Forest Tent Caterpillars: Identification and Control

In the last two years, we have encountered many complaints from people about caterpillars stripping off the leaves from desirable trees and leaving behind an unsightly mess of fecal matter, which rendered their backyards useless. These troublesome insects are typically forest tent caterpillars (Malacosoma disstria Hübner) or eastern tent caterpillars (Malscosoma americanum (F.)). These bugs are sometimes wrongly called armyworms (armyworms are the caterpillars of several species of night-flying moths in the family Noctuidae) due to the caterpillar’s habit of crawling together over the ground while searching for food.

Tent caterpillars do not kill trees, but they do defoliate them. Unfortunately, their damage is not limited to one specific tree species; they are can attack a variety of host trees. The forest tent caterpillars (FTC) larvae feed on leaves of a variety of deciduous hardwood trees, such as oaks, gums, basswood, cherry, plum and trembling aspen. During outbreaks, large caterpillars can be seen feeding on citrus, pine, loquat, azalea, and rose (Dixon and Foltz 1991). Larvae of eastern tent caterpillars (ETC) prefer to feed on black cherry, chokecherry and apple. But given no other alternative, they will feed on hawthorn, pear, plum, and flowering fruits (Day, 2002). Since FTC and ETC are native to the United States, harsh environmental conditions (extreme cold and heat) along with natural enemies (parasites, predators and diseases) keep them at or below a tolerable level. Outbreaks of these defoliators are among major disturbance factors in our naturally balanced forests and backyards. Normally, the FTC is one of the most known species that undergo regional/natural outbreaks. The outbreaks occur at 6-16 year intervals, each outbreak lasting up to 6 years (USDA Forest Service 1996). During an outbreak, only a few trees die, but several acres of trees can lose their leaves by mid-summer. This may lead to a significant damage to affected trees (Drooz 1985).

Correct pest identification is extremely important to select the most proper treatment options. Here are some useful tips to identify these two pests.

ETC

  • The larvae of ETC are brown hairy caterpillars with white stripes on the back that are bordered by yellow and brown dark lines with a row of blue spots down each side.
  • The adult moths are dark tan in color with two pale/creamy-white stripes running diagonally across each of the front wings.
  • The most distinguished identification feature of ETC damage is the presence of tents/webs in branches. The newly hatched caterpillars make these webs for protection. They usually leave the tent for feeding during the day and crawl back to it at night (Day, 2002).

FTC

  • The larvae of FTC 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 and they are lightly covered with whitish hairs, and reach about two 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.
  • 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 (USDA Forest Service 1996).


Timing is an important factor in managing tent caterpillars. The best time to treat for these pests is when the caterpillars are small and the leaves begin to open. When a pesticide application is necessary, contact your licensed pest management professional (PMP) to apply a proven contact residual pesticide directly to your trees. Applications should target the foliage, caterpillars and the trunk of the infested tree to the point of runoff. 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 large number of caterpillars are still observed four to five days after treatment, communicate with your PMP and have them repeat the pesticide application as indicated on the product label.

Finally, long-term management of tent caterpillars requires the implementation of both chemical and non-chemical approaches, including, but not limited to, promoting all factors leading to healthy tree growth and environment, removing egg masses after the leaves have dropped in the fall, and collecting caterpillars and dropping them into soapy water, etc.

References
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.

Day, E. 2002. Eastern Tent Caterpillar. Virginia Polytechnic Institute and State University. Virginia Cooperative Extension. http://www.ext.vt.edu/pubs/entomology/444-274/444-274.html

Drooz, A.T. (ed.). 1985. Insects of eastern forests. U.S. Department of Agriculture, Forest Service, Washington, D.C. Miscellaneous Publication 1426. 608 p.

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. 

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