Know Your Friends

Entomophaga maimaiga, A Fungal Pathogen of Gypsy Moth

Entomophaga maimaiga is a common disease in gypsy moth populations in its native country of Japan. The fungus was first released into the United States near Boston in 1910 as part of a program to introduce natural enemies of gypsy moth. Scientists could find no evidence that the fungus had become established and the project was abandoned a few years later. However, the fungus appeared unexpectedly in several northeastern states in 1989 and caused high mortality in many gypsy moth populations. Although scientists have several theories, the strange reappearance of the fungus is still a mystery.

E. maimaiga passes the winter as a tough, thick-walled "resting spore" in the soil and on tree bark. In May or June, resting spores germinate and produce sticky spores at the end of a stalk that grows just above the soil surface. Gypsy moth caterpillars come into contact with these spores as they search for suitable leaves to feed on. The fungus digests its way through the exoskeleton of the caterpillar and grows inside the body of the caterpillar. Infected caterpillars may die within one week.

When young caterpillars are affected early in the summer, the fungus will produce a second type of spore called conidia. These microscopic spores are spread by the wind and can infect other caterpillars. The cycle of conidia production and infection may occur four to nine times during the summer. When the fungus develops in large caterpillars, it produces the overwintering resting spores.

Weather plays an important role in determining how effective E. maimaiga will be. Like most fungi, its spores need moisture and high humidity to germinate. Frequent rainfall during May and June contributes to the start and spread of E. maimaiga through a gypsy moth population. Temperatures of 50 to 80 degrees F enhance fungal growth.

Entomophaga maimaiga can be distinguished from another disease common in outbreak populations of gypsy moth. NPV (nuclearpoly-hedrosis virus) is a viral disease that often causes gypsy moth outbreak populations to collapse. One important difference between the two diseases is that NPV is seldom prevalent until gypsy moth populations reach very high levels. In contrast, E. maimaiga may be found even when gypsy moth populations are low. Caterpillars killed by NPV often remain attached to the stem or branches of trees. The bodies of dead caterpillars are soft, filled with a brown liquid and disintegrate rapidly. Usually, they hang limply in an inverted "V" position. Caterpillars killed by E. maimaiga will also remain attached to tree stems or branches. However, the bodies tend to be stiff and straight, and the legs extend stiffly from the body. Some of the dead caterpillars may have tiny white conidia attached to the hairs on the body. The cadavers may remain on the stem well into the winter.

E. maimaiga may become an important biological control of gypsy moth in both low and high populations. Infections may be more common in years with rainy spring weather than in years with dry spring weather. Scientists have found that the fungus is present in at least 11 states. Laboratory and field studies have shown that E. maimaiga is host specific and poses little risk to other insect populations. It will not affect other animals or humans. Although there is not likely to be any "silver bullet" for gypsy moth, E. maimaiga should improve our ability to manage this pest.

- Dave Smitley, Deborah McCullough, and Lyle Buss, Michigan State University. Adapted from "Entomophaga maimaiga - A Natural Enemy of Gypsy Moth", Michigan State University Extension Bulletin E-2604.

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