Lymantria dispar
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Authors: Trish Hanson and E. Bradford Walker, Vermont Department of Forests, Parks and Recreation; E.A. Roberts, Virginia Tech
Contents |
Hosts
The European gypsy moth has more than 250 known host plants but prefers oak. Other hosts include apple, birch, boxelder, hawthorn, linden, poplar, sweet gum, and willow.
Evidence
The distinctive felt-like, tan egg masses may be seen on bark, branches, and in other sheltered locations throughout the winter. In early May, shot hole damage to leaves by young larvae is apparent. Large larvae, which are hairy and gray with five pairs of blue dots and six pairs of red dots, consume all but the larger veins and midvein. They tend to feed at night, and gather in protected areas during the day. They may even gather in nests of the eastern tent caterpillar.
Life cycle
There is one generation per year, with winter spent in the egg stage. Eggs hatch at the time that serviceberry (Amelanchier spp.) begins to bloom, and larvae feed until July. Gypsy moth larvae tend to feed at night. Feeding by later instar larvae will consume all but the larger veins and midribs. Pupation takes place in sheltered locations on tree trunks or branches, or under other objects such as rocks and picnic tables. Adults emerge about 2 weeks later to mate and lay eggs. The heavier white females do not fly but do crawl a short distance from the pupation site, mate, lay eggs and die. The smaller brown males are very active fliers and often become a nuisance in infested areas.
Population cycles
Gypsy moth is an outbreak pest. It can remain at low levels for several years and then undergo large population increases in a very few years. Unless areas are actively monitored even moderate gypsy moth populations can exist unnoticed. Thus, population outbreaks sometimes appear to occur in only one or two years. Although these cycles are influenced by numerous factors, the low populations are believed to be the result, at least in part, of a gypsy moth disease caused by the fungus Entomophaga maimaiga. This fungus was introduced into the U.S. as a possible control agent in the early 1900s but never appeared to become established. However, it was "rediscovered" in 1989 in New England and has since been found throughout a large portion of the generally infested area. Gypsy moth populations are also affected by another disease caused by a virus. The virus is the usual cause of mortality in high populations, whereas E. maimaiga serves as a control agent at both low and high pest densities.
Management
Gypsy moth outbreaks occur periodically. Egg mass counts can be used to predict spring infestation levels, with 10 or more masses per tree indicating that severe defoliation may follow. As egg masses remain somewhat intact for several years, be sure that the count includes only viable eggs. Viable eggs should be firm to the touch and "pop" when crushed. Should you desire to provide relief by scraping off egg masses, be sure to scrape them into a container so that they can be destroyed, not onto the ground where they may still hatch. Bacillus thuringiensis applied to very young larvae in May can provide good control. Residual insecticides are necessary if applications are made after mid- to late-May. Although barrier bands can afford some protection to individual ornamental trees, they should be used with great caution. Any materials applied directly to the bark may be toxic to thin-barked trees. Diseases and starvation become important control agents when populations are high.
Asian gypsy moth
There is also an Asian strain of the gypsy moth that was identified in 1991. It has a much broader host range including larch, alder and some evergreens. The females are active fliers due to their larger wingspan and can fly long distances (up to 20 miles). These factors would allow it to spread much faster than the European strain and be even more damaging. This strain was detected in the Pacific Northwest in 1997 and eradication efforts were pursued through 1999.
References
- Drooz, A.T. 1985. Insects of Eastern Forests. USDA Forest Service Miscellaneous Publication 1426. p 229-233;
- Hanson, T., and E. B. Walker. 2002. Field guide to common insect pests of urban trees in the Northeast. Waterbury, VT: Department of Forests, Parks and Recreation.
- Johnson, W.T. and Lyon, H.H. 1991. Insects That Feed on Trees and Shrubs . 2nd edition. Cornell University Press. p 138-141;
- Martineau, R. 1984. Insects Harmful to Forest Trees. Agriculture Canada Government Publishing Centre, Supply and Services, Ottawa. p 112-114;
- Roberts, E.A. Gypsy Moth in Virginia: An Update. Publication Number 444-750, Posted February 2001. Virginia Cooperative Extension.
- Rose, A.H. and Lindquist, O.H. Revised by Syme, P. 1994. Insects of Eastern Spruces, Fir and Hemlock . Canadian Forest Service Publication. p 141 and 144-145.
- USDA APHIS PPQ. Asian Gypsy Moth. April 2003
Image Gallery
 Photo by USDA APHIS PPQ Archive, USDA APHIS PPQ, Bugwood.org |
 Photo by Jim Occi, BugPics, Bugwood.org |
 Photo by Connecticut Agricultural Experiment Station Archive, Connecticut Agricultural Experiment Station, Bugwood.org |
 Photo by Hannes Lemme, , Bugwood.org |
 Photo by Mark Robinson, USDA Forest Service, Bugwood.org |
 Photo by USDA APHIS PPQ Archive, USDA APHIS PPQ, Bugwood.org |
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