The Homeowners Column

The Homeowners Column

Plan Now To Control Bagworms

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

I'm convinced there is a direct correlation between the likelihood of a plan failing and the number of times "Trust me, this will work" is uttered by the plan's creator. Many years ago I received a phone call from a woman who wanted me to evaluate the intelligence of her husband's plan to rid their arborvitae evergreen of bagworms. I could hear her husband in the background saying, "Trust me, this will work." The plan was to cover the shrub with a plastic bag then start the lawn mower and put the running mower in the bag with the bagworm afflicted shrub. His plan was to use the exhaust fumes to "gas" the bagworms. As I choked back a chuckle, I pondered his plan. I had to give him points for creativity, but I had to subtract points for peril to plants and people. The arborvitae would be coughing long before the bagworms.

Bagworms feed on the leaves of evergreens such as blue spruce, arborvitae and red cedar. They can also be found on deciduous trees such as maple and crabapple. As bagworm caterpillars mature they hang like Christmas ornaments. Damage is most severe on evergreens since leaf loss can cause branch death. The trouble is we often don't notice the bags and the damage until late in the season when hand picking the bags is our only option.

The adult bagworms are interesting 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 is a true "bag lady". She never leaves the bag. The male moths are black and almost clear winged. The male moth emerges from his bag in late summer, 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 500-1000 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 early to mid June. Pesticides are most effective if applied two weeks after egg hatch. In central Illinois control should begin in late June. An additional application is advised one to two weeks later to control later emerging caterpillars.

Just how did the bagworm make it to your tree? 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 tree.

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 and in some Bayer products) and spinosad (Conserve and some Ferti-lome products). 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 works very effectively by contact and ingestion. Cyfluthrin is recommended for larger larvae, but again thorough coverage is essential.

Mark your calendar now to eliminate the brown bag party on your evergreens.

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