NDSU Crop and Pest Report

ISSUE 2  May 9, 2002



Last year, substantial defoliation of hardwood trees by Forest tent caterpillar occurred in eastern North Dakota and will likely continue this year. The infestations in many areas of northwest Minnesota appear to be waning, though. Outbreaks typically last one to three years

Observations about trees response last year were that basswood defoliation was moderate to complete in North Dakota, while bur oak, boxelder, and green ash usually were lightly defoliated. Most of the defoliated oaks and basswoods refoliated quickly, while aspens were slow to recover. Healthy green ash refoliated by mid summer.

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 are often found first near the egg masses. The caterpillars 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 grow, keyhole_shaped spots along their backs and broad bluish lateral bands become evident. These markings make identification relatively simple.

Forest Tent Catepillar image

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. In the past three years, hatching and feeding by larvae was underway by mid-May. The cold temperatures are delaying hatching this spring. Watch the trees to get an early indication of when hatch should begin. Hatching would not be expected until young leaves are present.

Mechanical/Cultural Control: 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 an area. By reducing moths, egg masses in an area decline and so should defoliation the next year.

If only a few caterpillars are found, they can be picked from trees by hand. When FTCs are migrating from heavily infested to noninfested trees, they can be stopped with Tanglefoot or other sticky material applied as a band around the trunk. The sticky material must be checked on a daily basis.

Ensuring that trees have adequate water can help trees recover from early season defoliation; however, it is recommended you do not fertilize trees during FTC outbreaks.

Insecticide Control: If treatments do become necessary, the bacterial insecticide, Bacillus thuringiensis (Bt), can be effective when applied while most caterpillars are still small (a inch or less). Bt must be ingested by the caterpillars to work. Feeding stops quickly, but it requires a few days before caterpillars actually die. Bt works well to control young caterpillars, but larger caterpillars may require different insecticides such as carbaryl, permethrin, pyrethrins or others currently sold for tree and shrub application.

For more information on Forest tent caterpillar, visit the North Dakota insect updates for Lawn and Ornamentals:



Phillip Glogoza, Extension Entomologist

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