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College of Food, Agricultural, and Environmental Sciences


Gypsy Moth Larvae Now Emerging

May 1, 2001

WOOSTER, Ohio - Gypsy moth larvae are beginning to emerge from egg masses throughout the state, and egg hatch will continue for the next couple of weeks, says an Ohio State University entomologist.

Dan Herms said larvae will hatch from egg masses over the next three weeks, then climb into the canopy of host trees and spend the next two months feeding on the leaves. "Mid-to-late June is when 80-90 percent of defoliation will occur," he said. "One caterpillar will eat 25-30 leaves in its lifetime. Generally, if you find one caterpillar per 25 leaves, there's the potential for 100 percent defoliation."

During early and mid-May, the Ohio Department of Agriculture will take steps to help slow the spread of gypsy moth outbreaks in Ohio by making aerial applications of a biological pesticide derived from the naturally occurring bacterium Bacillus thuringiensis, commonly known as Bt. Gypsy moth larvae are most susceptible to Bt when they are small, so applications will be made to susceptible trees in urban and forested areas of Ohio before damage becomes evident.

The gypsy moth, an invasive insect accidentally introduced to the United States from Europe in 1869, has spread to Ohio in recent years from Pennsylvania and Michigan. It feeds on more than 100 species of deciduous and evergreen trees, leaving them defoliated and susceptible to other stresses and diseases. Several years of defoliation will eventually kill a tree.
Oak trees are the insect's favorite host. Last year more than 20,000 acres of Ohio trees were damaged by the gypsy moth, and state agriculture and forestry officials fear this year's outbreaks could be more severe as the insect continues to spread through the prime oak forests that dominate much of eastern and southern Ohio.

A gypsy moth caterpillar is easy to recognize by the five blue dots and six red dots on its hair-covered back. After feeding in early spring, it pupates in late June or early July, and the adult moth emerges about 10 days later. The female moth, which cannot fly, lays its egg mass on any object it comes across, such as rocks, houses, recreational vehicles, trees or car tires. "That's how the moth can spread rapidly from one place to another," said Herms. "It's important to check your equipment and cars, if you going camping, for example, and destroy any egg masses you find."

Natural predators, such as birds and mice, have been found to have some impact on controlling the gypsy moth, but only in low populations. Entomophaga maimaiga, a natural fungal disease of the gypsy moth, substantially decreased gypsy moth defoliation in northeast Ohio in 2000. But its effectiveness is highly dependent on environmental conditions. The fungus, which affects the caterpillar stage of development, is easy to distribute and spreads rapidly under favorable conditions. But it is only effective during wet conditions in the spring.

Herms said that during gypsy moth outbreaks, the most effective means of preventing defoliation during outbreaks is to apply insecticides. Bt is the most environmentally friendly because it affects only caterpillars and is harmless to humans, pets and birds. But even Bt has met controversy because of toxic effects on native butterflies and moths.

"Bt can be very effective, especially if it's timed correctly," said Herms. "Studies have shown that Bt has very little impact on non-target organisms. It has less of an impact than other insecticides that would affect a much broader range of insects." Dimilin, a growth-regulating insecticide that interferes with exoskeleton formation during the molting process, is also used in the state gypsy moth suppression program, but to a lesser degree because it affects a wide range of immature insects and remains in the environment much longer than Bt.

The Ohio Department of Agriculture will begin spraying 38,000 acres in 19 state counties beginning the week of May 7. Counties targeted for aerial spraying include Ashland, Ashtabula, Coshocton, Cuyahoga, Franklin, Harrison, Holmes, Knox, Licking, Lorain, Lucas, Mahoning, Medina, Richland, Stark, Summit, Trumbull, Tuscarawas and Wayne.

Single property owners or groups of property owners with at least a 50-acre block of trees can apply to the department each year in September to have their property included in the state suppression program the following year. There is currently no charge to the property owner for participation in the program. Homeowners can also contract with private applicators to have trees sprayed from the ground or the air, depending on the number and area of infested trees.

For more information on the gypsy moth, log on to or

Candace Pollock
Dan Herms