Once gypsy moths become permanently established, it may take several more years for moth populations to build before residents see any noticeable defoliation. The first outbreak may last three to 15 years depending on weather patterns, local site conditions, and the presence of natural enemies. Typically, the first outbreak is by far the most severe because there are abundant host trees and very few natural enemies present. The first outbreak is when most environmental, social, and economic impacts will occur as a result of repeated defoliation and associated tree mortality.
Competition for food, increasing disease, and natural enemies eventually cause gypsy moth populations to collapse. After the first outbreak, gypsy moth populations behave more like those of native insects, with cyclic outbreaks every eight to 12 years.
Gypsy moths do not kill trees directly they defoliate them. Severe defoliation can add to other stresses such as weather extremes or human activities. This cumulative stress can leave trees vulnerable to disease or other pest infestation that can cause death. For example, stressed oak trees are susceptible to two-lined chestnut borer and the fungus that causes Armillaria root rot, both of which can lead to oak tree death. The trees most at risk of dying as a result of gypsy moth defoliation are those that are already under stress. Most trees die following a gypsy moth outbreak in:
|Category||Overstory species||Understory species|
|Preferred: Species readily eaten by all caterpillar stages||all oak
bigtooth and quaking aspen
paper and river birch
|Less preferred: Species fed upon when preferred species are unavailable and by older caterpillar stages||yellow birch
sweet and black cherry
American, Siberian, and Chinese elm
Norway, red, and sugar maples
|Avoided: Species that are rarely fed upon||all ash
eastern red cedar
black and honey locusts
mountain and striped maple