Test Your Knowledge - May

Eastern tent caterpillar nest and larvae
Eastern tent caterpillar nest and larvae
Tim Tigner, Virginia Department of Forestry, Bugwood.org

Yes, this webbing is the result of eastern tent caterpillars (visible in the webbing).

The eastern tent caterpillar (Malacosoma americanum) is a common pest of understory trees in eastern North America. This foliage feeder is active in the early spring through early summer and is seen primarily on wild black cherry trees.  However, many other tree species may occasionally be infested, such as ornamental cherry, apple, crabapple, plum, peach, pear, oak, maple and sweet gum.

Soon after hatching, these caterpillars congregate and begin construction of silken webbed tents in the crotches or forks of tree trunks and limbs.
Caterpillars in webbing.
Eastern tent caterpillar nest and larvae.
Robert L. Anderson, USDA Forest Service, Bugwood.org

These grayish-white nests or tents enlarge as the caterpillars grow. The caterpillars are hairy and black with white and yellow stripes and blue spots. At maturity they will reach 2 to 2½ inches in length.
An Eastern tent caterpillar
Eastern Tent Caterpillar larva.
David Cappaert, Michigan State University, Bugwood.org

The type and location of webbing distinguishes the eastern tent caterpillar from other foliage feeding caterpillars, especially the fall webworm, which produces webbing around foliage at the ends of limbs later in the summer. The occurrences of the eastern tent caterpillar and the fall webworm will overlap only briefly during the early summer, as the fall webworm tents are seen from early summer until fall.

Similar confusion occurs between the bagworm and the eastern tent caterpillar, probably because of the bagworm’s name. However, the bagworm makes no obvious silken webbing on limbs or foliage. The bagworm makes a small, 2-inch bag of needles or foliage that is only lined with silken webbing, and an individual caterpillar lives within this bag. Bagworms typically feed on conifer foliage, such as from the Leyland cypress or junipers.

Eastern tent caterpillars typically feed during the day time and return to the nest at evening. They may remain in the nest during bad weather. These caterpillars are aggressive feeders and may strip a tree or major branches of all foliage. If the trees are otherwise healthy, the trees will normally produce new leaves. However, overall growth may be retarded due to the defoliation. This damage may significantly stress trees after three or four years of leaf loss.

Larval (caterpillar) development only lasts for about 6 weeks, after which the full-grown caterpillars will climb down in search of a place to spin silken cocoons and pupate. During the caterpillars’ migration for a pupation site, they may be seen by the thousands.

Generally the moths emerge 3 to 4 weeks later in early summer, mate and begin laying eggs on twigs of trees that are suitable for the larvae. The shiny black egg masses may contain 150 to 400 eggs and encircle twigs that are pencil-sized or smaller.
Egg mass of the Eastern tent caterpillar
Eastern tent caterpillar egg mass.
Whitney Cranshaw, Colorado State University, Bugwood.org

Control

Control of the eastern tent caterpillar may not be necessary unless the trees are desirable specimens and the damage is becoming significant. If the webbing is low enough to the ground it can be pulled down with a stick. Do not use fire to destroy the webbing on the trees as this will likely damage the tree. Egg masses found on twigs can be pruned from the trees or simply scraped off and destroyed. During the winter these egg masses are more visible when the foliage has dropped.

The safest product to spray for caterpillar control is Bacillus thuringiensis (Bt). Bt  works well to kill caterpillars if applied while the caterpillars are in the early stages of development and therefore small. Sprays are best applied in the early morning or late afternoon. Direct the sprays onto the foliage around the nests, as these leaves will likely be their next meal. Bt is the safest product to use and only kills caterpillars. It will not affect birds or beneficial insects that may be in the trees or feed on the caterpillars. Bt is found under numerous brands, such as Green Light Worm Killer Concentrate, Safer Caterpillar Killer Concentrate, American Brand Thuricide Concentrate and Bonide Thuricide Concentrate.

Joey Williamson
HGIC Extension Agent

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.