Forest Insect and Disease Newsletter

Forest tent caterpillars

Forest tent caterpillarsThe forest tent caterpillar (FTC), Malacosoma disstria, is a native defoliator of a wide variety of hardwood trees and shrubs. It is often mistakenly called the armyworm. It can be found throughout the range of all hardwood forests in North America.

Defoliation starts in mid-May in central Minnesota and late May in northern areas of the state. Defoliation by FTC will normally be obvious by early June and finished by mid-June. These caterpillars feed primarily on aspen and birch trees in northern part of the state and on basswood and oaks in west-central and southern parts of the state. The only hardwood not regularly fed on is red maple. When populations are high, FTC will even eat tamarack foliage during outbreaks.

Aerial photo of Forest tent caterpillar defoliationNorth-wide outbreaks of FTC occur at intervals of ten to sixteen years and are three to seven years in duration. In any given location, defoliation is usually noticed for two to four consecutive years. In the last 120 years, outbreaks peaked in 1891, 1898, 1912, 1922, 1937, 1952, 1969, 1978, 1990 and 2001. See maps of peak years' defoliation below. In 2001, over 7.5 million acres of hardwoods were defoliated, the most acres ever recorded in Minnesota.

Forest tent caterpillar peak years from 1937–2001

click on maps to enlarge

Forest tent caterpillars outbreak map 1937


Forest tent caterpillars outbreak map 1952


Forest tent caterpillars outbreak map 1967


forest tent caterpillare outbreak map 1978


Forest tent caterpillars outbreak map 1990


Forest tent caterpillars outbreak map 2001


Acres defoliated by foreset tent caterpillars, 1985 to 2012

In the west-central counties, FTC populations may synchronize with northern outbreaks or they may have small, localized outbreaks that pop up and collapse quickly. These outbreaks occur in oaks, basswoods and aspens along lakeshores and woodlands and cover a relatively low number of acres.
Outbreaks can begin suddenly or develop slowly over a period of several years. Forest tent caterpillar populations collapse due to starvation, predation and parasitism. Populations of the friendly fly, a native parasite, build up as caterpillar populations peak and can themselves become a nuisance.


Roles in the forest

Forest tent caterpillar Forest tent caterpillars are native insects and are therefore an important part of our forest ecosystems. While temporary outbreaks may lead to severe defoliation and are a nuisance, their importance in Minnesota forests should not be ignored. Changes in forest dynamics and regeneration patterns have been linked to large outbreaks, and it is likely the forests we see in our state today are the result of periodic FTC outbreaks that have occurred over millennia. Weak, diseased, or stressed trees may be killed by FTC making way for other tree species better adapted for the site. Wildlife can also benefit from FTC; they are an important spring food source for squirrels, rodents, bears and many bird species.


Nuisance factor

Forest tent caterpillar feeding on leavesDuring severe outbreaks, FTC can number from one to four million individuals per acre. They create a nuisance to people living or vacationing in forested areas from late May to mid-June. Young caterpillars spin threads and fall from the trees onto picnic tables, patios, buildings and people. Large, mature caterpillars wander widely in search of food and often appear to migrate across roads and open areas. Resting caterpillars commonly form large clusters of hundreds of caterpillars on buildings, tree stems, cars, campers, and other stationary objects. Caterpillars often emit a greenish-black fluid when disturbed that stains paint and cloth. During the height of defoliation, insect frass (excrement) becomes a serious nuisance as it rains down from insects feeding in the tree crowns.

Mass flights of FTC moths are common during outbreaks. These flights can involve millions of moths over hundreds of miles creating a nuisance where the flight ends. Mass flights can trigger new outbreaks suddenly where the insect had not been a problem before.



Forest tent caterpillars resting in groupsIn the forest, defoliation from FTC usually causes little damage to aspen tree health and vigor. Most trees develop a second set of leaves in July, but these leaves are noticeably smaller and tend to cluster near the branch tips. Defoliation by FTC reduces tree vigor, but tree vitality recovers within a few years of the population collapse.

As defoliation intensifies during an outbreak, wood production decreases.For example, a single year of heavy defoliation may reduce radial stem growth by 50 to 60 percent, whereas, two years of heavy defoliation reduce growth by 90 percent. Fortunately, the growth rate recovers quickly after the end of the outbreak, returning to 80 percent of normal during the first growing season.

Aspen trees rarely die from defoliation by FTC. A Minnesota study of the 1948 to 1956 outbreak documented the death of less than 0.1 percent of the 4877 aspens studied. FTC defoliation does weaken other hardwood trees and makes them more susceptible to attack from a variety of other pests.

These pests are called secondary pests because they only attack weakened trees, do more damage than the FTC alone, and, eventually, kill the infested tree. Trees that are also stressed by prolonged drought, growing on poor sites, old age or those that have root damage are much more vulnerable to attack by secondary pests. Weakened aspen may die from subsequent attack by bronze aspen borer, poplar borer, Hypoxylon canker, or Armillaria root disease. Similarly, other hardwoods can be weakened by FTC defoliation. Commonly, oaks weakened by FTC defoliation and drought or root system damage suffer branch dieback or death from two-lined chestnut borer attack and Armillaria root disease, and paper birches may succumb to bronze birch borer attack.


Life Cycle

Forest tent caterpillars overwinter in egg masses on twigs of host trees. The eggs are extremely hardy and easily survive Minnesota winters. It has been shown that less than 10 percent of the eggs are killed when temperatures reach -40°F and only 50 percent succumb at -50°F.


Forest Tent caterpillars life cycle Newly emerged FTC from egg mass Forest tent caterpillar cocoon from Forest tent caterpillar Adult moth from Forest tent caterpillar Egg masses overwintering ontwigs of host trees

Eggs hatch in the early spring about the time of bud break, mid-April to mid-May. The caterpillars have five growth stages, each stage lasting seven to ten days, before eventually pupating into adult moths. Moths lay eggs in late June and into early July.

During the early growth stages, caterpillars remain in clusters on the leaves. Older caterpillars develop a deep blue velvet coloration with a sparse covering of long brown hairs. A line of white to cream colored spots runs down the back. These spots may look like footprints or a series of keyholes. As the caterpillars get larger, up to 2.5 inches long, they consume increasing amounts of leaves and can wander widely in search of more food. They often drop from the defoliated twigs to the ground. Large caterpillars are solitary feeders, but commonly rest in large clusters on tree stems. Mature caterpillars spin silk cocoons with white to yellow threads attached to vegetation, buildings and other stationary objects to begin pupation. This can occur from early to late June. The pupae form inside the cocoons. Seven to ten days later, adult moths emerge.They are buff colored and have broad brown stripe across the front wings. FTC moths are night fliers and are attracted to lights in large numbers. As a result of this, it is common to find high populations of FTC near populated areas and along highways where lights are often concentrated.

After mating, the female moth lays 30 to 50 eggs in ½-inch long clusters wrapped around a twig. Each female lays several clusters of 150 to 200 eggs by early July. A tough, bronze-colored casing covers the egg mass and protects the eggs from drying out until they hatch the next spring.

Phenology* of FTC life stages and host tree foliage


Host tree budbreak

April 15 to May 10

FTC hatch

April 15 to May 15

FTC peak defoliation

June 5 to June 25

FTC moths fly and lay eggs

June 25 to July 10

Host trees refoliate

July 1 to July 30

*= The timing of biological events varies from year to year because it depends on weather and location.For example, events will occur near the earlier date when spring is early, when May and June are warmer and drier than average or when the location is south of Mille Lacs Lake.


Natural Control

Forest tent caterpillars are native insects that have evolved in the forest ecosystem for thousands of years. Natural control mechanisms have also evolved which help to keep outbreaks from seriously damaging forested areas.

Late spring frosts that blacken all the emerging leaves effectively causes the tiny caterpillars to starve. However, hard spring frosts are not common. Defoliating frosts in the early spring force the young caterpillars to wait 7-10 days for trees to produce new leaves and replenish their food supply. Frost is the only factor that can cause a population collapse during the first years of an outbreak.

Starvation of mature caterpillars is the main mechanism for population collapse. Here's how it works. During the early stages of an outbreak, trees have enough foliage to support the increasing number of caterpillars. After a year or two of population buildup, however, the large number of caterpillars need more foliage than is available. They are unable complete all five of their caterpillar stages and starve to death before pupation occurs. Starvation typically kills up to 95 percent of the caterpillars during the last year of the outbreak.

Forest tent caterpillar cocoon

Cocoon on left is parasitized by larvae
of the friendly fly.

The remaining caterpillars are able to form cocoons and pupate. Then friendly flies, Sarcophaga aldrichi, kill the remaining FTC pupae inside their cocoons. Friendly flies are native to North America. Although the friendly fly often plays a significant role in the collapse of an outbreak, its population often increases to the point of becoming a nuisance to people, too. Several other parasitoids are active in controlling FTC populations includingnon-stinging wasps, Itoplectis conquisitor, which also parasitize the pupal stage.

Predatory beetles, ants, tree bugs, spiders, small animals and birds feed on caterpillars and pupae, but the extent of their control has not been quantified. Bacterial, fungal, protozoan and viral diseases become important late in the outbreak cycle. This is commonly due to the weakened state of the larvae as low-level starvation begins and is enhanced by the constant contact of the larvae with each other. Cool, wet spring weather also plays a role by slowing down the development rate of the insects while making disease transmission easier.


Since FTC has such a wide host range, silvicultural options are severely limited. Forestry practices such as thinning and pruning are not used in FTC management. Thinning of stands being defoliated by FTC is not recommended as this increases stress in the residual trees and usually leads to high levels of mortality in the thinned stand. Oaks and birches are particularly vulnerable. It is preferable to wait an additional growing season after the outbreak ends before doing any stand thinning.

Silvicultural actions are limited to planting non-host species such as red maple or conifers. In general, management options are limited to the acceptance of the growth loss and nuisance or to the improvement of tree vigor so that secondary pests do not attack the weakened trees. The use of insecticide treatments is usually limited to shade trees.

Insecticide Treatments

Forest tent caterpillars are native insects that have evolved with Minnesota's forests and are an integral part of the ecosystem. FTC rarely cause severe damage to trees and, as a result, the forest does not normally need the protection afforded by pesticides. Natural control systems cause the collapse of populations resulting in cyclical outbreaks.

Insecticide treatments only reduce FTC numbers and their defoliation during May and June the year of treatment.There is no effect on next year's FTC population because moths from outside the sprayed area enter and lay eggs on treated trees.

Private landowners may desire or justify spraying in order to protect the trees from defoliation or to reduce the nuisance caused by caterpillars during late May and early June. In making this decision, the landowners should consider their goals, neighbor's rights, environmental concerns and their ability to pay for the treatment. The DNR provides technical advice to landowners and landowner groups interested in undertaking FTC control actions, but does not oversee or manage them. That is the responsibility of the local landowners.

Insecticide treatments can be effective against defoliation by FTC when applied while the caterpillars are small. Treatment is increasingly ineffective as caterpillars mature in the late spring. It is difficult to achieve satisfactory control with insecticides on areas less than 10 acres or where less than 80 percent of the forested area will be treated. Several insecticides are registered for controlling the forest tent caterpillar including the biological insecticide, Bacillus thuringiensis var kurstaki (Btk). The DNR strongly recommends the use of Btk because of its environmental safety. Btk is a natural occurring bacteria effective against caterpillars that eat treated leaves. More information


Repeated insect outbreaks promote multi-cohort aspen mixedwood forests in northern Minnesota, USA by M. Reinikainen, A. D'Amato and S. Fraver.2012. Forest Ecology and Management 266 (2012) 148-159.

Influence of the forest tent caterpillar upon the aspen forests of Minnesota by D.P. Duncan, A.C. Hodson, and A.E. Schneider. 1956. Office of Iron Range Resources and Rehabilitation, St. Paul, MN. 45 pages.

Numerical analysis of a forest tent caterpillar outbreak in northern Minnesota by John Witter, W. Mattson and Herb Kulman. 1975. The Canadian Entomologist Vol. 107:837-854.

Mass transport of forest tent caterpillar moths, Malacosoma disstria, by a cold front by Clifford E. Brown. 1965. T The Canadian Entomologist Vol. 97:1073-1075

Cold-hardiness of the first instal larvae of the forest tent caterpillar, Malacosoma disstria, Lepidoptera: Lasiocampidae by A.G. Raske. 1975. The Canadian Entomologist Vol. 107: 75-80.

Heat units and outbreaks of the forest tent caterpillar by W.G.H. Ives. 1973. The Canadian Entomologist 105:529-543.