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Invasion of the Forest Tent Caterpillars

University of Minnesota Extension, Stearns County News
June 10, 2015

Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns, Benton & Morrison Counties

Invasion of the Forest Tent Caterpillars
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (06/10/15) — Have you seen your trees or shrubs under attack this June? Many mistakenly call them armyworms, but the wiggly, hairy worms eating your trees and shrubs are likely forest tent caterpillars this time of year. These defoliating pests seem to be in full force this year in many areas of Minnesota.

The forest tent caterpillars hatch when the aspen trees begin to leaf-out in the spring. The caterpillars then spin a silky mat in trees to protect themselves from birds and other predators as well as protect them from temperature extremes that occur at night or on cool, rainy days.  Caterpillars emerge from their silky mats in warm, sunny weather to feed on leaves of trees. They will feed on a variety of hardwood trees, but are known to feed on other trees and shrubs as well.

Mature caterpillars are hairy, velvety-black with bright blue stripes on their sides. An additional identifying feature is a row of yellow markings resembling a keyhole or footprint running down their back. The forest tent caterpillar feed for five to six weeks starting as early as May and running through June depending on the date they emerge. The caterpillars pupate in a silky cocoon that is about one-inch long in a protected area attached to objects like trees, houses, garages or other structures.  A yellowish-brown moth emerges approximately ten to fourteen days later and will lay eggs.  The eggs are laid in a shiny dark-brown mass encircling a small branch where they will overwinter until the next spring.

Forest tent caterpillars are considered more of a nuisance than a serious pest for trees or shrubs.  Defoliation is typically not significant enough to cause permanent damage or death to healthy, mature tree for most trees will replace lost leaves later in the summer.  However, trees and shrubs infested with tent caterpillars look ragged until new growth occurs. The forest tent caterpillar can be a nuisance as they can be found crawling on roads, sidewalks, and homes.

Outbreaks of this caterpillar usually occur every 10 to 16 years and last for three to four years before subsiding. During peak outbreaks forest tent caterpillars can number one to four million caterpillars per acre.  These caterpillars will prefer birch, aspen, basswood, and oaks, but will feed on many other broadleaf trees and shrubs including fruit trees and shrubs such as roses. 

Forest tent caterpillars can be controlled without the use of insecticides.  Removal and destruction of egg masses from trees during winter and removing cocoons in early summer can avoid the use of chemicals and defoliation of trees or shrubs.  In early spring, webs can be removed by hand, stick, broom or a heavy spray of water.  Caterpillars can be put into a pail of soapy water or squished but be aware they may smear and leave marks on plants or structures. 

In times of heavy infestation, chemical treatments can be done. Prioritize to ornamental and fruit trees and shrubs, as many larger deciduous trees will recover after feeding.  Bacillus thuringiensis var. kurstai (BT) is a bacterial insecticide that is very low toxicity and is effective if applied when the caterpillars are small; it can be found as Dipel, Thuricide or as Bonide Bacillus Thurengiensis.  Other insecticide products that are effective against tent caterpillars include acephate (Orthene), carbaryl (Sevin), permethrein (Eight), cyfluthrin, esfenvalerate and malathion. Be sure to follow all safety and application directions on the product label.

For more information about the forest tent caterpillar or other insects visit

PHOTO CAPTION: Forest tent caterpillars in their protective silky mat in early May
Photo Credit: Beth Berlin


Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
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