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Bagworms

Tim Gibb, Purdue University

Bagworms have become an increasingly serious concern to homeowners over the past few years. During July and August, bagworms may defoliate arborvitae, junipers and other trees and shrubs. Bag-worms are caterpillars that live inside spindle-shaped bags which they construct to protect themselves against birds and other enemies. These bags, composed of silken threads and bits of foliage, look so much like a part of the tree that they may go unnoticed until extensive damage has occurred.

Bagworms are most common in southern and central Indiana, but in recent years they also have been reported in northern Indiana. Early in June, the insects hatch from eggs which wintered in the old bags attached to tree.branches. As soon as the young worms appear, they start to spin bags and continue to enlarge these as they feed and grow. The caterpillars crawl part way out of the bags to feed. If disturbed, they retreat safely inside, and it is almost impossible to pull them out. Each female bag can produce hundreds of bagworms.

Bagworms mature in late August or early September. At this time the bags are about 2 inches long. The worms then attach the bags firmly to branches or other objects and change into the adult stage. The wingless female never leaves the bag and is fertilized by the winged male. The eggs are laid in the bag where they pass the winter. There is only one generation each year.

If only a few small trees or shrubs are infested, bagworms can sometimes be controlled by picking off the bags and destroying them. This method is most effective during the winter and early spring since it destroys the eggs before they hatch.

For more information, please refer to Purdue's Extension Publication: Bagworms, E-27 (PDF 29K).

Click on the small image to view a larger image.

Bagworms on Arbovitae
Bagworms on Arbovitae

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Last updated: 7 March 2002/tlm.
The Plant & Pest Diagnostic Laboratory at Purdue University.