What is the Gypsy Moth?

Lymantria dispar

Gypsy moth was introduced from Europe into Medford, Massachusetts in 1869 by Leopold Trouvelot, who was attempting to breed the insect for silk production. Unfortunately, some of the caterpillars escaped from his backyard rearing facility, and by the early 1900's they began defoliating large areas of New England.

Gypsy moth was first discovered in Pennsylvania near Pittston, in Luzerne and Lackawanna counties in 1932. During the 30's and 40's it continued to spread to Pike, Lackawanna, Wayne, Monroe, and Carbon counties. By 1969, it had spread west of the Susquehanna River, and by 1980, 38 Pennsylvania counties were infested. The gypsy moth has now moved beyond our state. It is now found as far west as Minnesota, with populations reaching outbreak levels every 5-10 years.

Gypsy moth has one generation per year, and includes egg, caterpillar, pupal and adult stages. Female moths lay egg masses on tree boles, branches, vehicles, houses, and other structures, and this aids their spread to new areas. Egg masses are buff-colored after they are initially deposited in late summer, but they become lighter in color as they bleach in the sun. Egg mass size may indicate population trends. When populations are declining, most egg masses are around ½ inch long and contain about 100 eggs, while building populations have 1 ½ inch long egg masses containing up to a thousand eggs. Gypsy moths survive the winter in the egg stage and hatch from mid-April to mid-May in Pennsylvania when temperatures are above 60 degrees Fahrenheit.

Caterpillars have five double rows of dark blue spots, followed by six double rows of brick red spots on their dorsal surface. They also have a thin yellow median stripe along the length of their back. Tiny, young caterpillars are windblown to their food plants, where they will feed day and night. Older stages of the caterpillars feed only at night to avoid drying out or being eaten by predators. During the day, they rest under leaf litter and bark crevices near the bottom of the tree. Older caterpillars are able to eat conifers, while younger stages are usually found on deciduous hosts.

Mature caterpillars pupate from mid June through early July in Pennsylvania. Mice, shrews, and ground beetles eat the pupae, and are an important regulator of gypsy moth in this stage. Adult gypsy moths emerge about two weeks after pupating. Adults only live about a week, and do not feed. Female gypsy moths use chemicals to attract a mate soon after they emerge. They lay eggs about a day after mating. Adult gypsy moth males have feathery antennae and brown wings and are able to fly, while cream-colored females of European gypsy moths cannot fly and have threadlike antennae. There is also an Asian variety of gypsy moth with flying females that have luckily been eradicated in Western North America on several occasions following accidental introductions.

Various natural environmental factors help control gypsy moth in North America. A disease-causing fungus known as Entomophaga maimaiga was first introduced in 1910-1911 to control gypsy moth. This fungus only affects select families of moth caterpillars that encounter infected soil and plants or through contact with other infected caterpillars. The spores of the fungus germinate in the spring and work best if rain is abundant. E. maimaiga was responsible for widespread gypsy moth mortality in 1989 and 1990, when wetter than normal conditions were reported in May. Since this time, E. maimaiga has become a significant regulator of gypsy moth populations at both low and high densities. Researchers are unsure whether the increased prevalence of the fungus is due to its initial introduction or if it is the result of a more recent reintroduction into the US. Older gypsy moth caterpillars that die as a result of the fungus die in a vertical position with their legs sticking outward.

A nucleopolyhedrosis virus (Borralinivirus reprimens) kills enough gypsy moth caterpillars when populations are high to eventually end an outbreak. Caterpillars must eat the viral particles in order to become infected. Caterpillars infected with NPV die in an inverted V position, which explains why the common name for NPV is "the wilt". The activity of NPV is specific in that it only kills gypsy moth caterpillars.

A large metallic green ground beetle known as Calosoma sycophanta was introduced into New England from Europe for gypsy moth control in 1906. It is now established throughout Pennsylvania. C. sycophanta larvae and adults eat older gypsy moth caterpillars that rest in the leaf litter during the daytime.

Other factors can influence the impact of oak. The effects of other insects, such as oak leafroller, oak leaf tier, two lined chestnut borer, and oak sawflies, as well as pathogenic fungi such as oak wilt and Armillaria can compound the impact of gypsy moth. In addition, high deer populations in Pennsylvania make oak regeneration a challenge since the arrival of the gypsy moth.