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Picture of the Week for 23 August 1999

Bagworms: What are they?

Corey K. Gerber, Extension Entomology Specialist

Adult bagworms are actually moths. Males and females of the same species have a distinctly different appearance. Males are blackish in color, hairy, and have four wings. Females are yellowish- white in color and have no wings.

Bagworm larvae (caterpillars) are dark brown in color and are virtually enclosed in bags composed of silk, small twigs and leaves. These bags may reach two inches in length.

Bagworms only have one generation per year. In early June, young bagworms hatch from overwintered eggs laid within the female's bag. As soon as the bagworms emerge from this bag, they will begin to construct one of their own. During this time, the bagworms feed on tree and shrub foliage. Once mature, bagworms pupate within the bags. Following pupation, the males emerge from their bags and seek out females, which never leave the bag, to mate. After mating, females lay eggs within the bag and die. The eggs overwinter in the bag and begin a new generation the following spring.

Bagworms can be found feeding on the foliage of arborvitae, junipers, red cedars, and other trees and shrubs. In high numbers, bagworm larvae can severely damage trees and shrubs. Adult females may feed on plant foliage, but not to the extent as larvae. Adult male bagworms do not feed on plant foliage. If only a few small trees or shrubs are infested, simply removing and destroying the bagworms may provide adequate control. Often, insecticidal applications are required for bagworm infestations. For additional information on bagworm control, refer to publication E-27, Bagworms.

Source: Down the Garden Path, Issue 95, May 1995.

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Last updated: 22 September 1999/tlm.