treating web worms - Knowledgebase Question

Wexford, Pe
Question by ladquinn
March 26, 2010
Several evergreen trees have what I think are web worms. How are they treated, and when?

Answer from NGA
March 26, 2010


Bagworm (Thyridopteryx ephemeraeformis) is an insect pest that can cause serous defoliation of plants. They feed on most conifers and many deciduous trees and shrubs. They can be especially damaging to conifers because unlike deciduous plants, the damaged foliage is not regenerated. Severe infestation may kill the tree or shrub. Bagworms are the caterpillar stages of moths. In our area the bagworms hatch the first week of June. After they hatch bagworms spin the "bag" around themselves. They move about on the plant carrying the bag with them. The bags are made of silk, needle fragments and leaves. The bag enlarges throughout the summer as the bagworm grows, reaching a length of 1-2 inches. By August or early September the bagworms have finished their growth and are permanently attached to the plant by a tough silken thread. In the late summer the males pupate inside the bags and turn into moths. They mate with the females who are wingless and remain inside the bags. The female then lays 200-1000 eggs in the bag and dies. The eggs remain there until hatching the following spring. To control the bagworms the easiest method is to hand pick or clip off the bags from the plants during the winter or early spring before the eggs hatch. I pick the bagworms off whenever I see them on a plant, no matter what the time of year. The bags should be destroyed by crushing or putting them in soapy water. When infestation is too heavy for hand picking a biological control such as BT spray can be used. BT (Bacillus thuringiensis) is a type of bacteria that only kills certain insects and does not affect humans or animals.

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