This time of year, the best strategy for bagworm control is to physically remove the bags from your trees and shrubs. The overwintering bags contain as many as 300 to 1000 eggs each. Removing the bags before the end of May can help reduce their summer populations. Crush the bags or place them in the trash in a sealed trash bag. If bags containing larvae are discarded on the ground, the larvae may return to host plants after hatching.
Insecticide control needs to be aimed at young larvae in mid to late June to be effective. Spraying trees is expensive. To get the best control for windbreaks or large stands of trees, late June to early July is the best time to apply insecticides for bagworm control.
Feeding by mature caterpillars slows in August before pupation into adults, so chemical control in late summer and fall is not effective.
Bacillus thuringiensis is available at nurseries and garden centers as Dipel or Thuricide. Other insecticides currently labeled for bagworm control include acephate, carbaryl, cyfluthrin, malathion and permethrin. Affected plants must be thoroughly covered with the insecticide in June so the insects ingest it as they are feeding. Be sure to read and follow all label directions.