Bagworms typically hatch in southern Illinois in early June and in central Illinois in mid-June. They are uncommon north of I-80. The last couple of weeks with cooler-than-normal temperatures probably has countered the warmer-than-normal temperatures earlier this spring. The result should be insect emergence that is close to normal.
Female bags from last year hanging on the tree contain 300 to 1,000 eggs each. Newly hatched larvae exit through the bottom of the bag, form their own tiny silk bags, and cover them with whatever is available. They climb high into the tree and dangle on 2- to 3-foot strands of silk. Winds catch and detach these strands, and the silk becomes a streamer that keeps the tiny bagworm larva aloft. These bagworms can float for long distances until the silk strand catches on an object. Many land on roads, buildings, and other inhospitable places, where they are likely to be run over by an 18-wheeler or die from some other means. Some catch onto trees and shrubs. The bagworms may climb to the top of the plant and repeat the ballooning process or settle down to feed on the plant. Feeding typically starts at the top of the plant.
Preferred hosts include eastern red cedar, other junipers, arborvitae, spruce, crabapple, hackberry, and oak; but this insect is capable of feeding on a wide range of plants. Initially, the young caterpillars feed on one epidermis and the mesophyll, leaving the other epidermis intact. This pattern results in leaves that are whitish and then turn brown. At this time, the caterpillars have cone-shaped, brown bags that are 1/8 to 1/4 inch long.
Older larvae eat most of the needle or leaf, stripping the branches at the top of the tree. As they age and keep running out of food, the larvae and their damage descends the tree canopy. Stripped conifer branches usually die, and severe attack can kill an entire tree. Deciduous plants usually re-leaf and survive.
In late summer, typically mid-August, the bagworm cases reach 1-1/4 to 1-1/2 inches long. They pupate within the bag. Black male moths with clear wings emerge through the bottom of the bag, frequently leaving the empty pupal case hanging in the end of the bag. Females stay within the bag as a larvaform adult without wings. Males mate with the females through the end of the bag, and the females lay their eggs inside it and die. The eggs overwinter.
Handpicking and destroying the bags from fall through midspring is effective in removing the eggs. However, larvae are likely to balloon in the spring from neighboring or even distant trees. Insecticide sprays are effective against the young larvae, but larvae whose bags are at least 3/4 inch long are very difficult to control. It is most effective to wait a couple of weeks after egg hatch to treat, allowing the larvae to finish ballooning. Treating too early requires a second treatment to control larvae that balloon onto the trees or shrubs. Wait until mid-June in southern Illinois and early July in central Illinois to treat.
Bacillus thuringiensis kurstaki (Dipel or Thuri-cide), cyfluthrin (Tempo), spinosad (Conserve), trichlorfon (Dylox or Proxol), and other labeled insecticides are effective against the young larvae. Cyfluthrin and trichlorfon are the insecticides of choice if the larvae get a little large. Actively feeding bagworms always have little pieces of green foliage around the top of the bag, and these pieces dry and turn brown in just a few days. Older larvae with no green around the top have probably already pupated, and insecticidal control is useless. After treatment, the bagworms hang on the tree. A useful method to determine control is to check for green foliage on the bag a few days after treatment. If the top of the bag is brown, the larva is dead.