DALY: Eastern tent caterpillars unsightly, but not harmful

Many homeowners are currently observing a strange type of webbing that has suddenly appeared in some of their trees. Although some may conclude the webs are caused by a spider, the real culprit is the eastern tent caterpillar.

Its name comes from the ugly tent-like nests the insects build in the branches of the host trees. This pest is more of a nuisance than harmful. Although heavy infestations can cause defoliation of many branches, the caterpillars seldom cause harm to the trees. In the landscape, however, nests can be ugly, particularly when they are exposed by excessive defoliation.

The insect's preferred tree is the wild black cherry, but they can potentially infest numerous other fruit trees and occasionally certain ornamental shade trees.

The eastern tent caterpillar overwinters within egg masses on the branches of the trees. They start hatching when the tree buds begin to open, usually in mid-to-late March. The insects are one to two inches long. They are brown to black in color with a white stripe running down their backs.

The caterpillars from one egg mass stay together and spin a silken tent in the crotch of the host tree. The tent is initially small, but they spin successive layers of webbing thus enlarging it. Their webbing is filled with dead leaves and droppings. In time they leave their nest and crawl onto other plants, walkways, and storage buildings. They are searching for a protected shelter in order to spin a cocoon to undergo pupation. They pose no harm to people or animals, but sometimes they can become a nuisance when large numbers crawl or cluster on building walls, lawns and sidewalks.

One control method is physically removing the nests out of the tree. Simply scrape it off onto the ground using a rake or pole pruners, and then smash the caterpillars or drop them into a pan of soapy water. Larger nests can be pruned out and destroyed. The ideal time of the day to do this is early morning or late afternoon when most of the caterpillars are in the tent. Also, they are preyed on by many species of birds and other insects.

Several pesticides are available for control. Biological insecticides, sold under brand names Dipel, Thuricide or several others, contain the bacteria Bacillus thuringiensis (Bt). It specifically targets the caterpillars and does not cause harm to other organisms.

Insecticidal soaps, Orthene, Sevin and several other insecticides can also be applied to control them. The best time for the application is when the larvae are small and easy to control. Control of the older caterpillars, especially once they begin to leave the nest, is not needed since the damage has already occurred. Spray the chemical directly onto the plant foliage and into the webbing. If possible, break open the web to improve penetration. As with all pesticides, please follow all label directions and safety precautions.

If you notice eastern tent caterpillars in some of your trees, remember they rarely do any permanent damage. Usually the best course of action is let them finish feeding, and they will eventually go away along with their webbing.

Timothy Daly, MS, can be contacted at 678-377-4010 or timothy.daly@gwinnettcounty.com.