Gypsy moths consume about 4 million acres of American forest per year. In heavily infested areas, large tracts of forest can be completely defoliated (stripped of leaves).

Partially or fully defoliated trees are prone to attack by other organisms, such as fungi and a variety of other insects.

Defoliation increases water loss and run-off, removes nesting sites for canopy birds, and encourages the growth of understory shrubs because more sunlight reaches the forest floor. Mammals are also attracted to the infested area, where they prey on ground-nesting birds.

Besides their negative impacts of forest ecosystems, gypsy moths are also a nuisance. Their frass (waste) covers picnic tables and some people have allergic reactions to the hairs covering the caterpillars and egg masses.