There has been quite the resurgence of the gypsy moth this year.
The gypsy moth (Lymantria dispar) is a non-native insect from France. Its caterpillar (larva) stage eats the leaves of a large variety of trees such as oak, maple, apple, crabapple, aspen, willow, birch, mountain ash, pine, spruce, and more.
Gypsy moths were accidentally introduced in 1869 when they were brought to the U.S. in the hope that they could breed with silkworms to create a hardier variety of silkworm and develop a silk industry in the US. Even though they failed as a textile producer, some of the gypsy moths escaped and established their first U.S. population in Medford, Massachusetts.
Damage That Gypsy Moths Cause
Gypsy moth populations rise and fall in cycles, and vary over the years from very few and not noticeable to large numbers and very noticeable tree defoliation. Gypsy moths have “naturalized” in our forest communities and so will always be around, sometimes in small, unnoticeable numbers. When populations are high, thousands of acres can be damaged. Although gypsy moths do not pose a major threat to New York’s forests, they are not native and their populations can reach high, destructive (outbreak) levels.
Gypsy moths eat young, tender leaves in the spring. Deciduous trees (trees that lose their leaves each fall) can regrow a new set of leaves by July and can usually withstand 2-3 successive years of defoliation (removal of leaves) without being killed. However, defoliation does reduce the vigor and resistance of the tree, and it becomes more susceptible to pests and diseases. Tree death can occur when other stresses such as disease or other insect outbreaks attack trees in the same year. When populations of gypsy moths are very high, they will even eat evergreen species. Evergreens do not regrow leaves as easily as deciduous trees and can die as a result of complete defoliation.
Gypsy Moth Identification
Gypsy moth adults and eggs
The larval stage of the gypsy moth is a caterpillar that hatches in the spring from eggs laid the summer before. It hatches about the time oak buds start to open. Young caterpillars need to eat tender new leaves. As a caterpillar grows, it can eat older, tougher leaves. They grow to about 2 1/5 inches in length. Gypsy moth caterpillars have five pairs of raised blue spots followed by six pairs of raised red spots along its back. Female moths are white with brown markings. Males are brownish. Females do not fly. Egg masses are light brown in color and appear as fuzzy patches on tree trunks, branches, firewood, or in a sheltered spot, even on lawn furniture. Each egg mass contains 600-700 eggs.
Tent caterpillars look similar to gypsy moths and also feed in the spring.
What You Can Do
When populations are low, gypsy moth caterpillars and adults can be killed by squishing them. Egg masses can be destroyed by scraping them off trees or other structures and dropping them in a container of detergent. During outbreaks, manual removal may not be practical.
Caterpillars are always being naturally controlled by birds, rodents, parasites, and diseases. Extremes in temperature can also reduce population numbers.
If dense concentrations of tent or gypsy moth caterpillars cause a decline in the trees’ health or threaten an economic resource such as a sugar bush, spraying may be an option.
gypsy moth feeding on branch
Tim Tigner, Virginia Department of Forestry,
Various insecticides for gypsy moths are available at garden centers. Insecticides are divided into two general groups: microbial/biological and chemical. Microbial and biological pesticides contain living organisms that must be eaten by the pest. They are most effective on small, young caterpillars. As they mature, caterpillars become more resistant to microbial pesticides. The most common microbial and biological insecticide is Bacillus thuringiensis (Bt). Bt occurs naturally in soil and on plants. It is harmless to people, animals, and plants, but does affect young moth and butterfly larvae. When Bt is eaten, the caterpillar becomes paralyzed, stops feeding, and dies of starvation or disease.
Chemical insecticides are contact poisons. These chemicals can have an impact on a variety of beneficial, native insects (such as bees), so they should be used wisely.
Professional pesticide applicators can be found in the yellow pages under Tree Service. In order to use restricted insecticides, applicators must be certified.
In certain cases it may be economically feasible to spray large areas. Maple syrup producers may be interested in aerial spraying since severe defoliations can reduce maple syrup production.
Spraying is not effective against pupae or egg masses, and is less effective once caterpillars reach 1 inch long. Keep in mind that nesting birds, beneficial insects, and other animals could be endangered by use of chemical insecticides.
Predicting Next Year’s Defoliation
If you are interested in sampling your woodlot or forest to determine the likelihood of defoliation by gypsy moths for the following growing season, sampling protocol is provided here (PDF). If you do sample your woodlot or forest, your data is valuable to us for tracking annual populations. We would appreciate if you mailed or emailed you sampling data to:
Division of Lands and Forests, Bureau of Invasive Species and Ecosystem Health
Albany, NY 12233-4253
Telephone: 518-402-9425 Fax: 518-402-9028
Email the Forest Health team