Gypsy Moth In North America

Susceptibility - Potential for Defoliation

The gypsy moth is extremely polyphagous; it is known to feed on hundreds of different tree species in North America. However, within most forests in the Eastern US, there are some species that are highly preferred by the gypsy moth and other species that are immune. Click here to see a publication that lists the preference category of each N. American tree species.

The relative dominance of highly preferred tree species appears to be the major determinant of forest susceptibility (defoliation potential). This means that stands with a large component of highly preferred species (e.g. oaks) are likely to be defoliated. Click here to see maps showing the distribution of preferred hosts and susceptible forests in the US.

There is also evidence that the abundance of small mammal predators affects forest susceptibility to the gypsy moth. Small mammals are the most important gypsy moth predators at low density gypsy moth densities. There is evidence that small mammals are more common in mesic (wet) oak-dominated sites than in dry oak-dominated sites (e.g. ridge tops, coastal New England) and consequently outbreaks are more common in these dry sites.

Vulnerability - Tree Mortality

During gypsy moth outbreaks, feeding by caterpillars may remove much, if not all foliage from trees. This defoliation represents another form of stress (in addition to drought, soil compaction, etc) that affects a tree's physiology and may ultimately result in death. When defoliation levels are high, hardwood trees will "refoliate" (production of a 2nd flush of leaves). But in anycase, the defoliation weakens the tree and may lower its susceptibility to secondary agents. For oaks, the secondary pests of most important are the shoestring root rot fungus Armillara spp. and the two-lined chestnut borer, Agrilus bilineatus. Both of these agents attackweakened oaks and are usually the direct causes of mortality. Extensive mortality of oaks usually occurs following two or more consecutive years of defoliation, though mortality can occur following only one year of defoliation if some other predisposing condition exists (e.g. drought).

The mortality associated with gypsy moth outbreaks can profoundly affect the composition of forests and affect succesional trends. Click here to see research information about the effects of defoliation on forest dynamics. Silvicultural methods are available for minimizing gypsy impacts on forests.

Armillaria rhizomorphs under bark of killed oak.

Armillaria fruiting bodies.