Outbreaks may build and decline faster in aspen dominated stands than in oak stands, according to recent observations in Michigan. Effects of gypsy moth defoliation are usually most severe during the initial outbreak in a newly infested area. Low rates of diameter growth are common during years of heavy defoliation and often for one or more years after defoliation ceases. Gypsy moth caterpillars severly defoliate a young oak tree, but did not feed on the adjacent red maple tree, which is not a preferred host. The amount of tree mortality and topkill sustained during a gypsy moth outbreak depends on the severity and frequency of defoliation, and on tree health.
Vigorous trees can often withstand severe defoliation for a few years.
However, each refoliation stresses the tree, reducing its energy reserves. Subsequent stress, including additional defoliation, drought, or frost injury, may kill the tree. Defoliated oak trees often become more susceptible to attack by the two-lined chestnut borer or Armillaria root rot. Suppressed, diseased, or low vigor trees may be killed after 1 year of heavy defoliation. Gypsy moth defoliation can reduce seed production and root-sprouting, resulting in poor regeneration.