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Forest Tent Caterpillar - also called 'Army Worms'

Forest Tent Caterpillar or Army Worm, Malacosoma disstria The Forest Tent Caterpillar, Malacosoma disstria, is an important leaf-eating (defoliating) caterpillar in Wisconsin. Wisconsin's forests have evolved with forest tent caterpillars and they are a natural part of the north woods. Some people call forest tent caterpillars "army worms" because as they travel across the ground they look like marching soldiers.

Forest Tent Caterpillars attack a number of broadleaf trees and plants like quaking aspens, balsam poplar, basswood, oaks, ashes, birches, alder and fruit trees. This caterpillar rarely feeds on red maple and conifers, such as pine and spruce.

Quick Facts

  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.

Identification

Forest tent caterpillars are about 2 inches long with colorful bodies covered by many hairs. The sides of their bodies are blue with narrow orange stripes. Their backs are black with white markings that resemble keyholes, penguins, or footprints. Newly-hatched caterpillars are all black and only about one-eighth of an inch long.

Life Cycle

Egg

Forest Tent Caterpillar Egg Mass on Tree Branch, Malacosoma disstria

Eggs occur in masses of 100 to 350, forming bands of up to 1 inch in length that encircle small diameter twigs. The eggs are cemented together and they are coated witb a frothy secretion that hardens and turns a glossy dark brown. Within 3 weeks the embryos develop into tiny caterpillars that remain inside the eggs through the winter to hatch the following spring.

Egg masses are laid in late June to mid-July and stay through the winter until warm temperatures cause them to hatch in the following spring. You might find some old egg masses from last year; they are lighter in color with small holes where the caterpillars hatched and left.

Larvae (Caterpillar)

In northern Wisconsin, the caterpillars hatch from egg masses by late April to mid-May, about the same time aspen leaves begin to open. They actively feed on aspen and other broadleaf trees for 5 to 6 weks.

Pupae

Forest Tent Caterpillar Adult Moth, Malacosoma disstria

After 5 to 6 weeks of feeding, the caterpillars spin yellow cocoons of silk colored in a folded leaf, bark crevice, or other sheltered place. In these cocoons, the larvae molt into pupae. The adults emerge in about 10 days

Adult

Forest tent caterpillar adult moths are tan to white-yellow colored with brownish-colored bands on the moth's front wings. In addition, the adult has a series of darker-colored hairs on its abdomen and has a wingspan ranging from 1 to about 2 inches. They live for about 1 month.

The moths are attracted to lights at night. Because of this, during population outbreaks many moths are found flying in well-lit places such as gas stations and parking lots where lights are kept on at night. The adult moths mate, lay eggs and die within a week of coming out of their cocoon.

Diet

Leaf damage from forest tent caterpillar

Young larvae begin to feed on fresh, unfolding leaves until the next stage of development. The larva is the only of the four life stage that feeds on foliage. These insects overwinter as eggs and larvae emerge in the spring and feed for about 2 months. They spend about a month as adults that mate, lay eggs and die.

Broadleaved hardwood trees such as sugar maples, oaks, birch, cherry, Aspen, Tupelo, cottonwood, elms, willow, ash, basswood and others are preferred hosts. Investigations suggest that red maples, sycamores and most species of coniferous trees are not fed upon.

Forest tent caterpillar infestations will cause twig and branches to die, but usually does not result in death of the tree. However, when recurring defoliation accompanies other factors that can weaken a tree's health – drought, growth in poor soils and late growing season complete defoliation – the result of the infestation may kill the tree.

Population Control: Natural Enemies

After 2 to 4 years of forest tent caterpillar outbreaks, populations of natural enemies increase in response to rising tent caterpillar numbers. These natural enemies help put the populations of the forest tent caterpillar in balance. Beetles, true bugs, birds and small animals feed on caterpillars and their pupae (cocoons). In Minnesota, crows eat about 20% of forest tent caterpillar pupae during outbreaks there.

Impact

A group of forest tent caterpillars on tree trunk

Forest tent caterpillar populations regularly boom and bust. When their numbers are high they can eat nearly all of the leaves (cause heavy defoliation) on broad-leaved trees and shrubs. During an outbreak, caterpillars may crawl on roads and buildings as well as trees.

Although caterpillars can eat all of the leaves off of a tree, most deciduous trees can survive 2 to 4 years of heavy defoliation by forest tent caterpillars. Heavily defoliated trees often grow a second set of leaves to make up for the leaves they lost to feeding.

Trees in poor health or under drought stress may die after several rounds of severe defoliation. Also the stress caused by defoliation in otherwise healthy trees may attract secondary pests. For example, oak trees that have severe defoliation by forest tent caterpillars are often later killed by two-lined chestnut borers or by root rot fungi called Armillaria.

Forest Tent Caterpillar Management

Remove Egg Masses

Forest tent caterpillar egg masses are laid in summer on twigs of the trees and shrubs and stay all winter. Each egg mass has up to several hundred caterpillars, so you can quickly reduce the number of caterpillars that may hatch the following spring by removing egg masses before they hatch. This works especially well to protect smaller trees and shrubs.

Sticky Band Around Trees

A sticky band around tree trunk to trap forest tent caterpillars

You can put a sticky or slippery band around tree trunks to prevent caterpillars from crawling up to the leaves. Sticky bands can be purchased at garden centers or can be made at home using duct tape and a waterproof sticky material such as Tanglefoot or petroleum jelly. Wrap the duct tape tightly around the trunk, at least a few inches wide, at chest height. Then paint the sticky material onto the duct tape. Make sure that the sticky material does not directly contact the bark of the tree because it could harm or kill the tree.

A sticky band will only protect trees from caterpillars moving up and down the trunk. It will not work for caterpillars that hatch from egg masses already high in the tree. The sticky material may need to be replaced often because it gets less sticky when covered by debris or dead caterpillars. Take bands down around mid-June when most forest tent caterpillars are done feeding.

Turn Off Exterior Lights

Moths of forest tent caterpillars are attracted to lights. When moths are abundant, you should turn off exterior lights. This may reduce the number of egg masses laid on nearby trees.

Spray With Soapy Water

A stiff spray of water may work on smaller trees to kill young caterpillars. To remove caterpillars from your picnic table or house, you can kill individual caterpillars by pouring or spraying soapy water on them or simply by putting them in soapy water. Soapy water must completely soak caterpillars to kill them because their hair can protect them against small amounts of water. There are no traps available to put in your yard to attract forest tent caterpillars or moths.

Further Information on Garden Pests:

 Groundhog Facts and Control
 YIKES! Jumping Worms
 How To Get Rid Of Ants
 Japanese Beetles: Facts and Control
 Voles — The Good and the Bad

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