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. Large, mature caterpillars wander widely in search of food and often appear to migrate across roads and open areas. Resting caterpillars commonly form large clusters of thousands of caterpillars on buildings, tree stems, cars, campers, and other stationary objects.
Caterpillars often emit a greenish-black fluid when disturbed which stains paint and cloth.
Mass flights of FTC moths are common during outbreaks. These flights can move millions of moths hundreds of miles creating a nuisance where the flight ends. Mass flights can trigger new outbreaks suddenly where the insect had not been a problem before. These flights are often associated with the passage of a cold front. In the forest, defoliation from FTC usually causes little damage to tree health.