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Theuri: Bagworms infesting trees and shrubs

Formations on conifer, arborvitae, spruce, eastern red cedar, other junipers or white pine trees have been mistaken for cones, or seed-bearing structures. Take a look at them and you may see the structures moving.

A worm is inside the “bag” and the structures are therefore called bagworms. A few of them and your trees would be OK, but a whole lot of them could eat down your tree or bush.

Bagworm young hatch in the overwintering bag and emerge in June and begin feeding. They are easily blown to other plants. As they feed, female worms construct their case, or bag, for about three months; sometimes it is possible to see the bags moving as the worms move.

In late summer, the mature worms pupate for seven to 10 days; winged males emerge and exit the bag, while wingless females stay and mate while still in the bag. One female lays up to 1,000 eggs in the bag, where it will stay until the following year. Meanwhile, the female dies.

Symptoms & Damage

Bagworms usually begin feeding at the top of the tree. When small, the worms feed in the layers of the leaf tissue, creating light patches on leaves. As they age, they consume entire needles or leaves. A severe infestation may defoliate plants, which can kill branches or entire plants.

A healthy deciduous tree or shrub that has been defoliated usually produces a new flush of leaves and survives. However, a defoliated evergreen cannot push out an additional set of leaves and may die.

Management Tips

• You can manually pick the bags off and kill the worms, but some could be hard to find at the young stage.

• Fortunately, the small young caterpillars are very susceptible to insecticide treatment. A single application of Bacillus thuringiensis kurstaki-BT (Dipel, Thuricide), spinosad (Conserve), cyfluthrin (Tempo), permethrin (Astro), and other pyrethroids are effective even on older larvae.

• Soil application of the systemic insecticides dinotefuran and, to a lesser extent, chlothianidin can increase larval mortality and reduced the feeding rate and growth of larvae, up to 50 d post-treatment.

The biological insecticide Bt is good as it affects only caterpillars. It is less effective once the bags are larger and the worms have pupated. As always, be careful when using any pesticide and read label directions carefully.

James Theuri is a University of Illinois Extension local food systems and small farms educator.

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