By Diana Alfuth, Horticulture Educator, UW-Extension
At this time of year, we often see large webs forming in trees, and close inspection will reveal that these webbed “tents” are filled with small caterpillars. The tents, and the caterpillars inside, will get bigger as spring progresses. These are “Eastern Tent Caterpillars”, which are native to Wisconsin. In fact, records of their existence go back as far as 1646!
The caterpillars in these tents are the larva of a reddish-brown moth. The adult moth lays its eggs on small branches of trees in late fall, and the eggs overwinter and hatch in May here in Western Wisconsin. Wild cherry, choke cherry, apple, plum and ornamental crabapple are the most frequent hosts, although tents can also be found on hawthorn, mountain ash, maples, birch, willow, poplar and even oak trees.
The dense, silken tents are constructed by the tiny caterpillars and are expanded as they feed and get bigger. The caterpillars will go out on the foliage and feed during warm sunny days, and return to the tent at night for protection. By early to mid-June, the host trees may be stripped of leaves and the caterpillars will begin to wander about for additional food. About 6 weeks after hatching, the larvae are full grown, getting as big as 2 to 2 ½ inches long.
When done feeding and growing, the caterpillars will spin a whitish cocoon on fences, tree trunks or other available surfaces. About 3 weeks later, an adult moth emerges. The moth is reddish-brown with two parallel whitish lines on the wings, and a wingspan of 1 ½ to 1 ¾ inches long. Those adult moths later lay eggs on the trees to start the lifecycle over again.
While these caterpillars may defoliate a branch or even a whole tree, there is no need to get too concerned about them. Trees are able to withstand defoliation, and will simply break new buds once the caterpillars are gone.
If you find tents forming in spring, you can easily remove them by hand with a stick or other tool, and “squash” the tiny caterpillars! Once the protection of the “tent” is removed, the caterpillars are easy pickings for a large number of birds. Or you can create a hole in the tent, exposing the caterpillars, and treat them with an insecticide labeled for use on caterpillars.
Young caterpillars can be controlled with a natural insecticide called Bt (Bacillus thuringiensis var. kurstaki). This bacterium is harmless to people, animals, and plants, but is lethal to many types of caterpillars. It is sold under a variety of brand names, and there are several varieties of Bt, so look at the active ingredients, and be sure to select kurstaki. Bt is mixed and sprayed on the leaves, and acts as a stomach poison when eaten by the caterpillars.
Often we hear that you should prune the branch out and/or burn the tent. Pruning may disfigure the tree, and burning is a hazardous and unnecessary procedure that seriously injures the tree. While these tents of caterpillars look bad, they really are not very harmful.
When you see these tents on fruit trees, it’s nothing to panic about! The easiest control is to destroy the tents and squish or disperse the caterpillars. In fall, we may see similar tents caused by the Fall Webworm, which can be treated in the same manner, by destroying the tent.
For questions about plants, gardening or landscapes, contact Diana Alfuth, UW-Extension Horticulture Educator in Ellsworth at 715-273-6781.