Gypsy Moth and Forest Relationships
Susceptibility - Potential for Defoliation
The gypsy moth is extremely polyphagous; it is known
to feed on hundreds of different tree species in
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
This means that stands with a large component of highly
preferred species (e.g. oaks) are likely to be defoliated.
here to see maps showing the distribution of
preferred hosts and susceptible forests in the US.
Defoliated ridge-top, Western Massachusetts.
There is also evidence that the abundance of small mammal predators
affects forest susceptibility to the gypsy moth.
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
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
to see research information about the effects of defoliation
on forest dynamics.
are available for minimizing gypsy impacts on forests.
Armillaria rhizomorphs under bark of killed oak.
Armillaria fruiting bodies.
Galleries and larva of Agrilus bilineatus
Cluster of trees killed following gypsy
moth defoliation, Central PA.
Return to Gypsy Moth in NA
Last modified 10-29-03 by Sandy Liebhold .