Two species of tent caterpillars are important tree pests in New York State. About every decade they become numerous enough in local outbreaks to cause concern. At times the epidemics reach statewide proportions. The buildup of natural control factors eventually reduces tent caterpillar populations to a low ebb, but in the meantime serious defoliation of many kinds of deciduous trees can occur.
The eastern tent caterpillar normally favors black cherry, choke cherry, scrub apple trees and many species of ornamentals in the family Rosaceae. When numerous, the insect defoliates many other fruit trees as well as deciduous forest and shade trees including birch, oak, willow, poplar, and beech.
The forest tent caterpillar prefers the foliage of sugar maple, poplar, ash, oak, birch, red gum, and black gum. The early spring feeding often results in destruction of the buds before they have a chance to open. It does not attack red maple. In sugar bushes, severe defoliations may reduce the vigor of trees and lower sap production and quality. Recent studies in New York showed that such damage can be of a permanent nature because branches are killed, thus reducing the sugar producing capacity of the bush. This is particularly true if defoliation is followed by excessively high temperatures, drought or disease. This species will also attack fruit trees.
Eastern Tent Caterpillar
Malacosoma americanum (F.)
These caterpillars produce the conspicuous silken tents commonly seen in the spring on branches of favored host trees. The tents consist of numerous layers of dense silk webbing which contain much excrement and numerous molted skins.
The female moths are dull reddish-brown with a wing expanse of 1 1/2 to 2 inches. Males are smaller. The front wings of each are crossed by two whitish, oblique, parallel lines.
Mature larvae, or caterpillars, are 2 to 2 1/2 inches long. The head and body are generally deep black. There is a white stripe along the back of the body and a row of oval, pale blue spots on each side. There are many short, irregular brownish markings on the side of each body segment. Long, fine, brown hairs sparsely clothe the body.
During June and July, the female moths lay egg masses in bands that encircle the twigs of food plants. Each mass contains 150 to 350 eggs which are cemented together and covered with a dark, glue-like substance. Larvae emerge the following spring at the time leaves are unfolding. They construct a tent of silk and enlarge it as they grow. The gregarious caterpillars leave the tent only to feed. In six or seven weeks, they are fully grown and migrate to find a suitable place to spin their cocoons. Each larva spins a double layered whitish cocoon about 1 inch long and composed of a coarse whitish silk filled with yellow powder. An inner tough, parchment-like lining protects the pupa (resting stage). After 10 days to 2 weeks the moths emerge. There is only one generation each year.
Forest Tent Caterpillar
Malacosoma disstria (Hbn.)
The name of this insect is confusing because this species does not make a tent. The caterpillars spin a silken mat on the trunk or branch where they congregate during molting periods or when at rest. Like M. americanum, the caterpillars stay together until nearly full grown. The moths are typically light grayish brown or tan with two brown lines on the front wings instead of the white ones characteristic of the adult eastern tent caterpillar. The wing span of the female is 1 to 1 1/2 inches.
When full grown, the hairy caterpillar is about 2 inches long. The head and body are generally pale grayish-blue with fine orange and black lines. There is a row of keyhole-shaped cream-yellow or white spots along the back.
The female moths lay 100 to 350 eggs in a mass during late June or July. The egg mass completely encircles the twig and is similar in shape to that of the eastern tent caterpillar, but smaller. The egg mass usually appears relatively square at each end, in contrast to that of the eastern tent caterpillar which has tapered or rounded ends. The caterpillars hatch the following spring and feed for 6 weeks or more before spinning a cocoon in a folded leaf, usually on or near the host. This cocoon is similar to that of the eastern tent caterpillar, except that the inner portion is light, translucent, and contains very little yellow powder. There is only one generation of these insects each year.
Natural Control In New York State, a parasitic fly (Sarcophaga aldrichi (Park) has been abundant in areas following severe forest tent caterpillar outbreaks. Populations of this fly and numerous other parasites and predators build up rapidly and normally help to bring tent caterpillar populations under control. A bacterial disease, known as “wilt”, is at times helpful in destroying numbers of caterpillars. Cold, wet, adverse weather conditions in early spring also play a part. Late spring frosts kill many larvae.
On Shade Trees Destruction of worthless apple and wild cherry trees will help reduce infestations of eastern tent caterpillar on nearby shade trees and ornamentals. Removal and burning of tents in early spring during daylight hours when the larvae are inhabiting these shelters is also good control practice. This approach should be used cautiously on small ornamentals. Removal of overwintering egg masses during late summer and fall may be an effective way to reduce populations on small trees. When necessary, the use of chemical or microbial sprays after egg hatch in April or early May gives good control.
Insecticidal Control Certified applicators operating in New York State under certificate issued by the Bureau of Pesticides, State of New York Department of Environmental Conservation, must use pesticides in accordance with state and federal labeling. Refer to Cornell Recommendations for Pest Control for Commercial Production and Maintenance of Trees and Shrubs.
Homeowners may use nonrestricted pesticides as indicated on the insert sheet or as recommended by other state agencies referred to in this leaflet. Follow manufacturer’s directions and other precautions to be observed in the use of these materials.
Forest Trees If large areas are to be sprayed, contact the Bureau of Forest Resource Management, New York State Department of Environmental Conservation.
CAUTION: Insecticides are poisonous. Follow manufacturer’s directions for mixing and additional precautions in the use of various materials available in your locality. Keep insecticides away from children and pets.
Prepared by Dr. Howard C. Miller and Dr. Douglas C. Allen of the Department of Environmental and Forest Biology, State University of New York College of Environmental Science and Forestry, Syracuse, New York 13210.
Published in cooperation with the Bureau of Forest Resource Management, New York State Department of Environmental Conservation, Albany, New York 12233.