Extension Educator, Horticulture
Bagworms can wreak havoc on evergreens in our area. Sadly I saw a 20-foot tall blue spruce killed by them. If bagworms have tortured your trees, now is the time for control.
As bagworms mature they hang like Christmas ornaments from evergreens such as arborvitae, red cedar, but also deciduous trees such as maples and crabapples. Severe damage may occur on evergreens since leaf loss can cause branch death.
The adult bagworms are interesting looking moths. The female won't be the first moth asked to dance at the next porch light dance. She is eyeless without wings, legs or antennae or functional mouthparts. She has a soft yellowish white, almost hairless body and never leaves the bag. The male moths are black and almost clear winged. The male moth emerges from his bag, flies to the female, mates and dies in a few days. I guess he doesn't have a great life either, but at least he gets to leave the house.
The female produces 300-1000 eggs in one bag, which can mean large populations on a single plant. They spend the winter as eggs in the bag.
The bag is made of silk and bits of twigs and leaves of the host plant. Active bags will have green leaves on the top. The bag enlarges as the caterpillar grows and everywhere the caterpillar goes the bag is sure to follow.
In central Illinois bagworm egg hatch occurs in mid June to late June. Control should begin in early July to allow full caterpillar emergence.
The newly hatched tiny caterpillars move around by "ballooning." The young caterpillars climb high into trees. They produce a 2-3 foot long silk streamer that acts as a balloon to keep them drifting on the wind. Eventually they hit the side of a building or your favorite juniper. Feeding usually starts at the top of the plant.
According to Raymond Cloyd, U of I Extension entomologist, insecticides recommended for controlling bagworms include Bacillus thuringiensis kurstaki (sold as Dipel or Thuricide), cyfluthrin (Tempo), trichlorfon (Dylox), and spinosad (Conserve). Insecticide sprays are effective against the young larvae but bags that are 3/4 inch long or longer are very difficult to control. The bacterium Bacillus thuringiensis is effective on young caterpillars, but the material must be ingested--so thorough plant coverage is essential. Spinosad very effectively works by contact and ingestion. Cyfluthrin and trichlorfon are recommended for larger larvae but again thorough coverage is essential.
In addition to the recommended insecticides, research has shown certain species of nematodes (Steinernema carpocapse) attack bagworms. When nematodes are sprayed onto the bags, they infect the female bagworms inside. It is important to apply the nematodes before females lay eggs.
Handpicking of the bags is the only control from fall through midspring so find a really bored child.