FOREST INSECTS - Sandy Liebhold238 FOREST INSECTS potentially severe ecological and economic consequences. This entry presents a brief overview of the pathways by which nonnative forest

238 F O R E S T I N S E C T S

potentially severe ecological and economic consequences. This entry presents a brief overview of the pathways by which nonnative forest insects arrive, the impacts of inva-sive forest insects, and potential options for managing invaders.

BIOLOGY

Invasion by nonnative forest insects is not a new phenom-enon. More than a dozen nonnative forest insects were damaging trees in the United States before 1850. How-ever, subsequent to 1850, more than 400 species of non-native forest insects, from virtually all insect orders and feeding groups, have become established in the United States and Canada (Table 1; Fig. 1). Among these species, sap-feeding insects such as aphids, adelgids, and scales, and foliage feeding insects such as sawfl ies and many spe-cies of Lepidoptera (moths and butterfl ies), are the most frequent invaders. Invasions by insects that feed under the bark on phloem or wood, however, have increased in recent years. This group includes bark beetles, ambrosia beetles, phloem-borers, and wood-borers. Several of these insects, such as the emerald ash borer Agrilus planipennis, the Asian longhorned beetle Anoplophora glabripennis, and the redbay ambrosia beetle Xyleborus glabratus, are capable of killing otherwise healthy trees and have been particularly destructive.

The biology of many forest insects is intimately inter-twined with microorganisms such as fungi, and often it is necessary to consider such species complexes together in order to understand these problems fully. For example, considerable damage has occurred in pine plantations in the southern hemisphere following invasion by the wood-wasp Sirex noctilio, along with its fungal symbiont Amy-lostereum areolatum. Female woodwasps infect trees with fungi as they lay eggs, and the woodwasp larvae feed on the fungi. The fungus invades the xylem of the tree, how-ever, which disrupts water transport and causes the tree to die. In a very different example, beech scale Cryptococcus fagisuga, a sap-feeding insect native to Asia, was intro-duced into eastern Canada around 1890. At high densi-ties, these tiny insects will cover the bark on the trunk and large branches of American beech trees. Feeding wounds of the scales enables native pathogenic fungi to enter and eventually kill the host tree. Over the past 120 years, beech scale has expanded across much of the range of American beech in Canada and the United States, killing millions of large beech trees and affecting the wildlife that feed on beech nuts or use beech trees for habitat.

In many parts of the world, large plantations of non-native trees have been established to facilitate effi cient

into new regions, such as northern Europe, and for den-gue and even yellow fever into North America. There is also the possibility of Japanese encephalitis virus spread-ing east across the Pacifi c into California.

SEE ALSO THE FOLLOWING ARTICLES

Mosquitoes / Pathogens, Animal / Pathogens, Human

FURTHER READING

Blitvich, B. J. 2008. Transmission dynamics and changing epidemiology of West Nile virus. Animal Health Research Reviews 9: 71–86.

Bryant, J. E., E. C. Holmes, and A. D. Barrett. 2007. Out of Africa: A molecular perspective on the introduction of yellow fever virus into the Americas. PLoS Pathogens 3(5): e75.

Gould, E. A., and T. Solomon. 2008. Pathogenic fl aviviruses. Lancet 371: 500–509.

Gould, E. A., X. de Lamballerie, P. M. Zanotto, and E. C. Holmes. 2001. Evolution, epidemiology, and dispersal of fl aviviruses revealed by molecular phylogenies. Advances in Virus Research 57: 71–103.

Gubler, D. J., G. Kuno, and L. Markoff. 2007. Flaviviruses (1153–1252). In D. Knipe and P. M. Howley, eds. Fields Virology, Vol. 1, 5th ed. Phila-delphia: Lippincott, Williams, and Wilkins.

Holmes, E. C., and S. S. Twiddy. 2003. The origin, emergence and evo-lutionary genetics of dengue virus. Infection, Genetics and Evolution 3: 19–28.

Kyle, J. L., and E. Harris. 2008. Global spread and persistence of dengue. Annual Reviews of Microbiology 62: 71–92.

Mackenzie, J. S., D. J. Gubler, and L. R. Petersen. 2004. Emerging fl avivi-ruses: The spread and resurgence of dengue, Japanese encephalitis and West Nile viruses. Nature Medicine 10(12): S98–S109.

Wilder-Smith, A., and D. J. Gubler. 2008. Geographic expansion of dengue: The impact of international travel. Medical Clinics of North America 92: 1377–1390.

FORBS

SEE GRASSES AND FORBS

FOREST INSECTS

ANDREW M. LIEBHOLD

USDA Forest Service, Morgantown, West Virginia

DEBORAH G. McCULLOUGH

Michigan State University, East Lansing

Compared to other land uses, forests are relatively stable and diverse ecosystems that provide habitat for a variety of organisms. Unfortunately, forest ecosystems on virtu-ally every continent are threatened by a variety of factors, including deforestation, climate change, and biological invasions. Among these factors, invasions by nonna-tive forest insects are a particularly serious problem with

06_Simberloff10_F_p223-263.indd 23806_Simberloff10_F_p223-263.indd 238 9/13/10 2:49:55 PM9/13/10 2:49:55 PM

From Daniel Simberloff and Marcel Rejmánek, editors, Encyclopedia of Biological Invasions,Berkeley and Los Angeles: University of California Press, 2011.

FIGURE 1 Images of some important forest insect pests and the damage they cause. (A) The hemlock woolly adelgid, Adelges tsugae. (Photograph

courtesy of USDA Forest Service, Region 8 Archive, Bugwood.org.) (B) Forest trees killed by the emerald ash borer, Agrilus planipennis. (Photo-

graph courtesy of Daniel Herms, the Ohio State University, Bugwood.org.) (C) The Asian longhorned beetle, Anoplophora glabripennis, adult. (Pho-

tograph courtesy of Daniel Herms, the Ohio State University, Bugwood.org.) (D) Larva of the gypsy moth, Lymantria dispar. (Photograph courtesy

of Daniel Herms, the Ohio State University, Bugwood.org.)

TABLE 1

Examples of Some Well-known Invasive Forest Insect Species

Species Common Name Native Range Invaded Range Host Damage

Anoplophora Asian longhorned Eastern Asia North America, Acer spp., Aesculus spp., Dieback, glabripennis beetle Europe Populus spp., Betula spp. breakage, mortalityCameraria ohridella Horse chestnut Southern Europe Central and Aesculus hippocastanum Defoliation leaf miner western EuropeAgrilus planipennis Emerald ash borer China, Korea, maybe North America, Fraxinus spp. Dieback, mortality

other areas of Asia Russia (Moscow)Dendrocotnus valens Red turpentine beetle North America China Pinus spp. MortalitySirex noctilio Sirex woodwasp Eurasia Australasia, South Pinus spp. Mortality America, Africa, North AmericaHyphantria cunea Fall webworm North America Eurasia Broadleaf trees DefoliationAdeleges tsugae Hemlock woolly East Asia, western Eastern North Tsuga spp. Defoliation, adelgid North America America mortalityGonipterus Eucalyptus weevil Australia Africa, North Eucalyptus spp. Defoliation scutellatus America, South America, EuropeLymantria dispar Gypsy moth Eurasia North America Quercus spp., Defoliation Populus spp., Larix spp. Xyleborus Redbay ambrosia Asia Southeastern Redbay (Persea borbonia) Mortality glabratus beetle United States


240 F O R E S T I N S E C T S

The International Plant Protection Convention recently enacted an international phytosanitary measure, ISPM 15, which limits the movement of raw wood. While this mea-sure and other regulatory efforts should help, expanding global trade and travel will almost certainly result in more introductions of nonnative forest insects.

IMPACTS

Only about 15 percent of the nonnative forest insects known to be established in the United States have ever caused noticeable damage. The majority of nonnative insects remain at relatively low densities and cause no appreciable effect on trees or ecological processes. While the number of damaging species may be limited, a few of these species have severely affected the forests they have invaded. Emerald ash borer, for example, is an Asian insect that became established in Michigan at least ten years before it was discovered in 2002. To date, tens of millions of ash trees in Michigan and the upper Midwest have been killed by this phloem-feeding insect. At least 18 ash species native to North America appear to be threat-ened by this recent invader.

While most nonnative forest insects appear to be innocuous, some of these species may be altering ecosys-tems in subtle ways that are not easily detectable. Recent surveys have found that, in some regions of the world, most ambrosia beetles (Scolytinae) are not native. Most ambrosia beetles colonize dying or dead trees and play important roles in the decomposition of wood and nutri-ent cycling. It remains to be seen whether the replace-ment of native ambrosia beetles by exotics may be causing some alteration of ecosystem processes that to date has not been investigated.

MANAGEMENT

Regulatory offi cials and forest entomologists employ a variety of tactics to protect native forests from invasive insects. Preventing the arrival and potential establish-ment of nonnative forest insects is ideal. Numerous regulatory efforts including quarantines, mandatory pesticide treatments, and inspections are used to pre-vent nonnative insects from arriving on cargo or in baggage carried by travelers. Early detection of newly established nonnative insects is also critical. A localized, low-density population of an invader can sometimes be successfully eradicated, while eradication of a widely distributed or high-density population will be much more challenging.

The availability of sensitive detection methods remains a critical limitation of detection and eradication strategies.

production of lumber and fi ber. Many of these exotic plantation forestry efforts have been highly successful. However, much of the high productivity associated with growing exotic trees can be attributed to the “escape” of tree species from their associated herbivores. Unfortu-nately, this escape may not last forever, and in many parts of the world, herbivores are catching up with their hosts. For example, species in the genus Eucalyptus are native to Australia but have been widely planted elsewhere in the world. Many Eucalyptus plantations are highly produc-tive, and several Eucalyptus species are used as landscape trees in urban forests. Recently, however, several Austra-lian insects associated with Eucalyptus have invaded these regions and caused considerable damage.

INVASION PATHWAYS

The problem of forest insect invasions can be attrib-uted largely to global trade. Many insects are inadver-tently transported either on or within their host plant or because they hitchhike on commodities or items trans-ported as cargo. Imported plants, destined for nurseries or landscapes, have historically been an important pathway by which plant-feeding insects have arrived in new habi-tats. Herbivorous insects that are transported with their host plant presumably have a high probability of becom-ing established because they need not search for hosts. Regulatory offi cials may require live plants to be treated or inspected either before shipping or upon arrival, to reduce the risk that nonnative insects or other organisms could become established. Despite these and other safe-guards, ornamental trees for retail sale are increasingly propagated overseas, providing abundant opportunities for new introductions of potentially damaging insects.

Many insects have dormant life stages that may be accidentally transported on items that are completely unrelated to their biology. For example, thousands of used automobiles are shipped yearly from Japan to New Zealand, and these automobiles frequently arrive with gypsy moth (Lymantria dispar) egg masses.

Solid wood packing material has become another important pathway for invasive forest insects. Wood crating, dunnage, and pallets are often used as packing materials in cargo ships, airplanes, and trucks. Inexpen-sive and readily available wood is typically selected for this use. Unfortunately, such wood often contains bark- or wood-boring insects. International trade and contain-erized shipping has increased dramatically in the past few decades, and wood packing material now represents one of the most important pathways by which forest insects are accidentally transported to distant locations.


F O R E S T R Y A N D A G R O F O R E S T R Y 241

CONCLUSIONS

Invasions by nonnative forest insects constitute a major threat to the sustainability of natural forest ecosystems. While most nonnative forest insects have had relatively minor effects, a small proportion of these species have had catastrophic impacts, altering ecosystem processes into the extended future. Over the last century, numbers of inva-sions have steadily increased, and presently, the majority of forest insect problems are the result of invasions. Given trends of increasing global trade, this problem is likely to expand. Increased resources and efforts to reduce arrival rates of nonnative insects, to intensify early detection and eradication, and to slow the spread of established invaders will be needed in the future.

SEE ALSO THE FOLLOWING ARTICLES

Eucalypts / Gypsy Moth / Hemlock Woolly Adelgid / Pathogens, Plant / Pesticides for Insect Eradication / Phytophthora

FURTHER READING

Brockerhoff, E. G., A. M. Liebhold, and H. Jactel. 2006. The ecology of forest insect invasions and advances in their management. Canadian Journal of Forestry Research 36: 263–268.

Haack, R. A. 2006. Exotic bark- and wood-boring Coleoptera in the United States: Recent establishments and interceptions. Canadian Journal of Forestry Research 36: 269–288.

Langor, D. W., L. J. DeHaas, and R. G. Foottit. 2009. Diversity of non-native terrestrial arthropods on woody plants in Canada. Biological Invasions 11: 5–19.

Liebhold, A. M., W. L. Macdonald, D. Bergdahl, and V. C. Mastro. 1995. Invasion by Exotic Forest Pests: A Threat to Forest Ecosystems. Forest Sci-ence Monographs 30.

Niemelä, P., and W. J. Mattson. 1996. Invasion of North American forests by European phytophagous insects. Bioscience 46: 741–753.

Wingfi eld, M. J., B. Slippers, B. P. Hurley, T. A. Coutinho, B. D. Wingfi eld, and J. Roux. 2008. Eucalypt pests and diseases: Growing threats to plan-tation productivity. Southern Forests 70: 139–144.

FORESTRY AND AGROFORESTRY

DAVID M. RICHARDSON

Stellenbosch University, Matieland, South Africa

Afforestation has a long history in the northern hemi-sphere, but it was only in the twentieth century that many tree species began to be planted over large areas in environments far removed from their natural ranges. A small number of tree species now form the foundation

Many species of Lepidoptera (moths and butterfl ies) pro-duce unique sex pheromones that attract other members of the same species from long distances. Some bark beetles (Scolytinae) similarly produce aggregation pheromones or are strongly attracted to specifi c compounds emitted by their host trees. Pheromones can often be synthesized, produced effi ciently, and used as lures in traps deployed across large geographic areas in order to detect newly founded populations. Many groups of insects, however, do not produce long-range pheromones, and detecting low-density populations of these insects is more diffi cult. Insects that spend most of their life beneath the bark, in roots or other hidden locations, are especially diffi cult to fi nd and monitor until they reach high densities. Some of the most damaging forest insect pests, including the hem-lock woolly adeglid, the Asian longhorned beetle, and the emerald ash borer, are very diffi cult to detect until dam-age becomes apparent.

Distinguishing nonnative species from native species can also be challenging for some groups of insects whose systematics are not well known. In one example, wood-boring beetles recovered from a pocket of dying spruce trees in an area of eastern Canada in the early 1990s were originally identifi ed as a native species. Roughly ten years later, widespread spruce mortality in the prov-ince was determined to be caused by the brown spruce longhorned beetle, Tetropium fuscum. When the origi-nal specimens were reexamined, they were also found to be the nonnative beetles. The availability of taxonomic expertise, therefore, can be an important component of early detection.

Containment, or slowing the spread of established species, is a management approach increasingly consid-ered for certain forest insect invasions. One of the best examples of this strategy is the Slow the Spread program currently implemented to reduce the spread of the gypsy moth in the United States. Under this program, a grid of pheromone traps is deployed annually along a 100 km band along the expanding invasion front in order to locate newly invaded, isolated populations. Once these populations are identifi ed, their spatial extent is delim-ited using additional traps, and ultimately they are sup-pressed through the use of mating disruption or multiple applications of a microbial insecticide. Suppression of these nascent populations along the invasion front pre-vents their growth and coalescence, ultimately resulting in slower spread into new regions. Quarantines to restrict transport of potentially infested trees, logs, fi rewood, or related materials into uninfested areas can also serve to slow the spread of an invader.


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FOREST INSECTS - Sandy Liebhold238 FOREST INSECTS potentially severe ecological and economic consequences. This entry presents a brief overview of the pathways by which nonnative forest