Homeowners in the northern portion of Illinois where gypsy moth is present often feel frustrated in how to deal with it; however, an understanding of the behavior of gypsy moth may provide a way to deal with this plant-feeding pest. During this time of year, the later-instar larvae crawl down trees during daylight hours. Wrapping burlap near the base or up to 6 feet from the base of trees provides shade and shelter for older larvae when they seek out resting places during the day. Later on in the year, the larvae crawl down trees to pupate. Physically removing caterpillars (and then the pupae) during the day and placing them into a solution of soapy water may reduce the number of damaging larvae, particularly under low populations, and potentially reduce the number of moths.

Questions are often asked by homeowners regarding the effectiveness of biological controls or natural enemies of gypsy moth. Gypsy moth is susceptible to attack by various natural enemies such as parasitic wasps (parasitoids), predators, and pathogens (fungi and viruses). The major parasitic wasps are the egg parasitoid, Ooencyrtus kuvanae, and a parasitic fly of the caterpillar, Blepharipa pratensis. A large predatory beetle, Calosoma sycophanta, feeds on gypsy moth caterpillars. A fungus, Entomophaga maimaiga, which has been found in Illinois, causes a disease in gypsy moth larva that kills them. It was introduced into the United States in 1909. However, it was undetected for over 80 years until it was recovered again in the late 1980s. This fungus overwinters as a resting spore within dead caterpillars. It infests live caterpillars in the spring. The abundance of the fungus depends on wet weather because the fungus performs best under moist conditions. The spores (conidia) are spread by wind and infect other caterpillars. The fungus can kill caterpillars within a week. E. maimaiga infects gypsy moths at low populations. It is possible that this fungus, which is found in the soil, may follow the spread of gypsy moth.

During outbreaks or when populations are high, gypsy moths may be killed by a viral organism known as nucleopolyhedrosis virus (NPV). Naturally occurring in all gypsy moth populations, the virus has the capability to persist in the soil. Infection occurs when larvae consume leaves contaminated with the virus particles. Once inside the gypsy moth larvae, the virus disintegrates the internal organs, resulting in death. Before dying, the larvae migrate to the tip of branches and hang down in a characteristic J shape. Eventually, the body becomes a mass of viral particles, which seep out and fall onto leaves below. Unlike E. maimaiga, the virus only occurs under outbreak conditions, because caterpillars are generally crowded and stressed from lack of food.

Additionally, vertebrate animals such as mice or shrews feed on gypsy moth caterpillars. However, like many of the natural enemies of gypsy moth, they don’t kill enough gypsy moth caterpillars to prevent or minimize defoliation of trees.