Widespread outbreaks of FTC occur at intervals of ten to twenty years and are three to five years in duration. Locally, outbreaks normally last for two to three years. Outbreaks commonly begin over large areas simultaneously, primarily the result of weather preceding the outbreak. Population buildups can be triggered by a cool winter followed by a warm spring. Cool springs and warm winters usually accompany a population collapse.
Outbreaks can begin suddenly or develop slowly over a period of years.
Defoliation starts in late May in central Minnesota and early June in northern areas. Defoliation will normally be obvious by mid-June and finished by late June. During outbreaks, Forest Tent Caterpillars can number from one to four million caterpillars per acre. They create an extreme nuisance to people living or vacationing in forested areas. Young caterpillars spin threads and fall from the trees onto picnic tables, patios, and people causing serious annoyance.