Dealing with forest tent caterpillars (FTC) can be very frustrating. Especially during the first three weeks of June, they can be a downright nuisance. They don't cause a health risk to humans, but the presence of hundreds or thousands of caterpillars can be a real headache. Fortunately, the nuisance associated with FTC outbreaks can be reduced by individual homeowners. Another consideration is the effect of FTC defoliation on shade trees, ornamental plantings, and gardens.
Forest tent caterpillars are native insects that have evolved in the forest ecosystem for thousands of years. Natural control mechanisms have also evolved that help to keep outbreaks from seriously damaging forested areas.
Simple physical procedures can help control FTC.
Remove egg masses—Remove and destroy overwintering egg masses from branches of small trees before eggs start to hatch in the spring.
Brush off or spray with water—Caterpillars and cocoons can be brushed off houses, picnic tables, or decks with a stiff broom or brush or knocked down with a forceful spray of water. Be careful not to crush too many caterpillars; they can smear and leave marks on some paints.
Turn off exterior lights—Moths of forest tent caterpillars are attracted to lights. When moths are abundant, turn off exterior lights. This may reduce the number of egg masses laid on nearby trees.
Insecticide use—FTC rarely cause severe damage to trees, and the forest does not normally need the protection of pesticides. However, for some landowners such as resort owners, aerial applications of an insecticide to control FTCs may be an option to consider.
Insecticide treatments only reduce FTC numbers and 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. The use of insecticide treatments is usually limited to shade trees. Several insecticides are registered for controlling FTCs, 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 naturally occurring bacterium effective against caterpillars that eat treated leaves. Btk has virtually no effect on birds, people, aquatic arthropods, other animals, and most insects.
Private landowners may want to spray to protect trees from defoliation, preserve trees' appearance, or provide nuisance control. In deciding whether to spray, landowners should consider their goals, neighbors' rights, environmental concerns, and their ability to pay for the treatment. The DNR provides limited advice to landowner groups interested in undertaking FTC control programs.
Predatory beetles, ants, true 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 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 of the insects while making disease transmission easier.
Late spring frosts that blacken all the emerging leaves effectively cause the tiny caterpillars to starve. Defoliating frosts in early spring force the young caterpillars to wait seven to 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. 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 needs 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 an outbreak.
The remaining caterpillars are able to form cocoons and pupate. Then "friendly flies"(Sarcophaga aldrichi) kill the remaining FTC pupae inside their cocoons. Although the friendly fly plays a significant role in the collapse of an outbreak, its population often increases to the point of also becoming a nuisance to people. Several other insects are active in controlling FTC populations, such as the nonstinging ichneumonid wasp, Itoplectis conquisitor, which parasitizes the pupal stage.