Sandra Mason: Don't let bagworms destroy trees, shrubs

Sandra Mason: Don't let bagworms destroy trees, shrubs

By Sandra Mason

They're baaa-aaack!

OK, their arrival may not be on the level of evil spirits invading suburban homes in "Poltergeist II"; however, bagworms can wreak havoc on trees as they devour every evergreen needle within munching distance.

In central Illinois, they started hatching the third week in June. The early bird catches the worm, so start planning now to control bagworms on your evergreen trees and shrubs.

Just to recap the life of a bagworm, they hang like Christmas ornaments from evergreens such as arborvitae, red cedar and blue spruce, but also deciduous trees such as maples and crabapples. Deciduous trees can handle the damage, but severe damage may occur on evergreens as leaf loss can cause branch death.

The adult bagworms are easily overlooked moths. The female won't be the first moth asked to cut the rug at the next porch light dance. She is eyeless without wings, legs, antennae or functional mouthparts, and never leaves the bag. The male moths are black and almost clear-winged. In late summer, 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 to 1,000 eggs in one bag, which can mean large populations on a single plant. They spend the winter as eggs in the female bag.

So now we are seeing the bagworm eggs hatch into tiny caterpillars that exit the bottom of the bag. As kids like to do, the newly hatched caterpillars leave home.

Bagworm caterpillars are quite the daredevils as they move around by "ballooning." They produce a 2- to 3-foot-long silk streamer that acts as a balloon to keep them drifting in the wind. Eventually, they hit the side of a building or your favorite juniper. The larva then crawls to the top of whatever the silk caught onto and may repeat the process for about two weeks until it lands on a suitable host. Eventually, caterpillars are faced with the choice of feeding on what they land on or starving to death.

Once the caterpillars land, they spin a silk tent adorned with camouflage of bits of twigs and leaves of the host plant. As the caterpillars eat and grow, they increase the size of the spindle-shaped bags up to 11/2 inches long. As they feed, you may see the head of the caterpillar sticking out of the bag. 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 has started and is continuing. Southern Illinois is generally two weeks earlier and northern Illinois two weeks later. We recommend that treatment be delayed for two weeks after hatching to allow ballooning to finish and to allow for late hatch. Insecticide control should begin in early July in central Illinois.

Insecticides for bagworm control include: bacillus thuringiensis kurstaki (Btk), sold as Dipel and Thuricide; spinosad, sold as Conserve; and cyfluthrin, permethrin and other pyrethroids sold in many different products. Treatment is recommended while caterpillars are young and small. Always read and follow all pesticide label directions and do not spray when temperatures are above 85 F.

Insecticides are only effective while bagworms are feeding. Once they start pupating in late summer, hand picking the bags is the only control from September through mid-spring, so find a really bored child.

For a really great source for up-to-date pest information, the free University of Illinois Extension Home, Yard and Garden Pest Newsletter is available at

If you find all this fascinating, join us for the Master Naturalist training. For more information, call 333-7672 or visit

Sandra Mason is unit educator, horticulture and environment, for the UI Extension, Champaign County. Contact her with questions or comments at 801 N. Country Fair Drive, Champaign, IL 61821, call 333-7672, email or fax 333-7683.

Sections (1):Living
Topics (1):Environment

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