Insect Fact Sheet: The Forest Tent Caterpillar
Forest tent caterpillar (Malacosoma disstria Hübner) is a native insect pest. It is one of the most
common defoliators of hardwood trees in North America and is found throughout New Brunswick.
Records of outbreaks date back as far as 1791 and are reported to generally occur every 10-12
years (Martineau 1984). The two most recent outbreaks in New Brunswick occurred from 1979 to
1984 (1.4 million ha) and from 1991 to 1996 (0.4 million ha). Due to a number of natural controls,
outbreaks of forest tent caterpillar generally last only three to four years.
Host Plants
The principal host of the forest tent caterpillar is trembling aspen. However, during outbreaks it will
feed on a number of other hardwoods including American beech, apple trees, basswood,
cherries/plums, gray and white
birch, sugar maple, red oak, white
ash, white elm, and willow.
Damage
Forest tent caterpillars eat the
leaves of host plants. During
outbreaks, entire trees can be
completely stripped of their leaves
with defoliation occurring over
widespread areas (Figure 1).
Because forest tent caterpillar is
an early season defoliator, trees
will produce a new crop of leaves
after they have stopped feeding.
Most trees can withstand several
years of severe defoliation without
dying, though growth loss can be
significant. Multiple years of
severe defoliation can weaken a
tree and make it more susceptible
to attack from other insect pests,
disease and abiotic factors such
as drought.
NB DNR
NB DNR
Figure 1. Defoliation caused by the forest tent caterpillar.
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Identification
Caterpillars (or larvae) of the forest tent caterpillar have a distinctive row of key-hole shaped white
dots down their backs bordered by two blue lines (Figure 2a). The caterpillars congregate in large
masses on the stems of trees (Figure 2b) and may be observed migrating between food sources in
large groups (giving them the nickname “armyworm”). Contrary to their name, forest tent caterpillars
do not form tents.
a) b)
L. Hartling, NB DNR
L. Hartling,
NB DNR
Figure 2. Larvae of the forest tent caterpillar.
Several other caterpillars can be mistaken for the forest tent caterpillar. They include:
• Eastern tent caterpillar larvae (Figure 3a) congregate in a silken tent and commonly
feed on apple and cherry trees. These caterpillars have a white line down their back
rather than key-hole shaped dots.
• The larvae of the gypsy moth (Figure 3b) feed on many hardwood trees and other
miscellaneous plants, especially oak, poplar, birch and apple. Caterpillars of the
gypsy moth are very hairy and have five pairs of blue and six pairs of red dots along
its body.
• The satin moth caterpillar (Figure 3c) feeds on many hardwood trees but prefers
ornamental species of poplars. These caterpillars have one row of oblong pale
patches along its back with two yellow lines. Each side of the caterpillar has tufts of
long brownish hairs sticking out from a row of orange bumps.
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a) b)
c)
NB DNR
NB DNR
T. Arcand, NRCAN
Figure 3. Larvae of a) eastern tent caterpillar, b) gypsy moth, c) satin moth.
Life Cycle
In late May and June, caterpillars (larvae) emerge from eggs that have been laid in bands encircling
the twigs the previous summer (Figure 4a). Each egg mass contains between 100 and 300 eggs.
Over the next
five to six
a) b) c)
weeks, larvae
feed on the
leaves of the
host plant.
When the
larvae have
completely
defoliated all
trees in an
area, they will
migrate in
large groups
in search of a
new food
supply. In late
NRCAN
NRCAN
NRCAN
June, the
Figure 4. Various stages of forest tent caterpillar life cycle; a) egg mass, b)
larvae spin a
cocoon, c) adult moth.
silken cocoon
(Figure 4b) in
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folded leaves or other sheltered locations and several weeks later, an adult moth emerges (Figure
4c). These moths mate within 24 hours and females begin laying eggs several hours later. These
eggs remain on the twigs over the winter and become the next generation of forest tent caterpillar.
Pest Control
Outbreaks of the forest tent caterpillar are usually quite short because of natural control provided by
predators, parasites and disease. However, defoliated trees can be unsightly until new leaves flush
and large masses of migrating caterpillars can be a nuisance. To help reduce the number of
caterpillars and severity of damage, the following measures can be taken:
•
•
•
•
Before egg hatch in the spring, check the crowns of small hardwood trees for the
presence of egg bands laid on branch twigs. Remove and destroy them.
To prevent caterpillars from infesting a tree, apply a pest barrier product such as
Tanglefoot® on the tree trunk in the spring after leaves flush.
Use a registered insecticide for use against this pest. Many products are available at
home and garden centres. Be careful to follow the manufacturers’ directions.
Commercially available insecticides containing the naturally occurring bacteria Bacillus
thuringiensis (Bt) can be used to control the forest tent caterpillar.
If trees are defoliated or damaged, keep them watered to prevent additional stress
caused by drought conditions.
It is very difficult to control large numbers of migrating caterpillars. Manual removal and/or killing
caterpillars (e.g., pressurized water sprayers) may provide some relief. Caterpillars may crawl onto
houses and buildings as they seek new food or a place to pupate. They are not attempting to enter
the house, but homeowners should take efforts to seal any openings (e.g., windows, chimney
dampers, etc.) to avoid intrusion.
For further information, see:
•
Canadian Forest Service (www.imfc.cfl.scf.rncan.gc.ca/insecte-insect/insecte-insecteng.html)
•
Pest Management Regulatory Agency (www.hc-sc.gc.ca/cps-spc/pubs/pest/_pnotes/tentlivreeamerique/index-eng.php
•
Martineau, R. 1984. Insects harmful to forest trees. Canadian Forest Service Publication
Fo 64-32/1984E. ISBN 0-919868-21-5.
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