White oak is the preferred host, but most other oak species (in the Northeast) are also highly susceptible, as well as many other deciduous species. This includes maple, birch, poplar, willow, apple, hawthorn, and many others. Conifers, such as pine and spruce, may also be attacked when the preferred host plants are in short supply. While many trees can survive a year of defoliation from gypsy moth if they are otherwise healthy, consecutive years of defoliation can highly stress host trees. These stressed plants then become more vulnerable to secondary pests, such as certain wood-boring insects and decay fungi.
The gypsy moth accidentally escaped the home of E. Leopold Trouvelot and was introduced into the US in Medford, Mass. in the late 1860’s. He had intentionally brought it to his home in Massachusetts, from France, to study the insect with an interest in silk production. Since then, gypsy moth has spread throughout the Northeast and well beyond. It can be a serious pest of trees and a nuisance due to the irritating hairs on its body and the copious amount of excrement (frass) that it produces in high population years.
The gypsy moth overwinters as an egg in a cluster of 500 or more eggs (Figures 1 and 2). Eggs typically hatch in the spring during the first week in May in Massachusetts, but variations in climate and spring weather can either accelerate or delay egg hatching. Once hatched, the tiny, hairy caterpillars may remain in the lower forest canopy or, when in high populations, migrate upwards to the tree tops, where each one then spins down on a long silken thread. The tiny caterpillars hang in the air waiting for a strong wind to break the thread and carry them to a new location. This process of dispersal is known as “ballooning” and is somewhat common in caterpillar species where the adult females do not fly. It is the only silk that this species produces. Gypsy moth caterpillars do not make silken webs or tents. This type of dispersal helps young larvae relocate to more favorable hosts, such as oaks, while factors such as food quality (species composition) and the availability of suitable areas to hide during the day (such as in rough oak bark) may affect gypsy moth dispersal patterns. Population sizes of this pest can change dramatically from one year to the next.
Once the caterpillars settle on a new host, they begin feeding on the foliage. Small to moderate sized populations will often feed at night and come down out of the trees during daylight hours to avoid predators and parasites. Caterpillars in high populations usually stay in the trees around the clock due to intense competition for foliage.
While gypsy moth caterpillars are only 1/16 of an inch in length when they hatch, they may exceed 3 inches in length by the time they pupate, usually about six weeks later. The caterpillars have hairy bodies; along the length of their backs, they have five pairs of blue dots followed by six pairs of red dots (Figure 3). The caterpillar stage typically lasts until about the third week in June in Massachusetts, whereupon they pupate (Figure 4). Adults start to appear by late June/early July. Neither the male nor female adult moths feed.
Adult male gypsy moths are brown with black markings and have highly feathered antennae (Figure 5). Female moths are white with black markings and have straight, threadlike black antennae; female gypsy moths do not fl y (Figure 1). Gypsy moth caterpillars have numerous hairs on their bodies, as do the adults. Many people experience allergy-type reactions to these hairs. Symptoms range from itchy skin irritation to sinus allergies with itchy eyes and a runny nose.