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Scientific name: Lymantria dispar
Common name: 
Gypsy Moth

(Information for the following species page was compiled by Christina Girdwood for Biology 220W at Penn State University in Spring 2001)

The gypsy moth has a natural range that includes most of Europe and Asia. It was introduced into North America in 1868 or 1869 by Leopold Trouvelot who hoped to use this moth species as the foundation of a silk industry in the United States. The cocoon threads of the gypsy moth did not prove to be a reliable source of silk, but the moths were able to escape Trouvelot’s Boston home-laboratory and were also able to survive and then thrive in the hardwood forest ecosystems of Massachusetts.

Dynamics of Species Introduction
Every species in its native ecosystem exists in a web of complex relationships with the other species in the ecosystem. These relationships have developed over many thousands or millions of years of adaptation and evolution and are essential for a species’ survival and also for the control of a population’s rate of growth and increase. Species introduced into non-native ecosystems seldom have “normal” population dynamics. Often these introduced species are unable to survive in the conditions and interaction webs of their new, alien environment. These introduced species, then, quickly die out or, at best, persist very marginally in their ecosystems. In some cases, however, the lack of controlling forces and interactions leads to massive population “explosions” of the introduced species. In North America the pestilences of Dutch Elm Disease and Chestnut Blight, the staggeringly huge densities of European Starlings and English Sparrows, and the explosive growth of knotweed and garlic mustard are just a few examples of this out-of-control proliferation of an introduced species. The gypsy moth, to the detriment of North America’s hardwood forests, and to the great dismay and disgust of those humans living in or around these forests is an extreme example of an introduced organism exploding out of control throughout an alien ecosystem.

Spread and Population Growth Cycles
The first outbreaks of gypsy moths occurred in Massachusetts in the late 1800’s. These outbreaks have since spread northward into southern Canada and south as far as Virginia. The westward spread of the moth extends across Pennsylvania, northern Ohio and throughout most of Michigan. It is estimated that the moth is spreading at a rate of twenty kilometers per year.

Populations of the gypsy moth “boom and bust” along regular yearly patterns usually in spite of all human efforts to regulate or eradicate them via pesticides, bacterial toxins, or parasitoid biological controls. Gypsy moth outbreaks usually last from one to five years and then subside due to disease build-up, starvation, and the impact of natural predation. Populations then remain low from four to twelve years before resurging into the “boom” part of the cycle. Outbreaks in oak forests seem to cycle and persist over longer time frames than, say, outbreaks in aspen forests.

Life Cycle
It is, of course, the larvae (caterpillars) of the gypsy moth that are the destructive agents. Larvae feed voraciously on the leaves of deciduous, hardwood tree species (especially oaks, but in actuality, any tree, deciduous or coniferous, or shrub may be consumed by gypsy moth larvae). The larvae hatch from egg masses in early May, although the exact hatching time is dependant upon temperature. After hatching, the larvae climb toward light (i.e. toward the canopy of the tree) where they spin thin support threads of silk. They hang from these support lines off of branches or leaves until a strong enough wind comes along to carry them off in a dispersal flight. The larvae are very lightweight and covered with hairs that facilitate their floating and gliding in the breezes. This wind-borne transport is a very effective dispersal mechanism and can spread the larvae over great distances. The larvae that land on suitable trees will begin to voraciously feed on the leaves. In this early larval stage the feeding is continuous (day and night). The larvae grow very rapidly and will molt four or five times.

After the fourth or fifth molt, the larvae undergo behavioral changes. They stop feeding around the clock and begin to only feed at night. At dawn, they crawl down the trunk of the tree in which they are feeding and locate a dry, cool resting place within which they will spend the day-light hours. Typically, a larva will use the same resting place day after day until it is ready to pupate. During “boom” years, the extremely high densities of larvae make competition for these resting places quite intense. Insufficient numbers of resting places may lead to the persistence of larvae in the canopy through the daylight hours. These larvae will then continue to feed both during the day and night. These later stage larvae are about two inches long, gray to brown in color, with three white lateral stripes on their bodies. They have long hairs (but not as proportionally long as earlier larval stages) and spots (blue in the front, red in the rear) which are observable at the bases of the hair tufts.

Larvae pupate in the same refugia they have used throughout their nocturnal feeding phase. The pupal stage lasts about two weeks and is followed by the emergence of the adult moths. Male moths are brown with darker bands across their wings and backs. They have wingspans of about two inches and are active, energetic fliers. Females are larger (wingspan about two and half inches) and are white with black wing markings. The variety of gypsy moth in North America has flightless females. This feature is thought to have slowed the potential dispersal rate of the gypsy moth in North America. Asian varieties, though, which are capable of interbreeding with the North American form, have females capable of flight. These Asian varieties represent a significant danger that could possibly accelerate the pace of the North American gypsy moth infestation.

The adult lifetime is short. After emergence from their pupae, females begin emitting a pheromone that attracts swarms of males. After mating, the female then lays her yellow-brown eggs in a single, spherical egg mass directly on the bole of the tree on which she has fed and in which she had pupated. The number of eggs in an egg mass ranges from fifty to fifteen hundred. The eggs will then remain dormant until the next May when they will hatch and begin the dispersal, feeding, pupation, and mating cycles all over again.

A variety of birds have been reported to feed on the gypsy moth larvae although the irritating hairs of the caterpillars do discourage extensive bird predation. Grackles have been observed in Armstrong County, PA beating gypsy moth larvae repeatedly across the rough surface of an asphalt street thus removing the hairs before ingesting the larvae. Downy woodpeckers, Baltimore orioles, crows and blue jays are a few of the avian species that have been observed eating gypsy moth larvae. Other vertebrates like the white footed mouse (Peromyscus leucopus) are active predators of the larvae. Gray squirrels and foxes feed opportunistically on the larvae. Ground beetles (Calosoma spp.) and spiders also prey on the larvae and a number of parasitoid wasps and flies lay eggs in the body of the larvae. The developing wasp or fly larvae, then, consume and kill the infected caterpillar.

Environmental Impacts
Impacts of the gypsy moth on the deciduous forests of the northeastern United States have been both obvious and subtle. Obviously, trees that are partially or completely defoliated are energetically stressed as they miss days of photosynthesis and have to spend energy reserves generating replacement leaves. These trees will be more susceptible to disease and environmental stresses like drought and will have reduced rates of growth. Most healthy trees, though, survive these defoliation events as long as they are not too severe or are not repeated over too many successive years. Second growth leaves (i.e. the replacement  leaves made in response to defoliation) are typically tougher and thus more resistant to leaf predators like the gypsy moth. These second growth leaves, though, are also more resistant to decomposition. Nutrient cycling, then, in both the forests and leaf-fed streams around the forests can be disrupted by these reverberative impacts of the gypsy moth larvae. Some have expressed the view that gypsy moth predation on trees is a positive force that selects for stronger, healthier trees. The preference of the larvae for oak species, though, makes the nature of this overall impact less obvious. The stimulation of the growth of forest under-story vegetation by the thinning of the leaf canopy is also a consequence of gypsy moth activity. What the long-term impacts of this altered and accelerated pattern of forest succession will be is not known.

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