Forest Tent Caterpillar

Forest Tent Caterpillars

The Forest Tent Caterpillars (“FTC”), Malacosoma disstria Hübner, sometimes wrongly called armyworms due to the caterpillar’s habit of crawling together over the ground while searching for food. This pest is an important defoliator (they deprive the plants of their leaves) of a variety of deciduous hardwood trees (deciduous refers to trees that lose their foliage at the end of the growing season).  Infestations of FTC are not restricted to a specific tree species. They have a selection of host plants to choose from, such as oaks, gums, basswood, cherry, plum and trembling aspen. In addition, during outbreaks, large caterpillars can be seen feeding on citrus, pine, loquat, azalea, and rose.

Besides stripping off the leaves from desirable trees, these bugs leave behind unsightly mess of fecal maters, which may render one’s backyard useless.

The larvae have pale bluish lines along the sides of a brownish body, and a row of keyhole shaped white spots down the middle of the back. They are lightly covered with whitish hairs, and reach about 2 inches at maturity. They have a row of 10-12 footprint-shaped markings down the middle of their backs. Adult moths are buff-brown, with darker oblique bands on the wings. Egg masses of 100 to 350 eggs encircle the twigs and are covered with frothy, dark brown cement .  One of the identifying features of FTC is their damage and symptoms. The caterpillars are destructive defoliators causing a growth loss of the branches. The crowns of heavily infested trees may appear thinner and stripped of their leaves. Furthermore, when feeding is combined with other factors, such as drought or disease, a tree may die. Outbreaks typically occur every 6-16 years.

In spring, eggs usually hatch when enough foliage of the host tree is available for the first larval instar feeding activity and when the temperature is at 50°F or more. Caterpillars go through four molts during five to six weeks. In the end of June, fully-grown caterpillars seek protected places around the building, bushes and lawn to spin their yellowish cocoons and pupate. At this point, the presence of these hard to remove cocoons may frustrate the homeowners. Adult moths emerge about 10 to 45 days later (usually by the mid of July). When first emerged, females will secrete a sex pheromone to attract males for mating. Mated females usually deposit their eggs in masses around small twigs (eggs look like a wide gray ring). Each female, may lay an average of 150 eggs per egg mass. However, adult moths will not last for more than two weeks. This insect spends the winter in the egg stage. These eggs will hatch next spring when the leaves of host trees begin to open (this usually about mid May in Minnesota).

Adults have siphoning mouthparts and do not feed. They are solely dedicated for laying eggs. Caterpillars have chewing mouthparts and are responsible for all the damage. They feed exclusively on leaves.   Adults are attracted to light; as a result, they could become a nuisance factor at night, especially when they are found indoor hovering around light sources.  Unlike Eastern Tent Caterpillars, the caterpillars of FTC do not build tents but leave silken threads on trees where they travel or rest. They normally congregate to molt or rest. When food is scarce, caterpillars congregate on side branches, move toward the main trunk, and then migrate downward and disperse on the ground to find more food.  Weather conditions and outbreak have an influence on FTC. Extreme temperature limits, such as freezing during mid-winter and low temperatures in early spring, as well as high temperatures later in the spring cause high level of caterpillar mortality. Outbreak may also cause a decline in the populations of FTC as a result of starvation (no leaves are left for the bugs to eat).   Forest tent caterpillars have food preferences. The caterpillars prefer hardwoods, such as aspen, birch, basswood, ash and oaks. Additionally, they are known not to feed on balsam fir, spruces, pines or red maples.