Bagworm Control in St. Louis: Identification and Management

A St. Louis resident asks how to get rid of bagworms.

Bagworm Biology

Bagworms (Thyridopteryx ephemeraeformis) are caterpillars that construct spindle-shaped silk bags camouflaged with plant debris. Bags reach 1.5–2.5 inches. Larvae are brown/tan with black mottling. Adult males are bee-like with clear wings; females remain larva-like inside the bag. Hosts include arborvitae, red cedar, juniper, pine, spruce, and many deciduous trees. Females don't fly, so populations build rapidly on preferred hosts, causing defoliation and branch dieback.

Control Methods

Cultural/Mechanical

• Hand-pick bags in late fall/winter when visible on evergreens or after leaf drop. Cut attachment silk to prevent girdling.

Biological

• Bacillus thuringiensis (Bt): Effective on young larvae. Apply late June after egg hatch and ballooning. Trade names: Dipel, Thuricide.

Chemical (Stomach Insecticides)

• Thorough foliage coverage needed (larvae protected in bags). Early application on young larvae is best.
• Registered products: acephate (Orthene), bifenthrin (Talstar), carbaryl (Sevin), chlorpyrifos (Dursban), diazinon, dimethoate (Cygon), malathion, nicotine sulfate, pyrethrum, rotenone.

Always follow label directions. Consider pollinator safety when plants are flowering.