NORTHERN "AG" EXPOSURE

by Mark Miller
Rolette County Extension Agent

Published in the Turtle Mt. Star, June 4, 2001

Forest Tent Caterpillar

Small pockets of forest tent caterpillars (FTCs) have been consistently causing problems in North Dakota since 1997. Most of the infested areas have been severely defoliated once during the last four years and defoliation has been occurring in scattered clumps of trees. Since defoliation is early in the year and healthy trees will usually develop a second flush of leaves, tree health is usually not affected. However, the insects can become a nuisance during early to mid June. Substantial defoliation of hardwoods by FTC has been reported in Canada, Minnesota, and eastern North Dakota in recent years. Basswood and aspen have recently been defoliated most (moderate to complete defoliation) in North Dakota, while bur oak, boxelder, and green ash have usually been lightly defoliated. Most of the defoliated oaks and basswoods refoliated quickly, while aspens were slow to recover. Healthy green ash should refoliate by mid summer. Outbreaks may last one to four years in North Dakota. Parasitic flies (also called "friendly flies") are apparently important in the collapse of individual FTC pockets in North Dakota.

Forest tent caterpillars overwinter as fully developed larvae in egg cases which are laid in masses encircling small twigs. The tiny caterpillars emerge in spring and can often be found first near the egg masses. Unlike other tent caterpillars, FTCs do not produce large, unattractive webs. The caterpillars do congregate on stems and branches during the day. Forest tent caterpillars feed on ash, aspen, basswood, birch, cottonwood, elm, maple, oak, poplar, and other hardwoods. As the larvae enlarge, keyhole_shaped spots along their backs and broad bluish lateral bands become evident. These markings make identification relatively simple.

Management. It is difficult to determine how much damage the forest tent caterpillars will cause this year and during the next few years in the state. Since the adults are moths that are attracted to lights, shutting off yard lights from late June through mid July may reduce the number of egg-laying adults in areas where the larvae could become a nuisance the following year. This may reduce defoliation the following year. If only a few caterpillars are found, they can be picked from trees by hand. When FTCs are migrating to noninfested trees, they can be stopped with Tanglefoot or other sticky material. The sticky material must be checked on a daily basis. If treatments do become necessary, Bt can be effective when applied early. Since the insects are already feeding, this product should be applied as soon as possible for best results. Bt works well to control young caterpillars, while permethrin, pyrethrins or other insecticides are needed for older larvae. Always follow pesticide labels. Ensuring that trees have adequate water can help trees recover from early season defoliation; however, do not fertilize trees during FTC  outbreaks.

Go to Archives

Go to Rolette County Home Page

Updated June 11,2001 Shelley Armstrong,Webmaster