One common horticultural pest our office receives questions on is bagworms. Bagworms feed on the foliage of a wide variety of trees and shrubs, but are of most concern for evergreens, especially junipers. Bagworms overwinter as eggs in their bags which are attached to tree branches. The eggs hatch in mid-May to early June. As bagworms grow, leaf fragments are added to bags which often grow to 2 inches in length by the end of the summer. The earliest signs of bagworm injury in evergreens are brown or stressed needles at the tips of branches. Heavy infestations of older bagworms may completely defoliate a tree or shrub and if severe enough can kill the tree or shrub. Less severe injury will slow growth and stunt plants.
To control bagworms on small trees or small infestations, remove the bags by pulling them off the branches and immersing them in soapy water. If you place the bags next to the tree, the larvae might return to the host plants. If you have bagworms in a windbreak or large tree, insecticides are most effective when applied during early bagworm development. For early season damage, insecticides from mid to late June when bags are less than ½ inch in length are effective. By late August, chemical control is no longer effective as the bagworms have ceased feeding and are enclosed within their bags.
Reduced-risk insecticides to use contain Bacillus thuringiensis (Bt) and insecticidal soaps are quite effective on young bagworm larvae but may require repeated applications. Additional insecticide options for bagworms include: acephate, bifenthrin, carbaryl, cyfluthrin, malathion or others. As always, be sure to read and follow all label instructions and use all insecticides with caution to avoid exposure to humans, pets, wildlife and other non-target organisms.