifoliae Ross

Populus deltoides Bartr. Cottonwood 1: Idiocerus moniliferae Osborn & Ball

Salix exigua Nutt. Wolf or sandbar willow 10: Empoasca albolinea Gillette, E. digita DeLong, E. exiguae Ross, Idiocerus freytagi Hamilton, I. ramentosus Uhler, I. raphus Freytag, Macropsis feminis Hamilton, M. rufescens Hamilton, M. rufocephala Osborn, Macrosteles major (Dorst)

SAXIFRAGACEAE

Ribes aureum Pursh Golden currant 1: Idiocerus interruptus Gillette & Baker


with highly contrasting host phenologies (Whitcomb et al. 1994). Because of glaciation, most species of leafhoppers in Canada have not been in one area long enough to develop regional adaptations to climate, in contrast to more mobile ground beetles (Carabidae).

A large percentage of northern species seem to disperse too slowly to have filled their host’s geographical range in the 10,000 years since deglaciation. The glaciation of Canadian grasslands has provided a natural experiment that demonstrates this dispersal rate (Hamilton 1999a: Fig. 2). As the Wisconsin glaciers retreated (see Chapter 1), a vast landscape became available for leafhopper recolonization. This natural process provides an opportunity to determine the dispersal rates of species that had been displaced by the glaciers. Half of the 24 species of arctic leafhoppers have dispersed across open tundra at a rate of less than 1 km/year from their glacial refugium in the first 5,000 years since deglaciation, and 30% have not crossed the 10-km-wide Mackenzie River valley in 12,000 years. One of the factors involved in slow dispersal is univoltinism. For example, almost all species of leafhoppers in Yukon grasslands (including 17 endemics) are abundant only from late June to August during the short growing season (Hamilton 1997). Only Cuerna septentrionalis (Walker) overwinters as adults and is active whenever the weather permits. This species has been observed (KGAH, unpublished) climbing on a fence in Winnipeg, Manitoba, on a warm day in February!

Leafhoppers farther south have a much greater opportunity to disperse, but other factors such as mountain barriers, phenological constraints, and behaviour may prevent them from doing so. For example, the polyphagous but flightless leafhopper Errhomus calvus Oman appears to have dispersed northward from the north side of the Columbia canyon only 150 km since deglaciation, at an average rate of 15 m/year (Hamilton and Zack 1999) and has invaded only the most southerly parts of the Okanagan Valley of British Columbia.

As noted earlier, leafhopper specialists must perforce develop dispersal mechanisms and host-finding sensory abilities if their host occurs as scattered colonies. Insect specialists of grasses and shrubs in sloughs, for example, usually feed on plants that grow in widely separated hollows and therefore can be colonized only by flight. Wetland-adapted plants, such as manna grass, mat muhly, spike-rush, or wolf willow (Table 2), are the most usual hosts for these insects. Plants adapted to saline conditions, (e.g., salt grass, salt-meadow grass, and wild barley; Table 1), are particularly common today in roadside ditches, but originally occurred in natural depressions such as salt pans on the prairies. These species of plants have small but distinctive endemic faunas on the Great Plains. Similarly, plants that grow only in widely separated sand hill areas of the Great Plains generally support only a few species of bugs, but wherever these grasses grow, we usually find at least one of the associated species of leafhopper.

How do leafhoppers and other bugs traverse enormous distances and find tiny pockets of grasslands in isolated valleys of the Yukon, on limestone outcrops around the Great Lakes, and on sandy ridges as far east as the Maritimes? Do they simply scatter to the winds, raining down on the unyielding forest in untold millions until a single gravid female chances upon a clump of grass? This strategy accounts for the wide geographical distribution of tramp species that are not choosy about their food plants. It is also the way some other insects, such as Delphacidae, have traversed thousands of kilometres of open ocean over the millennia to populate the Hawaiian Islands (Zimmerman 1948). However, such a strategy would seriously deplete the gene pool of a host-specialist leafhopper, whose individuals stand little chance of finding a host that occurs only in isolated stands.

Bugs such as Delphacidae that have the greatest dispersal abilities and a highly vagile lifestyle are overrepresented in peripheral grasslands. The grassland fauna of the


Atlantic coast is dominated by Delphacidae, whereas leafhoppers represent only 33% of the fauna, as compared with 80% on the Great Plains. The grassland fauna of Homoptera (Auchenorrhyncha) on isolated patches of bluestem grassland in boreal forest areas of central Minnesota and northwestern Michigan comprises only a single Delphacidae, Delphacodes parvula Ball, a wide-ranging specialist on little bluestem.

Distribution maps of Delphacidae sometimes show large disjunctions. Many of these disjunctions result from a lack of sampling in intervening sites, but some cannot be so easily explained. A curious delphacid, Parkana alata Beamer, occurs in Arizona and Utah but has also been discovered in two remote localities in Canada, one deep in an interior valley of British Columbia at Kamloops, and the other far up in the foothills of Alberta near Calgary. Other wide disjunctions present evidence of niche consistency such as Elachodelphax hochae Wilson that has been taken in the Peace River district and in a number of Yukon sites. This species is also known from two Great Plains localities, each of which is an island of aspen parkland in a sea of grasses (the crests of Cypress Hills and of Moose Mountain in southeastern Alberta and Saskatchewan, respectively). Both P. alata and E. hochae are strange-looking insects that any collector of tiny bugs would be delighted to find in a net or trap. Why then have they never been collected anywhere else in the thousand-plus kilometres of suitable grasslands that separate the prairie populations from their sources of origin? We do not know, but it seems likely that the current isolated populations were part of a single widely distributed population at a time when temperatures were higher than at present.

Leafhoppers, like Delphacidae, are frequently flightless. Individuals of most Canadian Delphacidae have shortened (brachypterous) front wings and tiny or absent hind wings. It is thought that flightlessness in Delphacidae is an adaptation to increasing egg production in which energy normally expended in producing flight muscles is conserved (Denno 1994). Flightlessness is therefore most adaptive when host plants are low growing and plentiful. Brachypterous forms of specialists that inhabit dominant and subdominant grasses often build up large populations on their host patch.

Flightless Delphacidae normally produce winged individuals when population density increases, resulting in the production of dispersing forms (Denno 1994). No similar environmental factor seems to determine macroptery in leafhoppers. Instead, the proportion of winged individuals is more or less fixed for each species. The spring generation may have a higher percentage of winged individuals, as in the early-season populations of Aflexia rubranura in Illinois, where this species is double brooded (R. Panzer, pers. comm.) Long-winged individuals comprise no more than 10% of the population there, but can easily ensure habitat connectedness, although patches as wide as 36 m may be devoid of the Sporobolus host plants (Panzer 2003). These long-winged forms are rare (<0.3%) elsewhere.

Females of many species of leafhoppers disperse widely only when they are sexually immature. By the time they become gravid, they have lost the ability to fly (Waloff 1973). This peculiarity may account for the presence of brachypterous females in so many monophagous species of grassland leafhoppers (Table 3). In some genera, such as Commellus and Extrusanus, males are usually flightless, whereas at least 10% of females can fly. The proportion of flying adults tends to increase in more northerly grasslands (Hamilton 1995b). The reason for flightless males is not clear. Males are more frequently capable of flight than females, with 50 to 90% of females being brachypterous (Hamilton 1999b).

Wing morphology gives important clues to a species’ dispersal strategy. Flightless morphs may be divided into brachypterous (the front wings are reduced to scale-like


appendages not covering the abdomen) and submacropterous (the front wings are only slightly reduced in length, exposing at most the tip of the abdomen, whereas the hind wings are distinctly smaller or reduced to tiny, strap-like remnants). Individuals with the shortest wings probably feed the lowest on a plant (Hamilton 2000), a strategy that suggests an adaptation to a cryptic lifestyle and low powers of dispersal. Conversely, leafhopper species that have long, narrow wings (e.g., the aster leafhopper; see Hamilton 1983b) are more likely to be migratory than are species with shorter wings. Similarly, the long wings of Flexamia grammica (Ball) suggest that this species flies actively to seek out its widely dispersed host, sand reed grass, whereas other species of Flexamia, which are usually short winged, can reach their dominant or subdominant grasses without relying on flight.

Biogeography

Range DeterminantsHost specificity is a primary determinant of leafhopper ranges, but many other factors limit the ranges of species of leafhoppers. Among these factors are diel temperature regimes, seasonal phenology, and historical development of the grasslands in which these species live. These factors contribute to making regional endemism of leafhoppers highest on the northern edge of the Canadian prairies, in the Aspen Parkland Ecoregion (Hamilton 2004a). Many other peripheral grasslands in Canada and the northern United States are colonized by endemic species of leafhoppers. For example, two undescribed oak-feeding species of leafhoppers of the genus Eutettix have been found in a single oak barren (savanna) in New Hampshire (KGAH, unpublished). Other notable examples are found in the interior valleys of British Columbia and the Peace River district of Alberta (Hamilton 2002) and in an alkaline fen in Michigan (Bess and Hamilton 1999). Some of the most localized of these species of endemic leafhoppers occur in areas outside glaciated lands, in the Yukon (Hamilton 1997), on the Queen Charlotte Islands (Hamilton 2002), in serpentine barren grasslands of Maryland (Hamilton 1994b), and on coastal grasslands (e.g., Blocker and Wesley 1985; Hamilton 2009).

TemperatureLeafhoppers, unlike ground beetles, do not seem to be niche hyperadaptive. The commonest species tend to occur wherever their host is found and not just in conditions of particular humidity, soil conditions, or shelter. For example, Commellus sexvittatus (Van Duzee) ranges from subarctic Yukon (Hamilton 1997) to sand dunes in Michigan (Hamilton 1994a). Of all the environmental factors limiting the dispersal of leafhoppers, only temperature regimes clearly govern the northernmost limits of the ranges of Canadian species (Hamilton 1997). This limitation is probably mediated by a reduction in the length of growing season and a consequent reduction of available degree-days for development, known to be a major factor in spittle bug distribution (Hamilton 1983a). Northernmost limits of ranges thus tend to follow latitudinal climate zones. Few species of grassland leafhoppers are found in the arctic, subarctic, or boreal zones. There are dramatically more leafhopper species in the transitional or hemiboreal zone that covers much of southern Canada. Another large suite of grassland insects is represented only from the austral zone. The grasslands of the austral zone are represented in Canada only by oak savanna on the southern end of Vancouver Island and its offshore islands on the west coast, in the southern Okanagan Valley of inland British Columbia, and in isolated


Table 3. Flightlessness and host associations of 111 monophagous grassland leafhoppers in southern Canada and (flightless females only) adjacent Pacific Northwest states. Cool-season and warm-season grasses are designated as C3 and C4, respectively. Pinumius, Telusus, and Twiningia are omitted because their hosts are not known.

% Winged Host Plant Leafhopper Monophages

Female Male

0(22 spp.)

0 C4 2 spp.: Unoka

20-70 1: Dicyphonia

100 Woody 1: Carsonus

Forb 18: Errhomus [only 1 recorded from Canada]

<1 0 C4 1: Aflexia

1-10(39 spp.)

4: Lonatura; [oligophages only: Attenuipyga (s.s.)]

1-10 13: Flexamia; 11: Athysanella

Forb 1: Driotura

Sedge [oligophages only: Extrusanus]

C3 2: Amblysellus, Mocuellus

100 1: Orocastus (s.s.)

C4 5: Memnonia

Forb 1: Neocoelidia

20-70(5 spp.)

1-10 C3 1: Commellus

20-70 C4 1: Destria, Neohecalus, Polyamia

100 C3 1: Auridius; [oligophages only: Attenuipyga (Dorycara)]

100(54 spp.)

3: Orocastus (Cabrulus), Rosenus, 2: Amplicephalus, Deltocephalus, Latalus; 1: Chlorotettix spatulatus, Laevicephalus saskatchewanensis, Paluda, Psammotettix; [oligophages only: Elymana, Gypona, Hebecephalus, Sorhoanus]

C4 3: Graminella, Laevicephalus [part]; 2: Paraphlepsius; 1: Chlorotettix fallax; [oligophages only: Pendarus, Stirellus]

Sedge 1: Hardya, Stenometopiellus; [oligophages only: Cicadula]

Spike-rush 2: Limotettix; [oligophages only: Dorydiella]

Forbs 3: Ballana, Norvellina; 1: Aplanus, Ceratagallia, Erythroneura, Mesamia

Woody 6: Idiocerus; 4: Empoasca; 3: Macropsis; 2: Oncopsis, Stragania, Texananus; 1: Frigartus, Macrosteles; [oligophages only: Gyponana, Prairiana]


Table 4. Phenology of 216 grassland leafhoppers in southern Canada, with polyphages designated by an asterisk (*). Northern leafhoppers have one generation per year, in July.

Broods In Winter Adults Prairie Endemics and Their Genera

2 As nymphs June, August 36 spp.: Amblysellus (5), Athysanella (13), Diplocolenus (1), Driotura (1), Hardya (1), Mocuellus (4), Psammotettix (4), Rosenus (3), Sorhoanus (2), Stenometopiellus (1), Stirellus (1)

Many As adults Continuous 7: Ceratagallia

1 August to April

5: *Cuerna (3), Erythroneura (1), Idiocerus (in part: 1 willow feeder)

As nymphs May 1: Errhomus

June 23: Attenuipyga (4), Ballana (7), Carsonus (1), Memnonia (6), Norvellina (3), Oncopsis (2)

Mid-June to mid-July

1: Neocoelidia

June to July 26: Auridius (4), Extrusanus (1), Hebecephalus (8), Hecalus (2), Orocastus (4), Paluda (1), Prairiana (3), Stragania (2), Telusus (1)

In south (migrant)

1: *Exitianus

August 1: Aplanus

As eggs July 23: Commellus (4), Destria (1), Elymana (2), Empoasca (in part: 4 sagebrush feeders), Graminella (3), Gypona (1), Lonatura (4), Macropsis (3), Pendarus (1)

Mid-July to mid-August

1: Gyponana

July to August

43: Amplicephalus (2), Deltocephalus (7), Flexamia (14), Frigartus (1), Idiocerus (in part: 6 cottonwood, currant, and sagebrush feeders), Latalus (6), Mesamia (4), Neohecalus (2), Pinumius (1)

August 51: Aflexia (1), Chlorotettix (2), Cicadula (1), Dicyphonia (1), Dorydiella (1), Empoasca (in part: 3 tree feeders), Laevicephalus (9), Limotettix (6), Macrosteles (3), Paraphlepsius (8), Polyamia (2), Texananus (7), Twiningia (3), Unoka (2), *Xerophloea (2)


grasslands of southern Ontario, from Ojibway Prairie in Windsor (Hamilton 1995a, 2002) to the Rice Lake Plains east of Toronto (KGAH, unpublished).

Grassland species of leafhoppers in Canada, including the specialists, often occur in an enormous range of habitats and environmental conditions. The majority have wide distributions across the continent (Maw et al. 2000). Many species likewise range far south of their northern limits, inhabiting two or three different latitudinal zones. Some, such as Psammotettix latipex (Sanders and DeLong), are found in arctic to austral situations (Hamilton 1997). Exceptions to this generalization include cases in which severe stress leads to extinction of local populations. For example, exceptionally cold temperatures or other stressors may extirpate isolated populations of Athysanella magdalena Baker at high elevations in mountains as far south as New Mexico. This species has enhanced levels of macroptery to compensate for such losses.

Local patches of favourable habitat (even if small) are significant for plants. Sun-warmed south-facing slopes are particularly important in Canada and Alaska (Ross 1970). Grasses and sage (Artemisia spp.) may flourish on such slopes, and their leafhopper biota follows along. This may account for the survival of much of the grassland fauna in deep valleys of the Yukon, including endemic species such as the sage-feeding Chlorita nearctica Hamilton (1997). South-facing slopes become increasingly arid through southern Canada into the United States. In the face of such aridity, leafhoppers favour west-facing slopes, where conditions are optimal for host growth, and shun east-facing slopes, where insolation occurs during the coolest part of the day. This may explain why, in the west, some species of leafhoppers seem to be confined to Cordilleran grasslands even when grassy passes have adjacent prairies on their eastern slopes, although the Pacific Northwest grasslands themselves are sometimes invaded by leafhoppers from the prairies (Hamilton 2002).

PhenologySpecies of leafhoppers with two or more generations per year, or whose adults live for two or more months, have the greatest opportunity to find new stands of their hosts. Species belonging to 14 genera may be double brooded on grasslands in southern Canada, but most (52 genera) have only a single generation per year (Table 4). The latter usually are represented by adults mostly during June (15 genera with overwintering nymphs) or August (17 genera with overwintering eggs). In favourable years, adults may appear as early as mid-May or continue into October. Many late-season species of leafhoppers feed on dominant grasses. These species tend to have longer adult lives than those that emerge earlier and presumably are better at dispersing. The last of the early-season species of leafhoppers are disappearing (the females are the last to go) at about the time that late-season leafhoppers are maturing. Sampling in mid-July therefore often yields a portion of both faunas, as well as species of the 11 genera that peak in mid-summer or the 9 whose adults survive through both July and August.

Leafhoppers that feed on birches and willows take advantage of the spring growth flush when the nutritional value of their host plants is high, as do many leafhoppers that feed on grasses. Conversely, the adults of sedge- and rush-feeding leafhoppers always appear late in the season. Presumably, the egg hatches of these species are delayed until the high water tables of spring recede.

Three genera of grassland leafhoppers use another strategy. They overwinter as adults so that they can lay their eggs early in the season such that their nymphs benefit from early summer growth. Grass-feeding insects that use this strategy profit by escaping


autumn and spring wildfires. This lifestyle is found also in certain willow-feeding leafhoppers of the genus Idiocerus, but is most suited for polyphagous bugs that feed on annuals. Flying adults are better suited for finding annual hosts that have survived the winter as seeds. In the case of Cuerna, the large adults are often long lived, with only a short gap between generations in mid-July (Table 4). Species of this genus overwinter as adults. The small but robust adults of Ceratagallia are continuously brooded on forbs throughout the summer. Their generations are seldom evident except by the abundance of maturing nymphs. These insects are the best dispersers and have wide ranges across Canada (Hamilton 1998).

Grasses are a much less reliable food source for specialists than woody plants in arid or strongly seasonal areas. Grasses grow rapidly when conditions (especially moisture) are just right. The timing of this growth flush is critical for both the grass and its insect colonists. Any leafhopper specializing on such a fleetingly available host must be able to adapt to the host’s seasonal growth cycles. In Canada, winter dormancy, the length of the growing season, and the photosynthetic pathways of the grass are the principal seasonal factors affecting leafhoppers. Cool-season grasses (tribe Poeae) with a C3 photosynthetic pathway take advantage of an early start to maximize the length of their growing season. By contrast, warm-season (chloridoid and panicoid) grasses make up for a later start by vigorous growth even during relatively dry periods, using a different photosynthetic pathway (C4 type) to minimize water usage and take advantage of heightened metabolism conferred by higher temperatures (Farquhar et al. 1989). This phenomenon is familiar to home owners who must cope with vigorous growth of crab grass, Digitaria sanguinalis (L.), an annual C4 grass, at the very time that lawn grass (Poa pratensis L., a perennial C3 grass) is drying up. Warm-season perennial grasses are usually dominants on prairies because they use moisture more effectively and rely on a deep root system—3 m or more—to garner scarce groundwater.

Leafhoppers residing in boreal to arctic areas use the early and sustained growth of cool-season grasses to fit their adult lives into the short northern summer. But farther south, where summers are longer and warm-season grasses are common, various life history strategies are found. For example, the two species of leafhoppers that specialize on prairie dropseed (a warm-season grass) occur on plains and alvars at different times of the year. Memnonia panzeri overwinters as late-instar nymphs and feeds as adults on the first flush of early summer growth. Aflexia rubranura, by contrast, overwinters as eggs and feeds on the fully grown plant in late summer. Aflexia takes advantage of the seasonal growth of the grass but at the expense of fire vulnerability. Wildfires in fall or spring (when grasses are dry and most flammable) can wipe out Aflexia eggs and decimate their populations. Thus, Aflexia is probably more prolific than Memnonia to survive, and this fecundity increases its ability to repopulate adjacent patches of its host.

Not all prairie faunal and floral elements are limited to low elevations in southern Canada and the United States. Cool-season grasses occur abundantly in the southwestern mountains of the United States, provided that the elevation is high enough. In these mountains, grasslands between the shrub steppes of the valleys and the high cool-season grass zones are composed largely of warm-season grasses. In the far north, for example, the Yukon, cool-season grasses predominate in the valleys and on mountainsides below the tundra. A few cool-season grasses, such as Stipa neomexicana (Thurb.) Scribn., are adapted to relatively warm climates of the southwestern United States and Mexico. Similarly, a few warm-season grasses are adapted to far-northern climates and occur together with cool-season grasses in the high-elevation grasslands of the United States and in northern


Canada. For example, Muhlenbergia richardsonis (Trin.) Rydb. has been found along with one of its specialist species of leafhoppers on the shores of Lake Manitoba in the boreal forest zone (KGAH, unpublished). This grass is so adapted to northern climates that the few remaining patches in southern New Mexico (which occur in the fir forest zone of the Sacramento Mountains) are dying out.

Specialists on grasses and shrubs in Canada are often limited to regions of the Great Plains where their hosts are dominant or subdominant. Thus, leafhopper specialists on blue grama, prairie sedge, spear grass, and wheat grasses (Table 1) are characteristic inhabitants of western plains dominated by Bouteloua, Elymus, and Stipa (Hamilton 2004a). Similarly, species of leafhoppers of bluestem, Indian grass, porcupine grass, prairie dropseed, and switch grass (Table 1) are restricted to the eastern prairies dominated by Andropogon-Spartina-Sporobolus associations and also differentiate between tallgrass prairie and oak savanna (Hamilton 2005).

Warm-season grasses and their leafhoppers generally occur together in single Canadian biotic provinces (Hamilton 2004b). By contrast, in USA grasslands, species of leafhoppers and their grass hosts co-occur over a wide geographical range that encompasses various ecoregions (Whitcomb et al. 1994), determined in large part by the amount and seasonal patterns of precipitation. The more widespread the host, the more likely it is that the ranges of its leafhopper specialists will not fill the entire host range. Ranges of leafhoppers probably reflect gene pools whose wild types are adapted to regional phenology. Leafhopper faunas in the southwestern states are particularly dynamic. Late summer rains are predictable in both Chihuahuan and Sonoran grasslands, but winter and spring rains are either absent (Chihuahuan) or unpredictable in some years, absent in others (Sonoran). Grass growth cycles (and those of their leafhopper specialists) have extremely different seasonalities in southwestern ecoregions, and the zones, influenced in one way or another by mountain topographies, are much smaller than Canadian ecozones. This circumstance has resulted in subdivision of the range of blue grama in New Mexico by Athysanella species (Whitcomb et al. 1994). By contrast, in Canada, selection for specific timing for emergence and development of leafhoppers that would coincide with the phenology of their grass hosts is generally unnecessary. Periodic droughts affect only the most arid regions of the western Canadian plains.

Historical Range DeterminantsThe leafhopper faunas of Canada and the southern United States differ more in one respect than in any other: the degree to which glaciation has driven Holocene faunal changes. Coastal grasslands probably have been least affected because maritime conditions buffer temperatures. Much of the Great Plains grassland biota likely found refugia on the emergent Gulf Coast. Some species of leafhoppers may have moved northward from their southern refugia through glacial-era grasslands on the exposed Atlantic plain, but not farther than Long Island (a glacial feature). The steeper Pacific coast, separated from montane grasslands by north–south trending mountain ridges, would be less suitable for grassland refugia. Nevertheless, both Vancouver Island and the Queen Charlotte Islands remained ice-free during the Wisconsin (Hendrix and Bohlen 2001) and have retained unique endemic species of leafhoppers. However, many species of grassland leafhoppers seem to have survived on south-facing slopes and arid basins of the Cordillera (Hamilton 2002), and, as noted earlier, on the south slopes of the southern Rocky Mountains of northern New Mexico and Colorado.

South-facing slopes of prairie coulees are also unusually dry microenvironments that support arid grasslands and their biota. The major west-to-east valley system of the South


Saskatchewan, Qu’Appelle, and Assiniboine rivers that extends from southern Alberta to the glacial-era delta at Spruce Woods Forest Reserve in central Manitoba thus serves as a major corridor for western species of leafhoppers expanding their ranges eastward (Hamilton 2004b). Cooler sites in the same valleys provide habitats for eastern prairie species expanding their ranges westward.

Spruce returned to eastern Canada at least 1,000 years after deglaciation, and other tree species lagged far behind (Hamilton and Langor 1987: Table 9). This long period without trees probably allowed grasses, for example, salt-meadow grass, Puccinellia nuttalliana (Schult.) Hitchc., which is adapted to wet soils, to invade the region around James Bay. The probable invasion route of grasses is still marked by patches of prairie grasses such as slender wheat grass, mat muhly, and even switch grass that persist along the Albany River system (Dore and McNeill 1980: Maps 186, 162, 241). In this way, Deltocephalus serpentinus Hamilton and Ross probably followed its host, salt-meadow grass, 1,000 km from the Great Plains to James Bay. This and eight other leafhopper species of prairie origin are now isolated in many glaciated, boreal sites far north of the prairie (Hamilton 1997).

The persistence of some leafhopper specialists cannot be accounted for by these refugia. Some specialists, particularly flightless species, such as Aflexia rubranura, must have survived in northeastern localities. This species invaded a present-day island in Lake Huron not later than 9,000 years ago, when rising lake levels cut it off from the mainland (Lewis and Anderson 1989; Hamilton 1994a). It must, therefore, have inhabited southern Ontario at the time of deglaciation and moved northward as the ice cap melted. A periglacial grassland (Catling and Brownell 1995), induced by summer sunshine under a permanent high-pressure system over the ice cap (Hamilton 2002), seems to be the best explanation for this and other relict grassland species and biotypes still found in and around southern Ontario.

The present biotic provinces of the prairies emerged after the Altithermal, when temperatures dropped to levels comparable to those that prevail today. Longer summers permitted multiple broods of specialist leafhoppers, enabling them to expand their ranges. The zone in which bi- or multivoltinism was a viable strategy shrank after the Altithermal, and the zone in which univoltinism was the optimal strategy expanded. Throughout all climatic shifts, ecoregions on mountain slopes moved uphill and downhill. At the same time, both northern and southern prairie faunas experienced strong stressors, both natural and invasive, which are the subject of the next section.

Stressors

Grasslands and their faunas are now undergoing the greatest environmental changes and pressures since the last glaciation, thanks to human activities such as farming, housing, and transportation corridors (see Chapter 1). Loss of most natural grasslands on a continental scale seems inescapable. If leafhoppers survive, must we accept substantial degradation of biodiversity as inevitable? We argue that if properly managed grassland reserves are established, they should have a full complement of characteristic leafhopper specialists.

If one considers not only human-induced stress, but also natural hazards, such as wildfire, flood, and drought (realizing that these stressors are increasingly dangerous as a landscape becomes more highly fragmented), one might wonder how highly specialized species of grassland leafhoppers will survive at all. But this pessimistic expectation is countered by certain recent studies that show amazing resilience of leafhoppers to


disasters, including flood, fire, drought, and environmental degradation, as detailed in the following subsections.

FloodingIn narrow river valleys with annual floods, such as those in southern Wisconsin, grasslands are effectively stripped of all but their most highly dispersing species of leafhoppers (KGAH, unpublished). However, leafhoppers appear to survive in wider flood plains. The Red River in Manitoba overflows its banks regularly and at times catastrophically, with a flood plain that is >70 km wide, yet its vicinity has a well-developed leafhopper fauna at St. Charles Rifle Range near Winnipeg (see Chapter 10 for a description of this study site). This fauna is little different from that found at Grosse Isle 20 km to the north, at the crest of the valley and thus above the flood plain. Evidently, leafhoppers can recover from floods in much the same period as they can recover from fire.

Fire Fire is an intrinsic natural force that maintains grasslands. However, annual burning by prairie managers to discourage tree growth and encourage blossoming of forbs in small reserves can have disastrous consequences for the invertebrate fauna if no section of the managed prairie is left unburned. Sadly, grassland fragments often lack diverse leafhopper specialists because prairie managers prefer to burn the entire reserve to promote showy flowers and butterflies over grasses and leafhoppers. However, it is not necessary to sacrifice biodiversity for showiness. Where grassland reserves are large enough, they can be subdivided so that the entire reserve need not be burned in a single year. Alternative management practices, such as grazing, mowing, and hand clearing of brush, will suffice on smaller sites in most years, and these activities do not affect leafhoppers.

Large leafhopper faunas are often found at small sites where fire management has been infrequent at best. For example, the tiny intact prairie remnant at Grosse Isle, Manitoba, supports 30 prairie-endemic bugs, rivalling or exceeding the faunas of much larger prairie sites elsewhere in Manitoba and Minnesota (Hamilton 1995a). The infrequency of fire on that site, combined with its moist peripheral areas, likely contributes to this unusually high species richness.

Prairie fires usually have hot and cool patches. When a fire occurs in extensive grasslands, there are many skips, each of which is a possible refugium for faunal elements. Life history strategies of leafhoppers take advantage of such fire patterns; thus, populations rebound quickly (Panzer 1988, 2002, 2003; Panzer and Schwartz 2000). Adaptations to fire may include oviposition close to the ground where heat is least uniform. Alternatively, the niche breadth is probably wide enough in most species to permit oviposition in moist areas that are apt to be skipped by fire.

In areas of frequent fires, selection for high fecundity may occur to compensate for a high proportion of fire-kill. Preliminary results from studies at the St. Charles Rifle Range site (KGAH, unpublished) indicate that most species of leafhoppers become dramatically more abundant within two or three years after a burn off. This time frame may reflect, in part, increased host fitness resulting from mineral recycling, or decimation of predators and parasites. Effectively, fires may restart predator-prey cycles, in the earlier stage of which the prey may outbreed the predators. After four or five years, the fauna reverts to its pre-burn population density. Farther south, where insects have more annual generations, the faunal turnover takes only one or two years to revert to equilibrium (Panzer 2003).


DroughtThe success of specialist species of leafhoppers depends very much on the regional success of their host plants. Nutritious and luxurious stands of grasses provide better opportunity for successful colonization and permanence of a colony once it is established. Succulent growth, such as that provided by annuals, or by perennials regrowing after fire, is certainly attractive to colonizing leafhoppers. But at any one site, growth flushes come and go during each growing season. After each generation, leafhoppers are in the air, often by the millions. A succulent patch of new growth can acquire a substantial leafhopper population literally overnight.

Drought is less dramatic than fire but perhaps more devastating to leafhoppers when it causes large areas of grassland to wither. Surprisingly, one finds numerous instances of large populations of specialist leafhoppers on drought-stressed host grasses. The stressed hosts are likely occupied not by choice but by necessity. Some small sites with severely stressed grasses, such as Ontario alvars, may have a leafhopper superabundance in both species richness and total populations. The alvar species are specialists confined to their vegetational island by their non-dispersive life history strategy. In the long term, periodic droughts favour the survival of alvars because wildfire and dieback are essential for maintaining such isolated grasslands in a surrounding sea of trees.

Whatever the degree of specialization, leafhoppers will accept unusual hosts if there is a stark choice of feed or die. If a grassland patch becomes unsuitable in the course of normal heat stress, leafhoppers move to more succulent plant species, even if they do not usually feed on them. Enormous numbers of leafhoppers that normally feed on grasses or forbs commonly gather on sedges and rushes around a water hole.

Generalist leafhoppers usually feed on the most succulent herbaceous plant available (Tonkyn and Whitcomb 1986). Such plants are often annuals, which have vigorous growth flushes in the early part of the season. However, as the season progresses, plants in the early stages of growth are no longer available. Generalists must then fend for themselves. This fending may lead to conspicuous migrations. Local drought conditions may account for reports of millions of swarming leafhoppers of the polyphagous genus Xerophloea in Nebraska in 1920 and again in 1924 (Lawson 1931). Such drought-induced migrations must be distinguished from the seasonal movements of some generalists, for whom swarming is a natural, regularly occurring phenomenon.

When leafhoppers are tramps, their generation times are short, and once mature, the insects usually migrate. In their second generation, they may find other annuals undergoing a growth flush. Several tramp species winter south of Canada (e.g., the aster leafhopper) and begin their first generation much earlier in the season than they could in more northern latitudes. Such species move north with each successive generation (Chiykowski and Chapman 1965), thereby increasing their chance of finding new growth.

Specialists that feed on hosts whose growth terminates each year before their own life cycle is complete follow a similar strategy to that of tramps. They feed on alternative hosts, but without dispersing from the vicinity of their usual host. For example, Ballana chelata DeLong feeds on balsamroot (Balsamorhiza sagittata (Pursh) Nutt.) in the spring, but by the time adults emerge, the balsamroot leaves have begun to wither in the summer heat. Adults will then leave the host to feed on a variety of other forbs in the vicinity of the balsamroot patch (KGAH, unpublished), maintaining sizeable populations of females that probably oviposit on balsamroot before they die.

Usually, high populations of leafhoppers do not cause obvious stress to their hosts, but anyone who has reared leafhoppers knows from hard experience that plants are


not an infinitely rich resource and that there is such a thing as too many leafhoppers on a host plant. Leafhopper damage on grasses (e.g., black grama) is detrimental to seed yield (R. Garner, pers. comm.) Because parasites normally keep overpopulation under control, damage from large numbers is scattered. In cases of superabundance on isolated patches of their host, conditions may occur in which competition, normally not observed in sap-sucking insects, operates between closely related leafhopper species (Hamilton and Zack 1999).

Habitat Fragmentation Native grasslands today are broken into numerous isolated fragments, much as they once were during Pleistocene glaciation. The principles of island biogeography (MacArthur and Wilson 1967) should apply to such grassland patches; that is, they lose species until they reach a point at which species richness is in equilibrium. However, grasslands appear to have increased in ecological complexity through the Ice Age. The Hawaiian island chain, which continues to expand in size and number (Zimmerman 1948), may be a better analogue of prehistoric interglacial prairies. Such grasslands expand greatly each time world temperatures ameliorate and their leafhopper fauna could diversify at the same time.

Retention of Canadian leafhopper faunas in undisturbed, though highly fragmented, sites seems not to be a problem. Specialist leafhoppers can sometimes persist on very small patches, and large differences in faunal suites have been noted between prairie sites separated by as little as 6 km (e.g., Freda Haffner Kettlehole Preserve and Cayler Prairie in Iowa). A thriving colony of Laevicephalus minimus (Osborn and Ball) was discovered near Belleville, Ontario, on only four tufts of its only known host grass, side-oats grama, Bouteloua curtipendula (Michx.) Torr. This leafhopper species also has persisted on small isolated host bunches as far east as the shale barrens of western Maryland (RFW, unpublished), whereas another side-oats leafhopper specialist, Flexamia pectinata (Osborn and Ball), appears to have been unable to track this patchy resource east of the prairie peninsula in Ohio (Whitcomb and Hicks 1988). Small grassland sites not subjected to stresses such as fire and drought appear able to provide refuges almost indefinitely, because endemics have survived in remote montane sites in the Pacific Northwest throughout Pleistocene glaciation (Hamilton 2002).

The situation of grasslands on sandy or gravelly soils left by glaciation is much more fragile. Erosion and revegetation of glacial deposits presumably fragmented the ranges of many species of leafhoppers that feed on plants, which are, in turn, found primarily on outwash substrates or on sand and loess. For example, Altithermal temperatures may have permitted such western species as the flightless Neocoelidia tumidifrons to move into alkaline bogs and sand ridges in eastern Ontario and Pennsylvania where the habitats are unstable and decreasing in area. Winged grassland insects such as the leafhopper Commellus comma (Van Duzee) and the spittle bug Prosapia ignipectus, by contrast, are more mobile and can still be found in extensive relict grasslands as far east as upstate New York and Maine (Hamilton 1982, 1994a).

Conclusions

Canadian grasslands differ in many ways from those of the United States. These differences appear to be both proximal (short growing season and more limited choice of host plants) and historical, superimposed on pre-existing patterns by glacial geography. Prehistoric origins determined to a large degree the pre-settlement characteristics of northern grasslands.


Glacial scouring and redeposition, followed by meltwater coulees and outwash deltas, laid the foundations for extensive postglacial grasslands in much of southern Canada. Severe winter conditions during the height of glaciation, combined with cooler, shorter summers, have limited the number of potential leafhopper hosts. One of the results of this limited plant diversity has been the evolution of numerous species of leafhoppers that are strict monophages adapted to dominant or subdominant grassland plant species. After the Altithermal, short summers in the north have permitted a limited number of leafhopper generations each season. Many species in Canada have only a single brood. Univoltinism discourages dispersion but permits insects to more fully adjust to the seasonality of their hosts. By contrast, long summers farther south, combined with the vast geographical extent of the prairie, probably have fostered broader host and geographical ranges in the United States. Comparatively small leafhopper ranges in the desert plains grasslands are accounted for by distinctly different host phenologies in small ecoregions.

The leafhopper fauna of Canadian grasslands is unexpectedly biodiverse. The reasons are not entirely clear, but this biodiversity can be partially explained by host specificity, glacially driven habitat change, and persistent isolated grasslands. This situation contrasts with isolated grasslands in forested areas farther south, which are mostly the result of frequent but scattered local disturbance, such as wildfire, and usually harbour only widely dispersing leafhoppers.

Monophagy is common in species of leafhoppers that feed on perennial grasses and prairie shrubs. These plants, like trees, provide a generally stable, abundant food source that is easily located by ovipositing females. On the other hand, no single grass clump is sufficient for the long-term continuity of a leafhopper colony on the prairies. Whereas it is possible for a single tree to host a leafhopper colony for many decades or even centuries, this is not true for a single grass plant, which makes the fauna of very small prairie fragments vulnerable over time to extirpation. However, some leafhopper populations have developed compensatory mechanisms that make them remarkably resilient to such stressors as fire, floods, and fragmentation of landscapes. Like snails, leafhoppers have persisted for thousands of years in isolated grasslands much too small to preserve relict populations of larger or more vagile animals.

Present distributions of species of leafhoppers in Canadian grasslands reflect past distributions of both prairie ecoregions and glacial-age ecosystems. Cool-season grasses naturally proliferated during deglaciation, and relict postglacial flora and fauna associated with moraines and sand ridges are found as far east as the Atlantic coast. Several warm-season grasses (e.g., mat muhly) became adapted to periglacial grasslands. Similarly, the flightless leafhoppers Aflexia and Memnonia appear to have come north to islands in Lake Huron with prairie dropseed, a warm-season grass, at least a thousand years before the Altithermal. The presence of Memnonia panzeri on an alvar near Almonte, Ontario, indicates that a periglacial grassland formerly extended at least as far east as the vicinity of Ottawa. Prairies at their maximal (Altithermal) extent probably never reached farther east than Windsor, Ontario, the easternmost site for a tallgrass prairie remnant with a rich fauna of insect specialists (see Chapter 9). Thus, even before the Altithermal, postglacial patches of grasslands have been an integral part of the ecology of eastern Canada.

The future of many prairie species of leafhoppers is in doubt. Although they can survive many stressors, their host plants cannot, and global warming may well decimate the floras of many prairie preserves over the next century. If and when this happens, specialist leafhoppers probably will not be able to find and colonize remote patches.


Acknowledgements

Studies on the impact of fire on fauna at the St. Charles Rifle Range in Winnipeg were conducted by R. Roughley in 1997–2000, and his Homoptera samples were donated to the Canadian National collection in Ottawa. It is important for us to also acknowledge the inspiration of H. H. Ross, who interested us, as students, in grassland leafhoppers. In this chapter, we have attempted to synthesize the results of our joint studies and the personal communications left to us by Ross, which represent more than 60 years of study. We have worked for years independently on different aspects of this study: one of us (KGAH) on the biogeography and systematics of leafhoppers in Canadian and northern USA grasslands, and the other (RFW) on the ecology of more southerly USA grasslands, and, to a minor extent, those of northern Mexico. Our very different perspectives have greatly enriched this synthesis.

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Documents

Leafhoppers (Homoptera: Cicadellidae): a major …...history of North American grasslands and their insect fauna is intimately tied to glaciation. This is particularly true of interglacial