Forest Tent Caterpillar - also called 'Army Worms'
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.
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.
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.
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.
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.
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
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.
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.
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
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 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.
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
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.
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.
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