Frederick County Master Gardener Program
Bagworm (Thyridopteryx ephemeraeformis (Haworth)) is an insect pest that can cause serous defoliation of plants. They feed on most conifers and many deciduous trees and shrubs. They can be especially damaging to conifers because unlike
deciduous plants, the damaged foliage is not regenerated. Severe infestation may kill the tree or shrub.
Bagworms are the caterpillar stages of moths. In our area the bagworms hatch the first week of June. After they hatch bagworms spin the "bag" around themselves. They move about on the plant carrying the bag with them. The bags are made of silk, needle fragments and
leaves. The bag enlarges throughout the summer as the bagworm grows, reaching a length of 1-2 inches. By August or early September the bagworms have finished their growth and are permanently attached to the plant by a tough silken thread. In the late summer the males pupate
inside the bags and turn into moths. They mate with the females who are wingless and remain inside the bags. The female then lays 200-1000 eggs in the bag and dies. The eggs remain there until hatching the following spring.
To control the bagworms the easiest method is to hand pick or clip off the bags from the plants during the winter or early spring before the eggs hatch. I pick the bagworms off whenever I see them on a plant, no matter what the time of year. The bags should be destroyed
by crushing or putting them in soapy water. When infestation is too heavy for hand picking a biological control such as BT spray can be used. BT (Bacillus thuringiensis) is a type of bacteria that only kills certain insects and does not affect humans or animals. It works
well only on young bagworms and must be applied between June 15th and July 15th to be effective.
Read other articles about controlling insects & garden pests