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Gypsy Moth: Part One

May 8, 2002

Gypsy moth, Lymantria dispar, eggs have hatched in the northern portion of Illinois, and the caterpillars are active. The gypsy moth is a leaf-feeding insect that is a serious pest to many forest and ornamental trees. Lake and Cook counties in northeastern Illinois contain too many gypsy moths for meaningful eradication. Overwintering egg masses have been found in Winnebago, DuPage, and McHenry counties. The Illinois Department of Agriculture (IDOA) predicts that by 2005 most of Chicago, as well as other surrounding counties, will be quarantined. Gypsy moths migrate at a rate of about 15 miles per year. Because gypsy moth is emerging as an important pest in northeastern Illinois, we will have several articles on it as it develops this year.

Gypsy moth undergoes four developmental life stages: the egg, larva (caterpillar), pupa, and adult. Gypsy moth females lay between 500 and 1,000 eggs in sheltered areas, such as underneath the bark of trees. The egg mass, which is about 1 inch long and 1/2 inch wide, is covered with a dense mass of tan or buff-colored hairs. The eggs are the overwintering stage of the insect. Eggs may be attached to trees, houses, or any outdoor objects. The eggs hatch in spring (April) into caterpillars.

Gypsy moth caterpillars are hairy and easy to identify because they possess characteristics not found in other leaf-feeding caterpillars. They have five pairs of blue dots, followed by six pairs of red dots, lining the back. Young caterpillars, which are 1/4 inch long and blackish, primarily feed at the top of trees during the day, whereas older caterpillars feed at night. The large caterpillars, which are 1 to 1-1/2 inches long, crawl down from the tops of trees during the day to hide in protected places to avoid predators. When present in large numbers, the older caterpillars feed both day and night. A young caterpillar spreads to new locations by crawling to the tops of trees, where it spins a silken thread and is caught on wind currents. Gypsy moth caterpillars do not produce a web, which distinguishes them from web-making caterpillars such as the eastern tent caterpillar and fall webworm. The gypsy moth larval stage lasts about 7 weeks. Male larvae undergo five instars, females six, before pupating.

Treating localized infestations of gypsy moth with an insecticide can slow the spread. However, this only temporarily reduces the number of caterpillars. Pest-control materials are best applied when the caterpillars are small. This enhances the effectiveness of these materials, as they are less effective as caterpillars increase in size. One commonly used pest-control material for managing gypsy moth is the bacterium Bacillus thuringiensis (Bt). Bt, sold as Dipel and Thuricide, must be consumed to kill Gypsy moth larvae. The appropriate time to spray with Bt is soon after egg hatch, which is generally when oak leaves are about one-half expanded. The young caterpillars are much more susceptible to Bt than are older, larger caterpillars. Bt is not harmful to beneficial organisms such as honeybees. Additional insecticides effective on the larvae include cyfluthrin (Tempo), difluben-zuron (Dimilin), and tebufenozide (Mimic). Soybean oil (Golden Natur'L Spray Oil) is effective in destroying Gypsy moth egg masses.

For more information, contact your local U of I Extension office or consult the gypsy moth fact sheet (Entomology Fact Sheet, NHE-153).

Author: Donna Danielson of The Morton Arboretum Raymond A. Cloyd


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