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Gypsy Moth Activity in Illinois

July 11, 2001

We have received a number of inquiries on the status of gypsy moth activity during this time of year and what to expect in the next few months. As a result, this article addresses in detail gypsy moth activity occurring in Illinois.

The larvae (caterpillar) stage of the gypsy moth has already completed most of its feeding. By the last week in June or early July, larval development is complete and the larvae seek sites such as bark crevices where they spin silky threads. They then enter a pupa or transitional stage. The male gypsy moth has five instar stages, whereas the female has six. As a result, the first initial pupal cases, which can be very noticeable on trees, are males. Eventually, the females undergo pupation. The pupa stage lasts about 2 weeks.

Besides the presence of pupae during this time of year, dead caterpillars may also be evident hanging upside down from leaves, twigs, or branches. These caterpillars more than likely died from a natural disease such as a virus or bacteria. One of the common mortality factors that can impact gypsy moth populations is a virus called NPV, or nuclear polyhedrosis virus. 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. Although the virus is present in the gypsy moth larvae, it causes very little mortality when populations are low. However, when gypsy moth populations are high, the virus multiplies rapidly, resulting in very high levels of larval mortality.

Adult gypsy moth emergence generally begins the first week in July and continues into August. The adult stage does not feed. Males appear 4 to 5 days before females. Following emergence, adult females locate themselves on tree trunks and release a sex pheromone, or odor that attracts males. Males fly around until they contact the pheromone odor. Males can detect the odor from a mile away. They then fly upward or upwind in a zigzag pattern to locate females and begin mating. After mating is complete, the females begin to lay eggs. Both sexes live only for about 7 to 10 days in the adult stages. Females are unable to fly because they are heavily laden with eggs. Eggs are normally laid from July through August, with adult females capable of laying up to 1,000 eggs within a brown mass. The egg mass is normally found close to where the mature female pupates, such as on tree trunks, on the undersides of branches, under bark or rocks, on buildings, on vehicles, or in debris. Gypsy moth overwinters in the egg stage. The eggs hatch in the spring. There is only one generation per year in Illinois.

The pheromone released by gypsy moth females has been synthesized, and the synthetic pheromone is used in surveys to detect the presence of male moths. Each September, the number of male moths in traps (described in the previous issue) are counted to determine the extent of the gypsy moth program. This helps determine the need to implement quarantines the following spring, as well as the need for a more concentrated spray program. If you have concerns regarding gypsy moth, contact your local University of Illinois Extension office.

Author: Raymond A. Cloyd


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