The Homeowners Column

The Homeowners Column

Managing bagworms and tent caterpillars

Photo of Sandra Mason

Sandra Mason
State Master Gardener Coordinator

Tents and sleeping bags commonly house people this time of year. We have to buy ours while some caterpillars weave their own bags or tents. But when is a bag really a tent? After twenty years of helping home gardeners with their questions, I have come to the conclusion that the term "bagworm" can conjure up different images.

What most horticulturists call bagworms are quite visible now as dangling brown ornaments from the bare branches of evergreens such as eastern red cedar, other junipers, spruce or arborvitae. Bagworms left unchecked can devastate evergreens.

However May is too early to spray for bagworms. The bagworms seen now are old bags. These developed from caterpillars that emerged last year in June. Caterpillars fed all last summer and the bags got bigger as the caterpillars got bigger. In mid to late August the bagworm caterpillars zipped up their bags, quit feeding and formed pupae so they could develop into a moth. Once they zip up their sleeping bags and quit feeding, spraying with insecticides is useless until the young caterpillars emerge the next season.

The best time to spray for bagworms is while the caterpillars are small and actively feeding, generally late June with a second spray in early July. With our below normal temperatures this spring it may be even later so scouting for additional caterpillars two weeks after spraying is recommended.

Insecticide options for bagworm control include: Btk (Bacillus thuringiensis kurstaki) sold as Dipel, Thuricide and others; spinosad sold as Conserve; or cyfluthrin sold as Tempo and various Bayer products. Read and follow all label directions.

Right now removing the bags by hand is effective to reduce the numbers of caterpillars emerging. Every other bag is a female and each female bag can contain 300-1,000 eggs. Just introduce your family to the exciting new reality show "Let's collect the bag ladies off our trees".

You may wonder how do bagworm caterpillars find new trees to infest. They obviously aren't going to win any races even with all those legs. They have an ingenious method of transport called "ballooning". Bagworms hatch in late spring, exiting the bottom of the bag. The tiny caterpillars climb to the top of the tree where each one spins out a silk strand 1 to 3 feet long. The silks catch in the wind and carry the tiny caterpillars wherever the wind blows. Caterpillars then crawl to the top of whatever object caught the silk. Caterpillars may repeat the process until they land on a suitably tasty tree. The bagworm population continues this process for about 2 weeks after emergence.

Sometimes eastern tent caterpillar and fall webworm are also called bagworms. I have seen Eastern tent caterpillars developing their tents right now so it is not too early to control these defoliators. These caterpillars gather at a fork of a tree and build a web or "tent," but they exit the web to feed except on cloudy or rainy days and at night. The tent enlarges as the caterpillars eat. Tent caterpillars feed on leaves of crabapple, hawthorn, mountain ash, flowering cherry, and other trees and shrubs in the rose family.

Fall webworm is another caterpillar that forms a tent but they never leave their tent. As the name implies fall webworm usually occur later in the season becoming especially noticeable in August and September.

Tent caterpillars and fall webworm can also be controlled with Btk products when the caterpillars are young. With fall webworm and tent caterpillar use high spray pressures to break up the web and get the insecticide inside to the caterpillars and the leaves. If the caterpillars are not there, than insecticide sprays are useless. Tearing or pruning out the webs and their resident caterpillars from the tree can also control tent caterpillar and fall webworm.

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