Forest Tent Caterpillars
Overview
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) (Batzer and Morris 1978). 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 (Dixon and Foltz 1991).
Besides stripping off the leaves from desirable trees, these bugs leave behind unsightly mess of fecal maters, which may render one’s backyard useless.
Identification
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 (USDA Forest Service 1996). 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 (Drooz 1985). Outbreaks typically occur every 6-16 years (Myers 1993).
Biology
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 (Furniss and Carolin 1977). These eggs will hatch next spring when the leaves of host trees begin to open (this usually about mid May in Minnesota).
Habits
- 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) (Drooz 1985).
- 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.
Biological Management
There are varied identified natural enemies of FTC. These include, but are not limited to, 14 species of Hymenoptera egg parasites, 52 Diptera and 61 Hymenoptera species parasitic of larval and pupal stages, and 18 Hemiptera, nine Coleoptera and one Dermaptera that are predators of various life stages. In addition, frogs, mice, skunks and 60 species of birds all are known to feed on the caterpillars of this pest (Witter and Kuhlman 1972). Besides predators, FTC is also vulnerable to a variety of diseases-causing pathogens occurring alone or in combination in nature, especially the epidemic proportions of a nuclear polyhedrus virus (NPV) (Myers 1993).
Mechanical Management
Along with natural enemies, the egg masses of FTC are easy to spot and remove after the leaves have dropped in the fall. Tree trunks can also be banded to restrict the movement of caterpillars prior to or following emergence. Another management approach is to avoid transporting the caterpillars as much as possible (i.e., via infested firewood).
Chemical Management
The caterpillars can be managed by a number of insecticides approved for such use. If a pesticide application is necessary, contact your Pest Management Professional ("PMP"). The PMP will apply an approved contact residual pesticide directly to the foliage, caterpillars and the trunk of the infested tree to the point of runoff. The best time to treat for FTC is when the caterpillars are small and the leaves begin to open. The reappearance of caterpillars within 24 hours on several trunks after treatment indicates that the PMP has missed a patch or more during the pesticide application. If a big number of caterpillars are still observed four to five days after treatment, communicate with your PMP to repeat the pesticide application as indicated on the product label.
Management Summary
Long-term management of FTC requires the implementation of both chemical and non-chemical approaches, including promoting all factors leading to healthy tree growth and environment.
References
Batzer, H.O., and R.C. Morris. 1978. Forest Tent Caterpillar. U.S. Department of Agriculture, Forest Service, Forest Insect and Disease Leaflet 9. http://willow.ncfes.umn.edu/fidl-ftc/tentcat.htm (31 December 2000).
Dixon, W.N., and J.L. Foltz. 1991. Caterpillars that are not the gypsy moth caterpillar. Some forest Lepidoptera in Florida (Lepidoptera: Arctiidae, Lasiocampidae, Lymantriidae). Florida Department of Agriculture and Consumer Services, Division of
Plant Industry, Gainesville, FL. Entomology Circular No. 270. 2 p.
Drooz, A.T. 1985. Insects of eastern forests. U.S. Department of Agriculture, Forest Service, Washington, D.C. Miscellaneous Publication 1426. 608 p.
Frank, J.H., and J.L. Foltz. 1997. Classical biological control of pest insects of trees in the southern United States: a review and recommendations. U.S. Department of
Agriculture, Forest Service, Forest Health Technology Enterprise Team, Technology Transfer, Biological Control. Morgantown, WV. FHTET-96-20. 78 p.
Furniss, R.L., and V.M. Carolin. 1977. Western forest insects. U.S. Department of Agriculture, Forest Service, Washington, D.C. Miscellaneous Publication 1339. 654 p.
Myers, J.H. 1993. Population outbreaks in forest Lepidoptera. American Scientist 81: 240- 251.
U.S. Department of Agriculture, Forest Service. 1996. Pest Alert: Forest Tent Caterpillar.
U.S. Department of Agriculture, Forest Service, Northeastern Area and Region 8. NA-PR-02- 96. 1 p.
Witter, J.A., and H.M. Kuhlman. 1972. A review of the parasites and predators of tent caterpillars (Malacosoma spp.) in North America. Minnesota Agricultural Experiment Station. Technical Bulletin 289. 48 p.
