Nearly five hundred varieties of trees and shrubs are host plants to gypsy moth insect, with Oak and other forest hardwoods being the preferred hosts. Damage to the plant is done during the larval stage. The larvae begin to emerge just as the plant’s leaves are beginning to develop in the spring.
Gypsy moth larvae eat the entire leaf, excluding the veins, and a heavy infestation is capable of completely defoliating a tree. Larvae feed as they develop through instar stages. During early instar stages, larvae will feed during the day and rest at night. As they mature, they continue to feed day and night until they reach pupation. Trees which are healthy prior to infestation can be killed by three or four years of continuous defoliation. A tree which is stressed prior to infestation can be killed by a single heavy infestation.
Buff colored egg masses can be found on trees, building structures, fences and even the underside of cars—almost anywhere that the egg masses are somewhat protected from the elements. The larvae/caterpillars emerge in late April-early May, depending upon daily weather temperatures. As they mature through the instar stages they can be identified by their double row of five or six red dots down their back. They pupate beginning June and continue through July. During the month of August, the adults emerge as moths. The females, white in color, are large, heavy and cannot fly. The male is gray in color, half the size of the female, has feathery antenna and is a strong flyer. The male is attracted by pheromones emitted by the female and mating takes place, thus the cycle starts over.
Treating the egg masses can be an effective, environmentally friendly means of control. Egg masses can be destroyed by physically removing them from whatever they are attached to, scraping them into a soapy water solution or saturating the egg mass using a soybean oil and water solution. Biological control, Bacillus thuringiensis, commonly referred to as Bt, is effective if used when the larvae are young, through the third instar stage. There are a large number of insecticides that will kill the caterpillars while they are feeding. Direct trunk injections are effective but need to be timed so as to be active at the time of feeding.