|Lymantria dispar, the gypsy moth, is
responsible for millions of acres of defoliation annually.
Although white, chestnut, black and red oak are preferred,
gypsy moth caterpillars also eat hundreds of other tree
and shrub species including oak, apple, alder, aspen,
basswood, birch, poplar, willow, hawthorn, hemlock,
tamarack (larch), pine, spruce, and witch hazel. Gypsy
moth usually avoids ash, butternut, black walnut, locust,
sycamore, and yellow poplar (tuliptree). Although it
usually takes more than one year of defoliation before
trees die, conifers that are defoliated may be killed
after a single season of defoliation.
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
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.