Bagworms on our evergreen trees have spread to other trees in our yard. How can we control them? —Kathy Stelzel, Danville, IN
Bagworms (Thyridopteryx ephemeraeformis) are caterpillars that feed on both evergreens (junipers, pines, spruce, fir, and others) and deciduous trees and shrubs (black locust, maples, sweet gum, sycamore, and others). Bagworms can defoliate plants in July and August, which can be fatal, particularly for evergreens.
As they feed, bagworms construct protective bags of extruded silken “thread” with bits of foliage mixed in. These bags hang from the trees and look like natural growths. Caterpillars crawl partly out of the bags to feed, retreating to the bags if they’re disturbed. They mature into adults in late summer, then lay eggs that overwinter in bags and hatch the following spring.
There are several ways to control bagworms. You can remove the bags early in spring, dropping them into a bucket of soapy water to kill the eggs before they hatch. Or you can spray young caterpillars soon after they hatch, when new bags are very small. It’s best to use a biological product such as Bacillus thuringiensis because it won’t harm beneficial insects, just young caterpillars. Once the bags are closer to full size—2 inches long—spray with an insecticide containing acephate, carbaryl, malathion, or any insecticide that’s labeled for controlling bagworms on trees and shrubs.