Have you noticed an evergreen tree or shrub starting to die and turn brown? This is the time of year when we should be scouting for bagworms. Bagworms disfigure evergreen trees and shrubs by feeding on leaves and needles and girdling twigs. As worm-like larvae they spin silken bags around themselves, to which they attach pieces of the leaves they are eating. They carry this bag with them as they feed, it acts as a protective armor. A fully developed bag is about two inches long. These tough, spindle-shaped bags hang from the branches of infested trees like holiday ornaments, so they are easy to spot. Bagworms are the larvae of moths. Full-grown, these worms measure 1 to 1 1/4 inches. Their bodies are brown with that portion inside the bag lighter than the rest. Adult male moths have black wings, but the females are wingless. The females lay their eggs in fall, and the eggs hatch the following May or June. As the young worms begin to feed, they start doing their damage to tree and shrub foliage. They will continue eating for several months, maybe as late as August in our area. Proper insecticide use will effectively control their damage in late spring. Bagworm caterpillars make distinctive 1.5 to 2 inch long spindle-shaped bags that can be seen hanging from twigs of a variety of trees and shrubs. Sometimes the bags are mistaken for pinecones or other plant structures.
Bagworms prefer juniper, arborvitae, spruce, pine, and cedar but also attack deciduous trees in large infestations. Female moths cannot fly but the larvae can disperse. Very small caterpillars can spin strands of silk and be carried by wind, an activity called “ballooning”. Larger larvae may crawl to adjacent plants.
Bagworms pass the winter as eggs (300 or more) inside bags that served as cocoons for last year’s females. The eggs hatch in mid- to late May to early June in Indiana and the tiny larvae crawl out to feed. Each uses silk and bits of plant material to make a small bag that protects and camouflages it as during feeding and growth. This is the window of opportunity for pesticide use if you had a large bag worm problem the prior year.
Bagworm caterpillars feed for about six weeks, enlarging the bag as they grow and withdrawing into it when disturbed. When abundant, the caterpillars can defoliate plants. Heavy infestations over several consecutive years, especially when coupled with other stresses, can lead to plant death.
Bagworm control depends upon how many bagworms are present. If only a few small trees or shrubs are infested, handpicking and destroying attached bags may provide satisfactory control. This must be done during fall, winter or early spring before the eggs hatch. Destroy the picked-off bags by burning them, as they will overwinter on the ground and hatch in the spring.