Forest tent caterpillar is a Minnesota native that feeds on (defoliates) the leaves of a wide variety of hardwood trees and shrubs including aspen, birch, basswood, and oak.
Forest tent caterpillars are often mistakenly called armyworms because of the way they congregate in groups to feed. Despite the common name of tent caterpillar, they do not make tents or webs, but make silk mats they rest on during molting.
Forest tent caterpillar population outbreaks happen every 10-16 years and may last from two to four years. When there is a large outbreak, thousands of caterpillars congregate on trees and buildings from late May to mid-June, defoliating trees and creating a nuisance, making outdoor activities unpleasant and sometimes causing slippery conditions as caterpillars migrate across roads.
According to normal predictions, we expected an outbreak of forest tent caterpillar beginning in 2013. The chart below shows a population increase in 2013, but it didn't become what we usually think of as an outbreak. Based on forest tent caterpillar population dynamics, the next population peak may occur sometime between 2023 and 2029 (assuming that a true population peak occurred in 2013). Find detailed accounts of damaged acres in the Forest Health Annual Reports. In the last 120 years, populations peaked in 1891, 1898, 1912, 1922, 1937, 1952, 1969, 1978, 1990, and 2001. In 2001, more than 7.5 million acres of hardwoods were defoliated, the most ever recorded in Minnesota.
Forest tent caterpillar peak years 1937–2001
click on maps to enlarge
Forest tent caterpillar is found throughout the range of hardwood forests in North America.