Agriculture and Agri-Food Canada Insect Identification
Sheet No. 33
Forest Tent Caterpillar
The forest tent caterpillar is a major pest of deciduous trees
in many parts of Canada and the United States. A gradual buildup
in the population of the pest in an area may lead to an outbreak
that lasts 2 years or longer; outbreaks may occur at intervals
of 6 to 16 years.
trembling aspen is the principal host of this pest, forest tent
caterpillars will also attack and defoliate most other
species of deciduous trees, including balsam, poplar, birch, maple,
ash, oak, elm and basswood. A damaged tree will grow new foliage
later in the summer, but the growth of a severely defoliated tree
will be retarded for 2 years.
is a yellow or yellow-brown moth with a wingspread of 2.5 to
4cm. The forewings are marked with two brown stripes. The moths
mate within 24 hours after emerging from the pupal stage and
the females, whose adult life span is about 5 or 6 days, begin
laying their eggs several hours later. Egg-laying takes place
in July and early August and the eggs are deposited in masses
on twigs of the trees. Each mass of 100 to 300 eggs appears as
a narrow, dark brown band around a twig and is protected by a
covering of spumaline, a frothy substance secreted by the females.
Hatching occurs the next year (May to July, depending on weather
conditions and locality) and a new generation of the pest is
born. The young caterpillars, or larvae, migrate and as they
grow they attack and defoliate trees. When they reach full growth
after 5 to 8 weeks, they spin silken cocoons within which they
remain in the pupal stage for about 10 days before emerging as
factors — weather, disease, birds and parasites — contribute
to the natural control of the forest tent caterpillar. Common
parasites of the species are the flesh fly (Sarcophaga aldrichi)
and the ichneumonid wasp (Itoplectis conquisitor). If an outbreak
occurs, consult your provincial Agriculture Department for recommendations.
Agriculture and Agri-Food Canada
Name: Gilkeson, Linda A. Title: Integrated Pest Management Coordinator
Company: Ministry of Water, Land and Air Protection
Reference Text: CATERPILLARS
feed together in groups on deciduous trees. Tent caterpillars
are hairy and have various
color markings. They cannot be mistaken for gypsy moth caterpillars,
however, which are distinctly marked with 6 pairs of red dots
and 5 pairs of blue dots on their backs. Northern tent caterpillars
spin large, conspicuous silk nests in crotches of tree branches
in spring and early summer; forest tent caterpillars do not spin
these silken mats. Tent caterpillar populations ri se and fall
in 5-10 year cycles. When their numbers become too high, the
are eventually killed off by diseases and parasitic insects.
For home gardeners, the best control for tent caterpillars is
the nests out of trees (wear gloves) or prune them out and drop
the caterpillars in a bucket of soapy water. Start inspecting
for tents or colonies of small caterpillars in early spring before
they cause much damage and while they are easier to remove. On
trees with severe or inaccessible infestations, spray with BTK
or products containing pyrethrins and insecticidal soap. Before
spraying, make sure caterpillars are still present the empty
remains in the trees long after the caterpillars are gone. Old
webs can be removed with a strong stream of water from a hose.
In the winter, scrape or prune away eggs masses, which look like
a silvery-brown, hardened band of foam (about 1 cm wide), partially
encircling small tree branches. Dormant oil sprays also kill
tent caterpillar eggs.
THE FOLLOWING (UNTIL OTHERWISE NOTED) IS FROM THE UTAH STATE UNIVERSITY
ON-LINE INSECT FACT-SHEETS:
Subject: Tent Caterpillars
Tent Caterpillars Fact Sheet No. 28, Revised December 1988 Dr.
Jay B Karren, Extension Entomologist
Periodically, tent caterpillars cause considerable defoliation
of native trees and shrubs in the canyons and mountains of Utah.
These outbreaks often extend into residential areas, causing alarming
defoliation of ornamentals in the homeowner's yard. Healthy trees
can usually withstand heavy infestations for 2-3 years without
suffering permanent injury. Trees under other forms of stress or
young, recently planted trees could be severely damaged from a
single year's infestation. Trees and shrubs with extensive defoliation
will have reduced growth and vigor. In addition the caterpillars
produce unsightly webs, or
tents which detract from the home landscape. The key to eliminating
tent caterpillar problems is early detection and use of appropriate
cultural and chemical control measures.
Biology, Description, and Habits:
In Utah there are four species and a subspecies that attack various
trees and shrubs including maple, poplar, cottonwood, plum, cherry,
choke cherry, aspen, alder, shadscale, willow, birch, apple, and
others. Conifers are not usually fed upon. After the caterpillars
have stripped the trees of their leaves, they feed on wild and
ornamental shrubs and even the leaves of cultivated fruits and
vegetables. The different tent caterpillar species are not easy
to distinguish, but their life cycles and the damage they produce
Adults are stout-bodied moths about 1/2 inch long with a wingspan
of 1 to 1 1/3 inches. Wings are light yellow to reddish-brown in
color with a broad, dark strip across the middle third. They are
not generally observed and do not feed on leaves.
The egg masses
are laid on trees branches in the fall and usually contain from
150 to 200 eggs. The egg are covered with a dark,
reddish brown, frothy material that soon hardens. The eggs may
be deposited in a band that completely encircles the twig or
in a flattened mass on limbs and boles. Within 2-3 weeks, the
caterpillars are fully formed within the eggs, but they do not
hatch until spring. In northern Utah, they chew their way out of
the eggs during May, about the time new leaves appear. Noticeable
defoliation is reached the latter part of June when most of the
larvae reach maturity. At arial large groves of trees on the hillsides
are completely stripped of their leaves, leaving the mountainside
with a gray cast.
The young larvae are gregarious (gather in groups). Some species
construct web tents while others merely cluster together on limbs
when they aren't feeding. Mature larvae are about 2 inches long
with blue, black, and orange markings. Full grown larvae disperse
and spin one-inch long, dusty, silken cocoons in folded leaves,
under loose bark, or in litter on the soil surface. New moths emerge
in about two weeks, mate, and deposit eggs for the following year.
Egg masses appear as swellings around bare twigs.
Cultural control consists of hand removal of the egg masses during
winter pruning or removal of the young larvae and tents before
significant injury occurs. This is not feasible on larger trees
or when a large number of trees are involved. In many cases, however,
it provides a simple and safe solution. It should be done whenever
If chemical controls are required, consult your local pest management
thuringiensis (BT) causes a bacterial disease of caterpillars.
Commercially available pesticides contain the actual bacterial
spores. For Bt to be effective, the caterpillar must consume foliar
material that the spores have been sprayed or dusted on. Feeding
stops shortly after ingestion, but it may take up to five days
for the caterpillars to die. This material has the advantage of
being very selective; it only affects certain types of caterpillars
and will not harm other animals or even other types of insects,
when used as directed.
Cultural or chemical control should be applied as soon as the
tents or caterpillars are noticed. The caterpillars are easier
to kill Usually, the caterpillars foray from the tent to feed on
the surrounding foliage. The tent is expanded as the caterpillars
grow and as the adjacent foliage is consumed. If all of the foliage
is consumed, the caterpillars may migrate to nearby trees or shrubs
and continue feeding. The insecticide application should be directed
to the tent, and the foliage immediately around it.
REFERENCE: INSECT PEST MANAGEMENT GUIDELINES FOR CALIFORNIA LANDSCAPE
ORNAMENTALS C.S. KOEHLER. PUBLICATION: OPC87
Forest Tent Caterpillars (Malacosoma
L. A. Gilkeson
The forest tent caterpillar is mainly found in the interior regions
of the province. It prefers trembling aspen, but in outbreak years
will feed on a variety of broad leafed trees and shrubs. Despite
forest tent caterpillars do not make webs or "tents".
A species that does make a tent, the northern tent caterpillar
(M. californicum pluviale), occurs mainly in the southern half
of the province and occasionally in northern deciduous forests.
Fall webworms (Hyphantria cunea) are often mistaken for tent caterpillars
because they also make large silken webs in shrubs and trees. They
appear in late summer, however, 6-8 weeks after tent caterpillars.
Identification and Life Cycle:
Forest tent caterpillars are the
larvae of a large, reddish brown moth. They have one generation
a year. The adult moth lays its
eggs in late summer on twigs and small branches of deciduous
trees. The egg masses look like 1-2.5 cm wide masses of hard,
silvery brown foam. They remain on the trees all winter and
hatch in early spring, when the buds start to swell.
The caterpillars are dark-coloured, with rows of fine brown hairs.
They have a row of key-hole shaped white dots down their backs.
There are two fine orange lines on either side of the white dots
and a wider blue stripe below that along each side. The caterpillars
feed on buds and leaves from early spring into June, congregating
in large groups on trees. When they have eaten all the leaves on
the host tree, they move onto other trees and shrubs, somearial
travelling on the ground in large numbers.
When the larvae reach full size in June, they spin cocoons on
tree leaves or other protected sites. They pupate for 8-12 days
and then the adult moths emerge to lay the next generation of egg
Tent caterpillar populations naturally rise and fall over a cycle
of several years. After a couple of years of outbreak, the populations
collapse as the caterpillars are wiped out by naturally occurring
diseases and predators.
Is Control Necessary?
In peak outbreak years, stands of aspen
and other trees can be completely defoliated, but this usually
does not cause long-term
harm to the trees. Once the caterpillars leave, the trees produce
new leaves to fill in the canopy. The bare trees are unsightly,
however, and result in complaints from the public to municipal
and parks departments. Unfortunately, the caterpillars consume
leaves so quickly, and over such a short time, that by the
time damage is obvious it is often too late to do much about the
It is important to realize that, no matter what steps are taken
to control tent caterpillars on individual trees, that the overall
populations will increase over several years and then drop to low
levels naturally as diseases and predators catch up with the population.
For high visibility plantings, in outbreak years, it may be worthwhile
to control tent caterpillars. In that case, a monitoring program
should be put in place so that treatments can be done at the right
time to be effective.
Look for egg masses on dormant branches any time after
the leaves drop. In expected outbreak years, monitoring for hatching
masses should start by mid-April. Before leaves expand, the
hatching caterpillars are particularly easy to see.
Prune out egg masses and branches with colonies of caterpillars.
Burn egg masses and destroy caterpillars by crushing them or
dropping them in a bucket of soapy water.
When caterpillars are actively feeding in the trees, the microbial
pesticide Bacillus thuringiensis kurstaki (BTK) is useful to control
the caterpillars without harming other organisms. It must be applied
carefully to ensure good coverage of the foliage. It should not
be used if rain is expected within the next 24 hours as it can
be washed off before the caterpillars have a chance to eat it.
Safer's BTK Biological Insecticide is a product sold for home and
garden use. BTK products for Commercial use include Dipel (a wettable
powder formulation) as well as Thuricide and Foray (both liquid
Spraying other pesticides is usually not recommended because they
kill the natural enemies that eventually bring the caterpillar
populations under control.
Wood, C. S. 1992. Forest Pest Leaflet: Northern Tent
Canada. FRDA Publication No. 29-6/17-1992E