Generalist Natural Enemies of a Willow Leaf Beetle (Phratora vulgatissima): Abundance and Feeding Habits

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Journal of Insect Behavior [joib] pp1083-joir-477977 January 30, 2004 23:3 Style file version Feb 08, 2000

Journal of Insect Behavior, Vol. 16, No. 6, November 2003 ( C© 2003)

Generalist Natural Enemies of a Willow LeafBeetle (Phratora vulgatissima): Abundanceand Feeding Habits

Christer Bjorkman,1,2 Peter Dalin,1 and Karin Eklund1

Accepted July 16, 2003; revised August 18, 2003

The natural enemies attacking eggs (and young larvae) of the willow leafbeetle Phratora vulgatissima were identified in the field. Three heteropteranswere common natural enemies. The mirid Orthotylus marginalis was the mostabundant and had an intermediate consumption rate in the lab, whereas themirid Closterotomus fulvomaculatus was the least abundant but had the high-est consumption rate. The anthocorid Anthocoris nemorum was intermediatein abundance but had the lowest consumption rate. However, the experimentalsituation (in petridish or on shoot) affected the ranking of the predators andillustrates behavioral differences. The anthocorid was very mobile and couldbe characterized as a “run and eat” predator, whereas the mirids were lessmobile and behaved to a “find and stay” principle. Possible consequences ofinterspecific variation in behavior, from a biological control perspective, arediscussed.

KEY WORDS: Chrysomelidae; predation; Anthocoridae; Miridae; omnivorous; generalistpredators.

INTRODUCTION

Successful biological control of insect pests depends on a number of fac-tors. Knowledge about the key elements in the population dynamics of the

1Department of Entomology, Swedish University of Agricultural Sciences, P.O. Box 7044,SE-750 07 Uppsala, Sweden.

2To whom correspondence should be addressed. Fax: +46 18 672890. e-mail: [email protected].

747

0892-7553/03/1100-0747/0 C© 2003 Plenum Publishing Corporation

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748 Bjorkman, Dalin, and Eklund

pest needs to be acquired (Dent, 1991). Heavy defoliation by leaf beetles,attacking coppicing willows (willows grown for biomass production andcleaning of soil and wastewater), can cause a 40% reduction in plant growthcompared with undefoliated control plants (Bjorkman et al., 2000b). We havefound indications that generalist natural enemies could be of great impor-tance in determining the abundance of the beetles (Bjorkman et al., 2000a).An important step toward successful biological control is to identify the im-portant natural enemy species. The traditional view has been that naturalenemies should be specialists to reach efficient biological control (DeBachand Rosen, 1991; Hoy, 1994). However, recent findings have altered thisview—more and more data point to evidence that generalist natural en-emies may be very effective in pest control (Symondson et al., 2002, andreferences therein). Arguments in favor of generalists are, for example, thatthey often show (1) temporal persistence—they can survive on other fooditems than the target pests (many even on plant tissue, i.e., omnivores); (2)opportunistic feeding—they switch to the most common prey; and (3) goodcolonizing ability (Ehler, 1990). Generalist natural enemies may, therefore,be present in advance of pest establishment and thereby prevent increasesin pest populations.

Sustainable control is more likely if more than one enemy species is uti-lized because it becomes less likely that fluctuations in enemy density leads touncontrolled population growth of the pest. Thus, to focus on just one enemyspecies, often a specialist, has seldom proven a good strategy (Symondsonet al., 2002). Two of the main reasons why several species of generalisticnatural enemies may function as effective biological control agents are thatthey may act (1) in concert and (2) synergistically. That they act in concertis an important, but not always acknowledged, consideration because thereare often several enemy species feeding on the same prey species. The netoutcome from several natural enemies may be either additive, the sum ofparts, (Samways, 1986) or synergistic, greater than the sum of parts (Loseyand Denno, 1998). However, negative interactions between natural enemiescould result in antagonistic (less than the sum of parts) effects (Spiller, 1986;Ewans, 1991; Ferguson and Stiling, 1996). One concept is that species withvery similar behaviors and feeding habits would be more likely to interactantagonistically, whereas the chances of finding additive or even synergisticinteractions between species would increase with increasing differences inbehaviour (Sih et al., 1998). In the system we are working with (willows, leafbeetles, and natural enemies), we have observed that some natural enemiesdiffer in theie behavior. Differences in behavior could also imply differencesin enemy efficiency in finding prey and thereby their consumption rates,which is an important consideration when trying to find efficient predatorsfor biological control programs.

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Feeding Habits and Abundance of Generalist Natural Enemies 749

The first aim of this study was to identify the natural enemies attackingeggs (and larvae) of Phratora vulgatissima L., the leaf beetle which has causedmost of the damage in willow plantations. One reason for identifying theenemies are results indicating that predation on eggs and larvae may be akey factor in determining abundance of this species (C. Bjorkman et al.,unpublished data) and another willow leaf beetle species (Bjorkman et al.,2000a). The second aim was to verify our observations that natural enemiesdiffer in their behavior. This was done by (1) observing natural enemies inthe field and (2) estimating the spatial distribution of predation on shootswith leaf beetle eggs in the laboratory. A possible consequence of variationin behavior among natural enemies could be that they have complementaryeffects on the prey. The third aim was to quantify the consumption rateof the enemies. As an extension of the third aim we also wanted to testwhether predation rates differed when predators were held on willow shootscompared to when they were held in petri dishes. Our prediction was thatthe predation rate would be higher in petri dishes than on shoots becauseenemies would not have to find eggs before starting to consume them inpetri dishes. In other words, the search time needed to locate eggs on theshoots would result in a lower overall predation. The fourth aim was to getindications whether natural enemies attacking eggs of P. vulgatissima alsofeed on their larvae. If so, their potential role in biological control would begreater. For this purpose we compared the egg and larval mortality in sixwillow plantations in 2 years.

MATERIALS AND METHODS

Identifying the Natural Enemies

The method used for identifying potential natural enemies was to noteindividuals, belonging to taxonomic groups known to include predatoryspecies, that were found in knockdown samplings in willow plantations and innatural willow stands with Phratora vulgatissima present. To find out if theyfunction as predators we (1) observed if they fed on eggs in the field and/or (2)tested if they fed on eggs in the laboratory. Only the most common ones (i.e.,the two mirids Orthotylus marginalis Reuter and Closterotomus fulvomacu-latus [De Geer] and the anthocorid Anthocoris nemorum [L.]) were tested inthe laboratory. Most observations were done in 1997 and 1998. The knock-down sampling method is described below under Abundance. In total, about30 willow plantations and 20 natural willow stands were sampled each year.At least 30 knockdown samples (a 40-cm piece of a willow branch) per plan-taion/stand were taken. The egg batches studied in the field were a haphazard

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selection and observations were made at several willow plantations and nat-ural willow stands. Several hundreds of egg batches were checked.

For practical reasons we have concentrated our studies mainly on nat-ural enemies feeding on eggs. However, we have strong indications that theenemies attacking eggs are also feeding on young leaf beetle larvae (seebelow).

Abundance

The abundance of heteropterans was estimated by knockdown samplingin 13 willow plantations around Uppsala, central Sweden, in 2 years (2000and 2001). Samples were taken at equal distance (15 m) along six transectsdistributed evenly in each plantation. Transects were evenly distributed toget samples from the whole plantations because (1) we know that leaf beetledensity varies spatially (Peacock et al., 1999) and (2) we used plantation asthe independent observation unit. Transect leangth varied with plantationsize. However, at least 30 samples per plantation were taken, which meansthat samples were collected with a higher spatial density in a few (n = 2)small plantations. The sampling was conducted in late May to mid-June, i.e.,the period when P. vulgatissima lay their eggs. Each knockdown sample wastaken from a 35-cm top part of a shoot. By taking into account the factsthat (1) willow stools normally grow at a density of about 10,000 per ha and(2) the average total length of green shoots on a willow stool is 11.0 m (SE,0.84 m), it was possible to calculate the abundance per hectare.

Data on abundance were log 10(x + 1) transformed before they wereanalyzed in an ANOVA (GLM procedure in SAS). The transformation wasperformed to reach nonsignificant differences in variance and normal distri-butions. Species was considered a fixed factor in the analysis, whereas yearand plantation were treated as random factors.

Behavior

Following Individual Enemies in the Field

During 1999, the activities of known heteropteran predator species wereobserved in the field. Observations were done by visually searching plantsfor predators and noting their behaviour. The behavior was divided into (1)standing still, (2) walking, and (3) feeding. The activity of each individualpredator, when it was first discovered on the plant, was noted. Thus, we gotan estimate of the frequency of observations and each predator individualwas registered only once. Chi-square tests were used to test if frequencies

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of observations in different observation classes differed among predatorspecies. The plant species searched for predators were two species of willows(Salix viminalis and S. cinerea).

Distribution of Predation Among Egg Batches Within Shoots in the Lab

To illustrate differences in behavior between the two most commonspecies of heteropterans (A. nemorum and O. marginalis), we used data fromthe consumption rate studies on shoots (see below for a detailed descriptionof the experimental design). For this purpose we estimated (a) the numberof egg batches (zero, one, or two) where there had been any eggs eaten and(b) the proportion of eggs eaten among the egg batches in which there hadbeen any predation.

Consumption Rates in Petri Dishes and on Shoots

The consumption rate (calculated as the average number of eggs eatenper hour) of individual predators, in particular, heteropteran predators, wasfirst studied in petri dishes (diameter, 9 and 12 cm). Heteropterans havepiercing and sucking mouthparts and, therefore, do not consume the com-plete egg with the eggshell. However, it is quite easy to recognize a piercedegg, as the contents of the eggs are usually completely ingested by thesepredators. We therefore used the number of eggs predated (number ofpierced eggs) as a measure of consumption. This study was done in thelaboratory at approximately 20◦C (L:D, 20:4h). Enemy individuals were fol-lowed from instar I/II to adults. Individual enemies were followed for twoor three instars and individuals had to be replaced due to mortality and es-capes. The number of individuals for each species in each life stage varied:O. marginalis I:0 (i.e., for instar I, n = 0), II:4, III:14, IV:13, V:12, and ad:3;C. fulvomaculatus I:0, II:2, III:7, IV:10, V:9, and ad:4; and A. nemorum I:6,II:11, II:11, IV:8, V:8, and ad:7. Because of these imbalances in the data nostatistical analyses were attempted. Predation of eggs was counted every 1–3days when new eggs were also provided. The enemies always had a surplusof eggs.

Knowing that results from petri dish studies may differ from studiesperformed under more natural conditions (Dalin and Bjorkman, 2003), wedecided to compare the consumption rate of the two most common het-eropterans (A. nemorum and O. marginalis) on willow shoots with that inpetri dishes. Thus, a second set of petri dish experiments was conducted.Late instars (fifth instar) or adults of O. marginals and adult individuals ofA. nemorum were used in the experiment, as they had similar consumption

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in the petri dish experiment described above. These life stages of predatorsappear in the field simultaneously and the predators were collected from thefield prior to the experiment. The on-shoot study was conducted in a climatechamber (temperature, 20◦C; L:D, 20:4h), whereas the petri dish experimentwas conducted in the laboratory at approximately 20◦C (range, 19–23◦C).In the on-shoot experiment, individual predators were released on pottedS. viminalis saplings containing one shoot (60 cm tall) with two egg batches(one toward the top and one toward the base), inside plastic cylinders (height,70 cm; diameter, 25 cm), to prevent the predators from escaping. In the petridish experiment, the predators were provided with willow leaves containingtwo egg batches to avoid all eggs being consumed during the experiment.The number of eggs eaten was checked every day and the experiment wasterminated after 2–3 days. The main difference between the on-shoot experi-ment and the one in petri dishes is that the latter includes only handling time,whereas the former also includes searching time. The density of egg batchesin the on-shoot experiment was low to moderate compared to the densitiesobsereved in the field. The predators were allowed to search and consumeeggs on the shoots for 48 h. The reason for terminating the experiment after48 h was that we wanted to avoid all eggs being consumed during the exper-iment. Thus, similar to the petri dish experiment, the predators always had asurplus of eggs. Because of lack of normality in the data, due mainly to a highproportion of zero values in the on-shoot experiment (i.e., no eggs eaten),a nonparametric Kruskal–Wallis test was done with a Tukey-type multiple-comparison test (Nemenyi’s test) for unequal sample sizes (Zar, 1999).

Relationship Between Egg and Larval Predation in the Field

Several of the natural enemies identified to feed on P. vulgatissima eggshave also been observed to feed on P. vulgatissima larvae. If the same preda-tors feed on both eggs and larvae, their importance in the regulation of preypopulations could be greater. If the same predators feed on both eggs andlarvae, we would expect a positive relationship between egg and larval mor-tality at specific sites (e.g., plantations). Two implicit assumptions for thiscomparison are that there is little dispersal and mortality among the naturalenemies. We have preliminary data that indicate that this is true during thetime of the year when the studies were done (C. Bjorkman et al., unpub-lished data). To make the comparison we conducted a field study 2 years ina row, 1997 and 1998. The approach was to correlate egg mortality of P. vul-gatissima with larval mortality in six individual willow plantations. A total of10 egg batches and 10 larval groups per plantation was placed out at randompositions. Eggs were obtained by keeping gravid females in sleeve cages on

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willow shoots. The cages and the beetles were removed after 2 days and theeggs counted. The eggs were brought to the laboratory after the experimentand kept in petri dishes until the eggs hatched. Mortality equaled the dif-ference between the number of eggs at the start and the number of larvaehatched. Larvae were added to shoots by attaching small pieces of leaveswith first-instar larvae onto leaves of willows growing in the plantations. Lar-vae normally crawl over from the leaf piece to the experimental leaf within acouple of hours (C. Bjorkman et al., personal observations). Larval mortalityequaled the difference in number of larvae at the start and at the end of theexperiment. Egg mortality was censused between June 11/12 and June 23/24in 1997 and between June 10–12 and June 24–26 in 1998. Larval mortalitywas censused between July 8/9 and July 11 in 1997 and between July 2/3 andJuly 6/7 in 1998. The mean from each plantation was used inthe statisticalanalyses and is presented in Fig. 5.

We decided not to include controls (i.e., caged eggs and caged larvae)in the experiment because we have previously found a high survival ofeggs (on average, >90% [C. Bjorkman et al., unpublished data]) and lar-vae (Bjorkman et al., 2000a) in cages.

RESULTS

Identifying the Natural Enemies

A large number of potential natural enemies were identified (Table I).However, only a few of these were observed to actually feed on P. vulgatis-sima eggs. In the field, only three species of Heteroptera (i.e., the two miridsOrthotylus marginalis and Closterotomus fulvomaculatus and the anthocoridAnthocoris nemorum) and syrphids (Syrphis spp.) were observed to feed oneggs. The syrphids were common (i.e., approx. 20% of the P. vulgatissimaegg batches had one or several syrphid eggs or larvae) in certain plantationsin certain years. Thrips were also observed to “feed” (i.e., sit on) on eggs inthe field but their consumption rate in petri dishes was very low. The miridLygus rugulipennis (Poppius) was never observed to feed on eggs in thefield even though it fed on eggs when kept in petri dishes. Among the otherspecies (group B in Table I) some were locally common in certain planta-tions and natural willow stands (e.g., ants), whereas others were more evenlydistributed (e.g., spiders). Among the enemies in group B, only spiders weretested in petri dishes in the laboratory and they were never observed tofeed on eggs. Most of the other enemies were considered to be so uncom-mon that they would play only marginal roles in the population dynamics ofP. vulgatissima.

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Table I. Potential Natural Enemies of the Willow Leaf Beetle Phratora vulgatissima

Predation observed in

Natural enemy taxon Field Laboratory

A. Observed to feed on eggsOrthotylus marginalis (Heteroptera: Miridae) • •Closterotomus fulvomaculatus (Heteroptera: Miridae) • •Anthocoris nemorum (Heteroptera: Anthocoridae) • •Thrips (Thysanoptera) — •Syrphus spp. (Diptera: Syrphidae) • •Lygus rugulipennis (Heteroptera: Miridae) — •

B. Not observed to feed on eggsSpiders (Aranae) •a ◦bEarwigs (Dermaptera) — —Assassin bugs (Reduvidae) — —Damsel bugs (Nabidae) — —Shield bugs (Pentatomidae) •a —Lacewings (Chrysopidae) — —Ants (Formicidae) — —Rove beetles (Staphylinidae) — —Soldier beetles (Cantharidae) — —Soft-winged flower beetles (Malachiidae) — —Ladybirds (Coccinellidae) — —Tits (Parus spp.) — —

aObserved to feed on adults.bTested in the laboratory but did not eat.

Abundance

The most common heteropteran predator was O. marginalis, followedby A. nemorum, then C. fulvomaculatus as the third most abundant (Table II)and the three species differed significantly in abundance (F2,60 = 23.73,P < 0.05). There was no significant difference between years (F1,60 =1.55, P = 0.22), nor was there any significant species ∗ year interactions

Table II. Mean Abundance (Number of Individuals per Hectare) of Three Heteropteran Natu-ral Enemy Species Attacking Eggs and Larvae of the Willow Leaf Beetle Phratora vulgatissima

in 13 Willow Plantations for 2 Years (2000 and 2001)

Year 2000 Year 2001

Species Meana SE Meana SE

Orthotylus marginalis 98,104α 46,337 55,293α 20,384Closterotomus fulvomaculatus 1,837γ 1,119 3,424β 2,388Anthocoris nemorum 22,256β 11,002 12,063γ 3,599

aMeans with the same superscript Greek letter are not significantly different (P > 0.05; Tukey’stest). Pairwise comparisons done within years.

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(F2,60 = 0.74, P = 0.48). However, abundance of heteropterans differedsignificantly among plantations (F12,60 = 3.22, P < 0.001).

Behavior

Following Individual Enemies in the Field

In total, 112 observations were made for Orthotylus marginalis, 182 forAnthocoris nemorum, and 26 for Closterotomus fulvomaculatus. Observa-tions on the two mirid species were mainly on late instars (III–V), whereasboth adults and small nymphs of A. nemorum were observed. The obser-vations on A. nemorum were therefore divided up into nymphal and adultstages to be able to compare differences in behavior between the two lifestages. The proportions of observations on behavior for the two mirid speciesand the two life stages of A. nemorum are presented in Fig. 1. The figureshows that the anthocorid A. nemorum was more often observed walking onthe plants compared with the two mirid species that were mainly observedstanding still. Chi-square tests revealed significant differences between adultA. nemorum and the mirid C. fulvomaculatus (χ2 = 13.4, P = 0.001) andbetween adult A. nemorum and O. marginalis (χ2 = 10.9, P = 0.004) inthe frequencies of observations in different activity classes (standing still,walking, and feeding). Also, significant differences were found between

Fig. 1. Behavioral differences among three common heteropteran predatorsattacking eggs of the willow leaf beetle Phratora vulgatissima illustrated bythe proportion of total time devoted to three behavioral categories. N = 56and 126 for adult and nymphal Anthocoris nemorum, respectively, 112 forOrthotylus marginalis, and 26 for Closterotomus fulvomaculatus. Based onfield observations.

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nymphal A. nemorum and C. fulvomaculatus (χ2 = 10.7, P = 0.005) and be-tween nymphal A. nemorum and O. marginalis (χ2 = 36.9, P < 0.001). Nodifference between the two mirid species was found (χ2 = 0.22, P = 0.89).We found significant differences between the two life stages of A. nemorum(χ2 = 10.9, P = 0.004). Figure 1 shows that adults of A. nemorum weremore active; i.e., adults were more often observed walking than the smallnymphs.

Distribution of Predation Among Egg Batches Within Shoots in the Lab

The anthocorid was more mobile than the two mirids on the shootswith two egg batches in the laboratory. Two patterns show this. First, moreA. nemorum individuals visited (i.e., ate eggs) from two egg batches than anyof the two mirids (Fig. 2, left; χ2 = 45.9, P < 0.001, df = 2, and χ2 = 34.8,P < 0.001, df= 2, when compared with O. marginalis and C. fulvomaculatus,respectively). The two mirids also differed significantly with respect to thenumber of individuals that visited zero, one, or two egg batches (Fig. 2, left;χ2 = 13.6, P < 0.001, df= 2). Second, the two mirids consumed most of theeggs in an egg batch when they found one before moving to a new feedingarea, whereas A. nemorum left a greater proportion of eggs uneaten in eachbatch and instead visited several egg batches. This difference is illustrated inFig. 2 (right). A. nemorum differed significantly from both O. marginalis andC. fulvomaculatus (χ2 = 15.8, P < 0.001, df = 3, and χ2 = 19.9, P < 0.001,df = 3, respectively). The distributions for the two mirids did not differ sig-nificantly (P = 0.23, Fisher exact test—called upon due to low expectedfrequencies, df = 3).

Consumption Rates in Petri Dishes and on Shoots

The first experiment, where the consumption rates of three heteropteranpredators were followed from I/II instar to adult stages in petri dishes, in-dicated that C. fulvomaculatus had the highest consumption rate, the miridO. marginalis was intermediate, and A. nemorum had the lowest consuptionrate (Fig. 3).

The next experiment, in which we compared consumption rates of field-collected O. marginalis and A. nemorum in petri dishes with those on shoots,revealed that A. nemorum had a higher consumption rate on shoots com-pared with that in petri dishes (Fig. 4; Q0.05,4 = 5.52, P < 0.001). No signif-icant difference was found in O. marginalis on shoots versus in petri dishes(Fig. 4; Q0.05,4 = 2.36, P > 0.05). Note that the results for the two speciesare mirror images of each other.

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Fig. 2. Behavioral differences among three common heteropteran predators attacking eggsof the willow leaf beetle Phratora vulgatissima illustrated by the number of individuals ofpredators that visited (i.e., at least one egg was consumed) zero, one, or two (of a totalof two) egg batches on a willow shoot (left) and the relative proportion of egg batches inwhich at least one egg was eaten divided into categories of percentage of eggs eaten peregg batch (right). N = 73, 45, and 50 individuals for Orthotylus marginalis, Closterotomusfulvomaculatus, and Anthocoris nemorum, respectively, on the left and N = 25, 32, and 64egg batches, respectively, on the right.

Relationship Between Egg and Larval Predation in the Field

As mentioned earlier we present only data from the field comparisonbetween egg and larval mortality here. There was a positive, significant rela-tionship between egg and larval mortality in individual willow plantations inboth 1997 and 1998 (Fig. 5; y = 1.98x + 0.084, r2 = 0.867, P < 0.01, n = 6,and y = 2.37x + 0.66, r2 = 0.880, P < 0.01, n = 6, respectively).

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Fig. 3. Mean consumption rate (number of eggs eaten perhour) of three common heteropteran predators attackingeggs of the willow leaf beetle Phratora vulgatissima. Exper-iments performed in petri dishes with single willow leaveswith a naturally laid egg batch. Data subdivided by lifestage. Error bars indicate SE.

DISCUSSION

The main result in this study was the observation that natural ene-mies belonging to the same category, generalistic omnivorous predators,

Fig. 4. Mean consumption rate (number of eggsper hour) of the two most common heteropteranpredators attacking eggs of the willow leaf bee-tle Phratora vulgatissima. Note that the ranking ofpredator species depends on the experimental sit-uation: on shoot versus in petri dish. Error barsindicate SE.

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Fig. 5. Relationship between egg and larval mortality in the leafbeetle Phratora vulgatissima in six individual willow plantationsin 2 years. r2 = 0.867, P < 0.01, in 1997 and r2 = 0.880, P >

0.01, in 1998.

showed clear variation in behavior. The anthocorid bug (Anthocoris nemo-rum) had active behavior compared to the more sedentary mirids (Orthoty-lus marginalis and Closterotomus fulvomaculatus). One way to capture theessentials in the two behaviors could be “run and eat” for the anthocoridand “find and stay” for the mirids. van Alpen and Jervis (1996) used a similarterminology but mainly for parasitoids. For example, many predator specieshave been designated to have “sit and wait” behavior. However, this is notapplicable to the mirids because they move around to seek their prey butstay to eat it all before they move on. We may ask whether such differencesmatter when trying to understand the role of natural enemies in the popu-lation dynamics of the prey. A scenario possible to imagine is that an activeand more mobile predator may search and consume prey over a larger spa-tial area compared to sedentary predators. One could, therefore, predict asedentary predator to consume prey within a restricted area, whereas anactive and more mobile predator might be better at finding more scatteredprey. Thus, depending on the distribution and density of prey, such predatorsmay have different effects on prey survival.

An unexpected result was the higher predation rate by A. nemorumwhen held on shoots than when kept in petri dishes. We expected to find a

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higher predation rate in petri dishes because the predators would not haveto search for prey before starting to consume them. When predation ratewas studied from first instars to adults in petri dishes, A. nemorum had thelowest consumption rate. However, when studied on shoots, the anthocoridhad a much higher rate compared with O. marginalis. The anthocorid alsohad a higher consumption rate on shoots than in petri dishes. A plausiblereason is that A. nemorum was much more active on shoots than in petridishes and therefore used up energy which was compensated for by con-suming more eggs. Feeding tests conducted in petri dishes should, however,sometimes be treated with caution because they may not represent a realisticforaging situation (Risch, 1985; O’Neil, 1989; Dalin and Bjorkman, 2003).For example, the density of eggs in the on-shoot experiment was lower andmore realistic than the one in petri dishes. The way predators respond toartificially high prey densities can vary, and consumption rates measuredin such circumstances may not correspond to those under field conditions(Wiedenmann and O’Neil, 1991, 1992). However, we suggest that the resultsfrom both designs (on shoots and in petri dishes) illustrate how behavioralvariation among predators can affect the consumption of prey. These resultsmay confirm our prediction above that the active and more mobile preda-tor A. nemorum is better at finding scattered prey within plants, whereasthe sedentary predator O. marginalis is effective once prey are located asindicated by the petri dish experiment.

When predators have been observed coming to egg batches in the fieldwe have noticed that sedentary mirids stay in the vicinity of eggs after feed-ing (“find and stay”), whereas the anthocorid left the eggs to search forfood at other parts of the plants (“run and eat”). In other words, sedentarymirids may visit and feed from the same egg batches several times, whereasthe active and more mobile A. nemorum may visit many egg batches butleave a greater proportion of eggs uneaten. Related pairs of feeding strate-gies have been observed within other functional groups (e.g., parasitoids,scavenging snails) where some species greatly exploit resources at a localscale, whereas others discover new regions more quickly but leave more re-sources behind (Wiskerke and Vet, 1994; Chase et al., 2001). From a biologi-cal control perspective, the consequences of these predator strategies on pestpopulations may depend on the distribution of the pest. For example, a find-and-stay predator may be effective in controlling spatially aggregated prey,whereas a run-and-eat predator may be effective in controlling more solitaryprey.

We found the mirid O. marginalis to be the most common heteropteranpredator attacking eggs of P. vulgatissima. It was followed in abundance bythe anthocorid A. nemorum and C. fulvomaculatus was the least abundant.There was a significant difference in abundance of heteropterans among the

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13 plantations studied, which indicates that there may be site-characteristicfactors that influence abundance. The same may be true for the syrphids,which were common in certain plantations in certain years but almost ab-sent in other plantations. The abundance of syrphids seemed on average tobe lower than that for at least the two most common heteropterans. How-ever, because different methods were used to estimate abundance, no directcomparison can be made. From a biological control perspective it is interest-ing to note that the more uncommon syrphids and C. fulvomaculatus werethe most voracious of the predators. This means that it would be profitableto try and find methods to increase their abundance.

One might ask whether we have missed any important enemies. Thereasons could be at least two: we did not look in the right places or at theright time. Starting with time, which can be further divided into time ofyear and time of day, the greatest risk seems to be that we did not makeany observations at night. Spiders may be more active during the night andlarger spiders have been observed capturing adult P. vulgatissima. Feed-ing tests in laboratory with spiders collected from willow stands revealedno evidence that they prey upon eggs. The identifications of natural ene-mies have been conducted in plantations of basket willow (Salix viminalis).It could be that other natural enemies are common and more importantin other willow systems (e.g., in stands of S. cinerea and S. aurita). Birdsand pentatomid bugs have been observed to eat larvae of the willow leafbeetle Galerucella lineola in wetland willows (Sipura, 1999; Bjorkman et al.,2000a). Predation by birds is difficult to measure but it is reasonable to as-sume that both birds and pentatomids do prey on larvae. Some larger birds,for example, pheasants, may even consume adult beetles and pupae. Stud-ies on densities of predators during the egg-laying period of leaf beetleshave shown low densities for pentatomids (P. Dalin et al., unpublished data).The relative importance of pentatomids for eggs and early-instar larvaemay therefore be low compared with the other heteropteran predators. Wehave made some observations that pentatomids do feed on older leaf bee-tle larvae (second to third instars), which may indicate that other naturalenemies may become more important later in the season (Bjorkman et al.,2000a).

As most of the plant damage is caused by the cohort of older leaf beetlelarvae (second to third instars), the survivorship of eggs and early-instar lar-vae will probably determine whether economically damaging populationswill be present. Predation on eggs and early-instar larvae could then repre-sent the most important contribution to pest control (Yeargan, 1998). Thegood correlation between egg and larval mortality in individual willow plan-tations (Fig. 5) implies, to us, that the same natural enemies were feeding oneggs and young larvae.

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The fact that there seem to exist few studies dealing with variationwithin functional groups could be taken as an indication of its unimportance.Some studies have shown that variation in behaviors can allow coexistenceof related consumer species (Wilson et al., 1999; Chase et al., 2001). Otherstudies have considered variation in predator/parasitoid behavior in rela-tion to prey/host range of the predator/parasitoids (Zhang and Sanderson,1993; Wiskerke and Vet, 1994). We believe that the lack of studies similarto ours is explained by the lack of attempts to study such variation and thatit may be important to take such variation into account to better under-stand the role of generalist natural enemies in the population dynamics ofprey.

To conclude, our methods for detecting enemies identified three het-eropteran species and syrphids to be the most common and probably themost important predators on the leaf beetle P. vulgatissima. Generalist om-nivorous predators that feed on food items other than the pest can havethe potential to be present in good numbers in advance of pest establish-ment and thereby prevent increases in pest populations (Coll, 1998; Coll andGuershon, 2002). However, a high abundance of predators may not neces-sarily lead to a better pest control (Symondson et al., 2002; Ostman, 2002).One important factor that can lead to enhanced prey survival is negativeinterference among predators. This is clearly undesirable for biological con-trol and it is considered that predators with very similar foraging behaviorsare more likely to interfere with each other (Sih et al., 1998).

One striking and interesting result in this study was that the predatorsshowed clear variation in behavior. The variation in behavior was mirroredin the consumption of eggs when studied in petri dishes and on willow shoots.That such variation in feeding habits among natural enemies may affect thepopulation dynamics of an aggregated prey such as the leaf beetle P. vulgatis-sima is indicated by results from a simple population model (P. Dalin et al.,manuscript under review). We would therefore like to encourage others totry and discover variation in behavior among species within groups of natu-ral enemies. Such knowledge could be used to better understand observedpatterns of prey abundance in time and space and to develop more efficientbiological control programs in the future.

ACKNOWLEDGMENTS

We would like to thank Solveig Hoglund and Adriana Jerlstrom-Maj fortechnical assistance. Eva Andersson, Barbara Ekbom, and Richard Hopkinsgave constructive criticism of the manuscript. Financial support was providedby the Swedish National Energy Administration.

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Generalist Natural Enemies of a Willow Leaf Beetle (Phratora vulgatissima): Abundance and Feeding Habits