Forest tent caterpillars

Quick facts 

Forest tent caterpillars in Minnesota attack a number of broadleaf trees and plants like quaking aspens, balsam poplar, basswood, oaks, ashes, birches, alder and fruit trees.

  • Feeding damage by these caterpillars slows down the growth rate of deciduous trees.
  • When target trees are defoliated, forest tent caterpillars may also damage other nearby plants.
  • Damage can be seen on vegetables, fruit trees and other small fruits, and nursery crops.
  • They are a nuisance when they are found around buildings or on roads.

The forest tent caterpillar (Malacosoma disstria) occurs throughout most of the United States and Canada wherever hardwood trees are found. This caterpillar rarely feeds on red maple and conifers, such as pine and spruce.

Blue-black forest tent caterpillar with hairs on the side of the body
Forest tent caterpillar larva

How to identify forest tent caterpillars

Full-grown caterpillars:

  • about two inches long

  • mostly blue and black

  • have a row of white, footprint shaped markings on their backs

  • many hairs along the edge of the body

Adult moths

  • tan moths are nocturnal and are attracted to lights at night

  • emerge from cocoons about two weeks later in mid-July

Several cylindrical, gray eggs wrapping a tree branch
Forest tent caterpillar egg mass

Life cycle of forest tent caterpillars

Larvae (caterpillars) emerge from egg masses in early to mid-May, about the same time aspen leaves begin to open.

  • feed actively on aspen and other broadleaf trees for five to six weeks

  • spin an unnoticeable silken mat where caterpillars group on the trunk and branches

In June, older larvae move around trees and other vegetation to find food, and may damage nearby plants.

  • end of June, full-grown caterpillars look for protected places to spin silky cocoons

  • in mid-July, adults come out of cocoons

  • adults live for about five days and deposit 100 to 350 eggs in gray, cylindrical masses surrounding small twigs

The eggs live through the winter and larvae hatch from the eggs next spring. There is only one generation per year.

Two blue-black forest tent caterpillars on partially eaten leaves
Partial defoliation by forest tent caterpillar

Damage caused by forest tent caterpillars

Feeding by forest tent caterpillars generally does not kill deciduous trees, as they can produce another set of leaves during the same season.

Healthy trees can tolerate two to three consecutive years of heavy defoliation.

Trees may be killed:

  • If the same tree has been heavily defoliated for four or more years.
  • When trees become stressed, such as during a drought.
Completely eaten up leaves with a forest tent caterpillar on a leaf vein
Complete defoliation by forest tent caterpillar

Forest tent caterpillars as a nuisance

Mature larvae may be seen on buildings and in yards, when they are looking for sheltered sites.

  • They do not bite people or harm animals or property.
  • Their cocoons on the sides of buildings are very difficult to remove and are a nuisance.
  • If forest tent caterpillars on streets get accidentally crushed, they can make surfaces greasy and slippery.

A large gray fly, Sarcophaga aldrichi, feeds on forest tent caterpillars and its population increases when large numbers of forest tent caterpillars are found.

This fly does not bite and is harmless, but is a nuisance since it lands on any object, including people.

This insect is very important for ending a forest tent caterpillar outbreak naturally.

How to protect your trees from forest tent caterpillars

Natural controls

  • Cold or damp spring, starvation and viral disease reduces forest tent caterpillar numbers.
  • Wasps and flies can grow inside forest tent caterpillar eggs, larvae and pupae to kill them.
  • An example is the large gray fly, Sarcophaga aldrichi, that is native to Minnesota.

Remove eggs and caterpillars as you see them

  • Remove and destroy egg masses from branches of small trees before eggs hatch in the spring.
  • Brush off caterpillars and cocoons from houses, picnic tables or decks with a stiff broom.
  • Use a spray of water to knock caterpillars off.
  • Be careful not to crush too many caterpillars as they can smear and leave marks on some paints.

Using pesticides

In addition to physical controls, pesticides may help in control of larvae.

Apply pesticides when larvae are small (1 inch or less), usually in early to mid-May. Larger larvae are difficult to kill and can continue to defoliate trees before pesticides have any effect.

Some available options are:

  • Bacillus thuringiensis (also referred to as BT), a microbial pesticide from a bacterium is effective and conserves beneficial insects.
  • Other pesticides that conserve beneficial insects: insecticidal soap, spinosad (microbial pesticide) and azadirachtin (botanical pesticide).
  • Chemical pesticides like: acephate, bifenthrin, carbaryl, cyfluthrin, diazinon, esfenvalerate, fluvalinate, lambda-cyhalothrin, malathion, permethrin and phosmet.

Check the label carefully when picking a pesticide, as the active ingredients are listed in fine print.

Forest tent caterpillars in wood lots

In wood lots, birds, rodents and even bears eat forest tent caterpillar larvae. Control measures should be implemented only in case of heavy defoliation for a number of years.

In wood lots, resort areas and campsites where large areas may need treatment:

  • aerial spraying of pesticide by aircraft is the most rapid, effective and economical method
  • spraying should not be done when breezes threaten to drift the pesticide over open water or other sensitive areas
  • Treat an additional strip about 400 feet wide adjacent to the area, when spraying in residential or recreational areas. This barrier strip controls migrating caterpillars.

The pesticide BT is preferred since it does not harm people, birds or beneficial insects and is generally considered first in aerial spray programs.

Do not use diflubenzuron near wetlands or water, as the pesticide may affect aquatic insects and other arthropods.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Jeffrey Hahn, Extension entomologist and Vera Krischik, Extension entomologist

Reviewed in 2018

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