Forest tent caterpillars

Forest tent caterpillars

The forest tent caterpillar (FTC), Malacosome disstria, is a native defoliator of a wide variety of hardwood trees and shrubs. The FTC's range in North America extends from coast to coast and from the tree line in Canada to the southern states. These caterpillars feed primarily on aspen and birch trees in northern Minnesota and on basswood and oaks in central and southern Minnesota. The only hardwood not regularly fed on is red maple. When populations are high, FTC will even eat tamarack foliage during outbreaks. It is often mistakenly called the armyworm.

North-wide outbreaks of FTC occur at intervals of five to ten years and are five to eight years in duration. In the last 120 years, outbreaks peaked in 1891, 1898, 1912, 1922, 1937, 1952, 1967, 1978, 1990 and 2001. See maps of peak year's defoliation below. In any given location, defoliation is usually noticed for two to four consecutive years.

Forest tent caterpillars peaks year's

Outbreaks can begin suddenly or develop slowly over a period of years. Outbreaks normally collapse quickly due to natural causes with defoliation reduced by as much as 80% in a single year. Defoliation starts in late May in central Minnesota and early June in northern areas. Defoliation will normally be obvious by mid-June and finished by late June.

In 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 on lakeshores and have relatively low acreages.

2001 northern outbreak map

 

FTC Nuisance

Forest tent caterpillars resting in large groups on a aspen.

During outbreaks, forest tent caterpillars can number from one to four million caterpillars per acre. They create an extreme nuisance to people living or vacationing in forested areas. Young caterpillars spin threads and fall from the trees onto picnic tables, patios, and people causing serious annoyance. 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 thousands of caterpillars on buildings, tree stems, cars, campers, and other stationary objects. Caterpillars often emit a greenish-black fluid when disturbed which 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 crown.

Mass flights of FTC moths are common during outbreaks. These flights can move millions of moths 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. These flights are often associated with the passage of a cold front.

 

Forest tent caterpillars feeding on aspen leaves.

Damage
In the forest, defoliation from FTC usually causes little damage to aspen tree health. Most trees develop a second set of leaves after attack, but these leaves are noticeably smaller and tend to cluster near the branch tips. The second year after the collapse of an outbreak, 80% of the trees have normal sized leaves. FTC defoliation reduces tree vigor, but vigor recovers within a few years of the population collapse.

FTC defoliation reduces aspen stem growth. As defoliation intensity and duration increase, stem growth decreases. For example, a single light defoliation does not reduce growth. However, a single heavy defoliation may reduce stem growth by 50 percent to 60 percent. Two years of heavy defoliation reduces growth 90 percent. Growth rate recovers quickly, returning to 80 percent of normal during the first year after the end of the outbreak.

Aspen trees usually do not die from FTC defoliation alone. A Minnesota study of the 1948 to 1956 outbreak documented the death of 396 aspen trees out of 4877 aspens. Identifiable problems other than FTC accounted for the death of all but four trees. So, in this instance, about one percent of the aspens died due to FTC defoliation alone.

FTC defoliation does weaken trees and makes them more susceptible to attack from a variety of other pests. These pests, called secondary pests, do more damage than the FTC and may kill the infested tree. Trees defoliated by FTC and suffering stress from other factors such as prolonged drought or defoliation due to late spring frosts, growing on poor sites or old age, they are much more vulnerable to attack by secondary pests. Weakened aspen may die from subsequent attack by Saperda borer, Hypoxylon canker, or Armillaria root rot. Similary, other hardwoods can be weakened by FTC defoliation. Commonly, oaks weakened by FTC defoliation and drought or root system damage suffer branch dieback or whole-tree mortality from two-lined chestnut borer attack or Armillaria root disease.

 

Life Cycle

The Forest tent caterpillar overwinters in an egg mass on twigs of host trees. The eggs are extremely hardy and easily survive Minnesota winters. It has been shown that less than 10% of the eggs are killed at -40 F and 50% survive at -50 F. Eggs hatch in the early spring about the time of bud break. The caterpillars have five growth stages, each stage lasting seven to ten days. During the early stages, caterpillars remain in clusters on the leaves. Caterpillars are blue-black with white spots down the back. Long brown hairs sparsely cover the body.

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 often look like footprints or a series of keyholes. As the caterpillars get larger, 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. Mature caterpillars spin silk cocoons with white to yellow threads on vegetation, buildings and other stationary objects to begin pupation. This can occur from early to late June. The pupae form inside the cocoons. Adults emerge seven to ten days later. The adult moth is buff colored and has a broad brown band 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.

After mating, the female moth lays 30 to 50 eggs in each of several clusters that about ½ inch long and are wrapped around a twig. Each female lays 150 to 200 eggs. A tough, bronze colored casing covers the egg mass and protects the eggs from drying out.

Forest tent caterpillars Life Cycle
Forest tent caterpillars egg mass on twig	Forest tent caterpillars	Forest tent caterpillars pupa inside its cocoon	Forest tent caterpillars as a adult moth

 

Natural Control

The FTC is a native insect and has 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. A natural control mechanism that causes population collapse is starvation induced by the caterpillars' feeding. During the early stages of an outbreak, the trees have enough foliage to support the increasing number of caterpillars. After a year or two of complete defoliation, the large number of caterpillars need more foliage than is available. Starvation typically kills 75-95 percent of the caterpillars.

Adult friendly flies.

Forest tent caterpillars pupa inside its cocoon parasitized by larvae of the friendly fly.

Late spring frosts that defoliate the trees have much the same effect. However, frosts hard enough to cause complete defoliation to all the trees species at the same time are not common. Defoliating frosts force the young caterpillars to wait 7-10 days for refoliation. Frost is the only factor that can cause a collapse during the first years of an outbreak.

Another significant natural control occurs near the end of the outbreak cycle. A native fly species, Sarcophaga aldrichi, kills many FTC pupae in their cocoons. Although the fly often plays a significant role in the collapse of an outbreak, its population often increases to the point that it also becomes a nuisance to people.

Predatory beetles, ants, tree bugs, spiders, and small animals and birds feed on caterpillars and pupae, but the extent of their control is not known. Bacteria, fungus, protozoan and virus 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. A stingless wasp, Itoplectis conquisitor, is another important parasitoid of FTC pupae.

 

Management

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. 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 caterpillar rarely causes severe damage to trees and, as a result, the forest does not normally need the protection of pesticides. Natural control systems cause the collapse of populations resulting in cyclical outbreaks.

Private landowners may desire or justify spraying in order to protect the leaves and preserve aesthetics. In making this decision the landowners should consider their goals, environmental concerns and their ability to pay. The DNR provides technical advice to landowners and landowner groups wanting to undertake control programs.

Insecticide treatments can be effective against the FTC defoliation. When applied while the caterpillars are small, insecticides can protect the foliage and the aesthetics of an area. It is difficult to achieve satisfactory control with insecticides on areas smaller than 10 acres or where less than 80% of the forested area will be treated. Several insecticides are registered for controlling the forest tent caterpillar including h the biological insecticide, Bacillus thuringiensis var kurstaki (Btk). The DNR strongly recommends the use of Btk because of its environmental safety. Btk is a naturally occurring bacteria effective against caterpillars that eat treated leaves and products contained BTK are formulated as insecticides by several companies. Btk has no effect on birds, people, other animals and most insects.

If you have any questions on or about the management of the forest tent caterpillar, please contact your local DNR forester.

 

References

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.