August 15, 2009
By Francesco Pacelli,
Nursery Technical Analyst

Insects and diseases can threaten plant health. As soon as you notice any abnormality in your plant’s appearance, begin a careful examination of the problem. By identifying the specific symptoms of damage and understanding the causes, you may diagnose the problem and select appropriate treatment. Each month Horticulture Review is publishing an article on pests and diseases to help growers understand pest and disease identification.

Malacosoma americanum (Eastern tent caterpillar)

The eastern tent caterpillar (Malacosoma americanum) is a pest native to North America. Eastern tent caterpillars produce only one generation per year, forming communal nest in the branches of trees. It is sometimes confused with gypsy moth, or the fall webworm (Hyphantria cunea), and maybe erroneously referred to as a bagworm, which is the common name applied to unrelated caterpillar in the family Psychidae.


The eastern tent caterpillar overwinters as an egg, within masses of between 200 to 300 eggs. The masses are covered with shiny, black varnish-like material, encircling branches that are about pencil-size, or smaller in diameter.


In the early spring, the newly-hatched caterpillars construct a silk tent soon after emerging. They typically aggregate at the tent site for the whole of their larval life, expanding the tent each day to accommodate their increasing size. The caterpillars feed three times each day, just before dawn, at mid-afternoon, and in the evening after sunset. During the heat of the day or rainy weather, the caterpillars remain within the tent. The larvae are hairy, black with a white stripe down the back, brown and yellow lines along the sides, with a row of oval blue spots on the side.


The pupa is about one inch long and made of closely woven white or yellowish silk and is attached on tree trunks or other objects by a few coarser threads.


The adult moth emerges three weeks later from the pupa stage. The moth is reddish-brown with two pale stripes running diagonally across each forewing. After mating, the female begins to lay eggs on small branches, with the hatch the following spring.


Malacosoma americanum has one generation per year. The eastern tent caterpillar is among the earliest of caterpillars to appear in the spring. Because the early spring is often cold, the caterpillars rely on the heat of the sun to elevate body temperature to levels that allow them to digest their food. Studies show that with temperature, below 15oC, the caterpillars are unable to process the food.

Hosts and damage

Most commonly affected are fruit trees, such as apple, cherry, flowering crabapple, plum and chokecherry. The pest may also defoliate other hardwood trees, including ash, birch, hawthorn, maple, oak, poplar and willow. Eastern tent caterpillar is more of a nuisance than detrimental to tree vigour. Feeding does not seriously damage healthy, mature trees; the damage is primarily cosmetic, with trees appearing ragged or unsightly. Even if completely defoliated, most trees will leaf out again within two or three weeks, since caterpillar feeding generally ends during vigorous leafing activity. Small trees cannot tolerate as much defoliation without health consequences, and yield on fruit trees will be reduced and trees already weakened by disease may be killed. The nests can also be an eyesore in the landscape, particularly when exposed by excessive defoliation.

Natural control

Natural enemies play an important role in reducing eastern tent caterpillar numbers in most years. Caterpillars are frequently parasitized by various tiny braconid, ichneumonid, and chalcid wasps. Several predators and a few diseases also help to regulate the populations. This, in part, accounts for the fluctuating population levels from year to year.

Biological control

Young caterpillars can be killed by applying an insecticide containing Bacillus thuringiensis var. kurstaki.

Mechanical control

Removal and destruction of the egg masses from ornamentals and fruit trees during winter greatly reduces the problem next spring. In the early spring, small tents can be removed and destroyed by hand. Larger tents may be pruned out and destroyed or removed by winding the nest upon the end of a stick. Burning the tents out with a torch is not recommended, since this can easily damage the tree.

Chemical control

The most commonly used chemical pesticides contain spinosad, carbaryl, diflubenzuron. malathion, methoxychlor, phosmet, trichlorfon, and synthetic pyrethroids (permethrin). Diflubenzuron represents a new class of pesticide called insect growth regulators. It kills eastern tent caterpillar moth larvae by interfering with the normal molting process. Diflubenzuron has no effect on adult insects. Aquatic crustaceans and other immature insects that go through a series of molting stages are often sensitive to this pesticide.
Francesco Pacelli may be reached at

Malacosoma americanum
Malacosoma americanum larva