University of Illinois Extension
Bruce Spangeberg

These articles are written to apply to the northeastern corner of Illinois. Problems and timing may not apply outside of this area.

Stateline Yard & Garden

Gypsy Moth Numbers Up in Illinois

April 16, 1998

Gypsy moths are serious insect pests of trees and can cause extensive damage to trees and forests. In Illinois, the gypsy moth exists, but for the past several years numbers have been low. In 1997, however, those numbers went up dramatically, and are anticipated to be high in 1998. Today and next week I'll discuss this pest and the situation here in northeastern Illinois.

Gypsy moths are an introduced species to the United States. Larva (caterpillars) are heavy feeders on many kinds of trees, capable of completely defoliating shade trees. Larva do not make silken webs or tents in trees. Silken tents visible in trees in spring are from Eastern tent caterpillar, and those appearing later in summer are the work of the fall webworm.

Gypsy moth caterpillars are brown, hairy, and have a double row of several red and blue dots on their backs. They usually feed in large groups, primarily at night, in May and early June. Among their favorite host trees are oak, apple, alder, aspen, basswood, hawthorn, and birch. Many other trees, including maple, pine, and spruce, may be damaged. Fully-grown caterpillars will migrate down from the trees sometime in June to pupate.

In July, adult moths emerge. The female has white or cream-colored wings with black markings (about 2-inch wingspan) but is too heavy to fly. Male moths are brown with black markings with a 1-1/2-inch wingspan, and fly. Female moths release pheromone to attract males.

Egg masses are deposited under loose bark of trees, in woodpiles, on outdoor furniture, and other concealed locations. Egg masses are about 1-1/2 inches long by 1/2 inch wide and are covered with buff or yellow hairs from the female moth. Egg masses hatch the following spring.

This egg case is often moved from infested areas into other regions. Moving with nursery stock, recreation vehicles, firewood, and outdoor furniture are among the ways egg cases may travel out of infested areas. Once in a new location, the eggs may hatch and larva start feeding on trees in the area. Caterpillars suspended by silk threads can be blown in the wind out of trees and spread into adjoining areas.

Next week, I'll discuss more on the gypsy moth and what can be done to help keep this pest under control.


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