Gypsy moth caterpillars consume as much leaf tissue as they can, as quickly as they can, to obtain nourishment to become reproducing adults. Since the caterpillars' feeding period lasts seven to ten weeks through spring and summer, they can do a lot of damage to young tree leaves. If a tree loses more than 50% of its leaves for more than two years in a row, it will certainly be weakened and may not survive.
A single gypsy moth caterpillar can consume 11 square feet of vegetation during its lifetime so the presence of millions of caterpillars can severly affect trees and forests. Although gypsy moths are capable of feeding on over 500 different species of trees and plants, they prefer oak trees.
Although gypsy moths can exist at relatively low population levels for years at a time, sometimes their populations explode. This occurs for various reasons (favorable weather conditions or a lack of predators, for example). This rapid swelling of population size is called an "outbreak".
During large outbreaks, trees are virtually stripped of their leaves by hungry caterpillars within a few days. Although most trees will re-grow new leaves before summer's end, the process stresses the tree and drains its reserves.
Weakened trees and shrubs, especially those in urban settings, are more susceptible to attack by opportunistic diseases and organisms such as armillaria root rot and two-lined chestnut borers. In addition, the tree's growth rate is impaired, which affects its chances for survival during the following years.