Biological Control of Pests in Forests of Eastern United States

 

 

Gypsy moth
(Lymantria dispar [L.])

Joe Elkinton and Roy Van Driesche, University of Massachusetts, Amherst (vandries@nre.umass.edu)

Range in North American and other Invaded Areas

Gypsy moth (Lymantria dispar [L.]) (Lepidoptera: Lymantriidae) was deliberatley brought to Massachusetts in the 19th century, and from there it has spread slowly to other parts of the United States and Canada (Fig. 2). It now occurs west to the Great Lakes region and south to North Carolina. Isolated infestations have been detected in various western states (CA, OR, etc) (Driestadt and Dalhstein, 1989) but have been successfully eradicated

Fig. 2. Approximate distribution of gypsy moth in the United States, exclusive of temporary infestations in the western United States (image from www.villageoflombard.org/.../images/spread.gif)

Damage

This species periodically reaches high densities that defoliate hardwood forest species, especially oaks, over large areas (tens of thousands of acres) (Fig. 3). Damage is especially severe in Virginia, Pennsylvania and other parts of the insect's range that are on the leading of its expansion. Large outbreaks in New England have not occurred since 1981, suggesting that the species is now effectively suppressed in that part of its range by natural enemies, in particular the newly present fungal pathogen Entomophaga maimaiga Humber, Shimazu, and Soper.

Fig 3. Defoliation by gypsy moth larvae in Pennsylvania forests (photo courtesy of Pennsylvania Department of Conservation and Natural Resources, Forestryimages.org)

Known Natural Enemies (native or introduced)

Parasitoids

In the course of a biological control program against this pest that began over a century ago, a large number of parasitoids and a smaller number of predators have been found and studied. Of these, the parasitoids that have established and become most comon are the encyrtid egg parasitoid Ooencyrtus kuvanae (Howard), the braconid Cotesia melanoscela (Ratzeburg), and the tachinids Compsilura concinnata (Meigen) (Figure 4), Blepharipa pratensis (Meigen), and Parasetigena silvestris (Robineau-Desvoidy), all of which attack larvae, and the chalcid Brachymeria intermedia (Nees), which attacks pupae (Montgomery and Wallner, 1988). The most recent species to establish is the ichneumonid Coccygomimus disparis (Viereck) (Coulson et al., 1986). Clausen (1978), Doane and McManus (1981), and Elkinton and Liebhold (1990) provide information on the biology and importance of these species.

Fig. 4. Compsilura concinnata, an important tachinid parasitoid of the gypsy moth (source of photo: www.vegedge.umn.edu/vegpest/beneficials/Comps.htm )

Predators

Rodents and other predators of gypsy moth larvae and pupae are believed to be important in suppressing the increase of low density populations (Montgomery and Wallner, 1988; Elkinton and Liebhold, 1990). The carabid beetle Calosoma scycophanta, as both adults and larvae, is an important specific predator of gypsy moth pupae, consuming as many as 40% of the pupae at some sites (Weseloh, 1985). Ants are believed to be important predators of young larvae (Weseloh, 1994). Bird predation is an important source of mortality of egg masses, with 67-89% of all egg masses being at least partially eaten at some sites (Cooper and Smith, 1995).

Pathogens

High density populations of gypsy moths were for many decades often greatly reduced by epizootics of a nuclear polyhedrosis virus, but not until defoliation had occurred. The dynamics of this pathogen in nature have been modeled by Elkinton et al. (1995). This virus has been reared in live hosts and tested for use as a microbial insecticide (Cunningham and Kaupp, 1991), but has not been commercialized because of high production cost and erratic demand. Since 1989 in New England, epizootics of a fungal pathgogen (E. maimaiga) (Figure 5) have occurred and have become an important source of mortality (Elkinton and Liebhold,1990). This pathogen now appears to be preventing populations from rising to outbreak levels. The bacterial pathogen Bacillus thuringiensis is effective against young larvae and commercial products have been developed for use against this pest (Reardon et al., 1994). In Europe, microsporidia appear to suppress gypsy moth populations and their introduction to North America has been suggested (Weiser and Novotny, 1987).

Fig. 5. Gypsy moth caterpillars killed by the fungus Entomophaga maimaiga Humber, Shimazu, and Soper. (Photograph by Charles Burnham, Massachusetts Department of Conservation and Recreation)

Biological Control Efforts Against the Pest, Worldwide

Attempts to obtain biological control in North America of this destructive forest defoliator have been underway for more than a century and this large body of work is summarized by Clausen (1978), Reardon (1981), and Griffiths and Quednau (1984). Over the course of these efforts, some 80 species of parasitoids were imported into North America for study and 10 have established in the field and seven are common enough to potentially affect gypsy moth populations (Ooencyrtus kuvanae, Cotesia melanoscela, Compsilura concinnata, Parasetigena silvestris, Blepharipa pratensis, Brachymeria intermedia, and, most recently, Coccygomimus (Pimpla) disparis).

Whether introduced parasitoids or predators have reduced the average density of the pest or lenthened the period between outbreak is difficult to determine. Literature on the population dynamics of gypsy moth has been reviewed several times (Doane and McManus, 1981; Montgomery and Wallner, 1988; Elkinton and Liebhold, 1990) but only a few data sets cover enough years to determine if this univoltine pest changed (in the years prior to the emergence of E. maimaiga as an important new biocontrol agent) in density as a result of these arthropod introductions. A study in Melrose, Massachusetts (near the point of introduction) from 1910 to 1930 reflects the value of the earliest introductions. At this site the number of egg masses per hectare decline from about 7000/ha from 1910-1921 to 100-1000 from 1922-1930.

Field studies have shown parasitoids to be important sources of gypsy moth mortality (Doane, 1971; Barbosa et al., 1975). Long term analyses of the impact of parasitoids at fixed study plots have shown that the egg parasitoid (O. encyrtus) caused an average loss of 26% to the egg stage (data for 19 years in 17 plots in New Jersey, Williams et al., 1990). Data on mortality rates in these plots for larval parasitoids is given in Willams et al., 1992). Lifetables for populations in various stages of the population cycle are given by Campbell (1981).

Experimental manipulations to asses the role of specific mortality factors, based on creation of local artificial gypsy moth outbreaks or deployment of gypsy moth pupae as food for predators, have revealed the importance of spatial aggregation of the tachinid C. coninnata to local outbreaks (Liebhold and Elkinton, 1989; Ferguson et al. 1994) and the denisity inverse nature of mortality to pupae from predation by small mammals such as mice.

Current Status of Biological Control Efforts in the United States

Work on gypsy moth dynamics declined sharply in the late 1990s after the establishment and spread of the new fungal pathogen E. maimaiga, which periodically causes extensive mortality to larvae (Webb et al., 1999). These outbreaks appear, at least in New England, to have prevented large scale gypsy moth outbreaks (Gillock and Hain, 2001), the last of which occurred in 1981. Control by this fungus is apparently not as effective in the peripheral parts of the infested area, to the west and south.

References Cited

Barbosa, P., J. L. Capinera, and E. A. Harrington. 1975. The gypsy moth parasitoid complex in western Massachusetts: a study of parasitoids in areas of high and low host density. Environmental Entomology 4: 842-846.

Campbell, R. W. 1981. Population dynamics, pp. 65-214. In: Doane, C. C. and M. L. McManus (eds.). 1981. The Gypsy Moth: Research Toward Integrated Pest Management. USDA Forest Service Technical Bulletin No. 1584, 757 pp.

Clausen, C. P. 1978. Introduced Parasites and Predators of Arthropod Pests and Weeds: a World Review. USDA Agriculture Handbook No. 480, 545 pp.

Cooper, R. J. and H. R. Smith. 1995. Predation on gypsy moth (Lepidoptera: Lymantriidae) egg masses by birds. Environmental Entomology 24: 571-574.

Coulson, J. R., R. W. Fuester, P. W. Schaefer, L. R. Ertle, J. S. Kelleher, and L. D. Rhoads. 1986. Exploration for and importation of natural enemies of the gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), in North America: an update. Proceedings of the Entomological Society of Washington 88: 461-475.

Cunningham, J. C. and W. J. Kaupp. 1991. Development of nuclear polyhedrosis virus for control of gypsy moth (Lepidoptera: Lymantriidae) in Ontario. I. Aerial spray trials. The Canadian Entomologist 123: 601-609.

Doane, C. C. 1971. A high rate of parasitization by Brachymeria intermedia (Hymenoptera: Chalcididae) on the gypsy moth. Annals of the Entomological Society of America 64: 753-754.

Doane, C. C. and M. L. McManus (eds.). 1981. The Gypsy Moth: Research Toward Integrated Pest Management. USDA Forest Service Technical Bulletin No. 1584, 757 pp.

Dreistadt, S. H. and D. L. Dahlsten. 1989. Gypsy moth eradication in Pacific coast states: history and eradication. Bulletin of the Entomological Society of America 35 (2): 13-19.

Elkinton, J. S. and A. M. Liebhold. 1990. Population dynamics of gypsy moth in North America. Annual Review of Entomology 35: 571-596.

Elkinton, J. S., G. Dwyer, and A. Sharov. 1995. Modeling the epizootiology of gypsy moth nuclear polyhedrosis virus. Computers and Electronics in Agriculture 13: 91-102.

Ferguson, C. S., J. S. Elkinton, J. R. Gould, and W. E. Wallner. 1994. Population regulation by parasitoids: does spatial desntiy dependence lead to temporal density dependence? Environmental Entomology 23: 1155-1164.

Gillock, H. H. and F. P. Hain. 2001/2002. A historical overview of North American gypsy moth controls, chemical and biological, with emphasis on the pathogenic fungus, Entomophaga maimaiga. Reviews in Toxicology 4: 105-128.

Griffiths, K. J. and F. W. Quednau. 1984. Lymantria dispar (L.), gypsy moth (Lepidoptera: Lymantriidae), pp. 303-310. In: Kelleher, J. S. and M. A. Hulme (eds.). Biological Control Programmes against Insects and Weeds in Canada, 1969-1980. Commonwealth Agricultural Bureaux, Farnham Royal, Slough, United Kingdom, 410 pp.

Liebhold, A. M. and J. S. Elkinton. 1989. Elevated parasitism in artificially augmented populations of Lymantria dispar (Lepidoptera: Lymantriidae). Environmental Entomology 18: 986-995.

Montgomery, M. E. and W. W. Wallner. 1988. The gypsy moth, a westward migrant, pp. 353-375. In: Berryman, A. A. (ed.). Dynamics of Forest Insect Populations: Patterns, Causes, Implications. Plenum Press, New York.

Reardon, R. 1981. Parasites, pp. 299-421. In: Doane, C. C. and M. L. McManus (eds.). 1981. The Gypsy Moth: Research Toward Integrated Pest Management. USDA Forest Service Technical Bulletin No. 1584, 757 pp.

Reardon, R., N. DuBois, and W. McLane. 1994. Bacillus thuringiensis for managing gypsy moth: a review. National Center for Forest Health Management, USDA Forest Service, Morgantown, West Virginia, USA, 33 pp.

Weiser, J. and J. Novotny. 1987. Field application of Nosema lymantriae against the gypsy moth, Lymantria diapar L. Journal of Applied Entomology 104: 58-62.

Webb, R. E., G. B. White, K. W. Thorpe, and S. E. Talley. 1999. Quantitative analysis of a pathogen-induced premature collapse of a “leading edge” gypsy moth (Lepidoptera: Lymantriidae) population in Virginia. Journal of Entomological Science 34: 84-100.

Weseloh, R. 1985. Predation by Calosoma sycophanta (Coleoptera: Carabidae): evidence for a large impact on gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae), pupae. The Canadian Entomologist 117:1117-1126.

Weseloh, R. 1994. Forest ant (Hymenoptera: Formicidae) effect on gypsy moth (Lepidoptera: Lymantriidae) larval numbers in a mature forest. Environmental Entomology 23: 870-877.

Williams, D. W., R. W. Fuester, W. W. Metterhouse, R. J. Balaam, R. H. Bullock, R. J. Chianese, and R. C. Reardon. 1990. Density, size and mortality of egg masses in New Jersey populations of the gypsy moth (Lepidoptera: Lymantriidae). Environmental Entomology 19: 943-948.

Williams, D. W., R. W. Fuester, W. W. Metterhouse, R. J. Balaam, R. H. Bullock, R. J. Chianese, and R. C. Reardon. 1992. Incidence and ecological relationships of parasitism in larval populations of Lymantria dispar (Lepidoptera: Lymantriidae). Biological Control 2: 35-43.