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Photo by: Gary Fewless

Location: Vilas Co., WI

Date taken: June 8, 2001

Camera: Olympus CL 2500L digital camera


Forest Tent Caterpillars on Sedges.

Forest Tent Caterpillars (Malacosoma disstria) feeding on sedges after poplars have been defoliated.

(Click on image to see other forest tent caterpillars (76K)

If you have been in the Vilas County area this summer you were probably surprised to see so many caterpillars covering the trees. The species you saw is the Forest Tent Caterpillar and is native to the area. It is easy to differentiate this species from the gypsy moth and from the eastern tent caterpillar by looking at its back. The forest tent caterpillar has white spots, the eastern tent caterpillar has an unbroken white line, and the gypsy moth has red and blue spots. Forest tent caterpillars are always present in hardwood forest, but occasionally there are "outbreaks" where the populations become very high. Outbreaks usually last about 3 years and then the populations decline again for between 6 and 16 years. Certain species of broadleaved trees like aspens and maples can be completed defoliated during outbreak years, but the trees usually suffer no permanent damage unless the outbreak lasts for 5 or more years. The trees usually produce a second smaller set of leaves after the June outbreak. In extreme situations the caterpillars will even feed on spruce and other evergreens.

Unlike eastern tent caterpillars, the forest tent caterpillars do not spin large silk tents. The very young larvae wrap up leaves or weave silken mats for resting or molting. If you look carefully you will see what appear to be small dead caterpillars trapped on the silk, but these are only the molted skins. After feeding for about 6 weeks the mature caterpillars migrate away from their host trees to spin yellowish cocoons inside folded leaves or under bark. They pupate for about 10 days and the tan colored moths that emerge only live a few days. The adult moths are also easy to find because they are attracted to lights.

If you have seen the forest tent caterpillar you have probably also seen their most important predator Arachnidomyia aldrichi. These large gray flies do not bite humans, but they are extremely important in controlling tent caterpillar numbers. Female flies lay live maggots on cocoons and the maggots eat through the silk and eat the pupating moth. Predatory beetles, ants, true bugs, spiders, birds, and small animals also feed on caterpillars and pupae. The caterpillars are really more of an annoyance than a serious threat to the trees survival. Only 1% of trees die from caterpillar outbreaks, although the stress of defoliation can make the trees more susceptible to disease or other pests. If necessary, sticky bands can be placed around tree trunks in June as the caterpillars begin to migrate. The egg masses are large and can be removed by hand.

Contributed by Vicki Medland

© 2001-2004 The Cofrin Center for Biodiversity and the University of Wisconsin Green Bay, All Rights Reserved
Last updated on May 12, 2014