Bagworm can be a serious pest, capable of rapid buildup and extensive defoliation. Annual surveys during fall, winter, or early spring are important to detect infested plants before serious damage results.
Bagworms do the most damage on juniper, Leyland cypress, arborvitae and redcedar trees
Larvae enclosed in cone-shaped bags consume foliage. Insect seldom seen, except head of larva protruding from the bag. The larva is mottled brown to black and encloses itself in a bag spun from silkstrands. Bits of leaves and twigs from the host are incorporated in the bag during its creation. During June bags are difficult to see, since they start less than 1/4" in length. Bags increase to 1 1/2" by late summer. Adult males fly and are the only form occurring outside the bags. Females develop inside the pupal case within the bag where the eggs are laid.
Feeding begins in June and lasts until late July, depending on the weather. After mid-August, when the insects are inside their bags, it is too late to control the pest with insecticides. The bag protects its inhabitant securely. Stripping of leaves is usually most noticeable in the uppermost parts of trees and shrubbery.
It is important to treat during mid-June. Small larvae are more susceptible to insecticides. Larger larvae and molting larvae are not easily killed. Insecticides with some residual are preferred. Picking off and burning bags from fall until spring will reduce populations, but is tedious. The presence of bags during winter is a good indication of which plants need to be treated the following year. One generation per year.
Actively feeding bagworms can be controlled with an organic caterpillar killer product that contains Bacillus thuringiensis (B.t.). Garden insecticides that contain acephate, bifenthrin, esfenvalerate or permethrin are also very effective.