Trees are host to a number of moth species that exhibit high population densities, leading to widespread forest health decline across North America and Europe. The most significant forest defoliators are the Gypsy Moths, Budworms, Tussock Moths, Tent Caterpillars and Loopers in North America, and Processionary Moths and Gypsy Moths in Europe.
Populations of forestry defoliators can display cyclical highs and lows (see figure below for gypsy moth) as do government appropriations for corresponding treatment programs. The population fluctuations may range from very low levels over wide areas for 3-10 years followed by rapid growth to epidemic levels lasting 2-4 years. In addition to affecting forest health, the caterpillars of Processionary Moths are covered with allergenic body hairs that can cause allergic reactions in humans.
Factors that contribute to population cycles of forest defoliators are complex. Tri-trophic interactions – interactions among the tree hosts, the insect defoliator, and natural enemies – all contribute to population fluctuations. For example, slow- growing older trees have weaker defenses than younger trees against defoliators, and wet springs favor diseases that reduce defoliator survival. Therefore, some conditions may favor caterpillar survival and population growth while other conditions may not.