Bagworms on Arborvitae: Identification and Late-Season Control

Those "miniature pine cones" dangling from your evergreens that move like slugs when they fall? Classic bagworms (Thyridopteryx ephemeraeformis). The bags are silk cases decorated with host plant debris—perfect camouflage.

Life Cycle & Timing

• Eggs overwinter inside female bags (1000+ per bag)
• Hatch late May-June — tiny larvae disperse on silk threads
• Feed & build bags all summer — this is when you see them
• Pupate August-September — bags firmly attached, sealed
• Adults emerge — wingless females stay in bag; males fly to mate

Late-Season Control (August+)

Insecticides are largely ineffective now — bags are sealed, larvae pupating. Your only options:

1. Hand-pick bags: Remove and destroy (burn, soak in soapy water, trash). Do this fall through winter.
2. Prune heavily infested branches: If localized, remove entire branches.
3. Orthene (acephate): Only option with some contact activity on late larvae, but poor penetration.

Next Year's Strategy: Early Intervention

• Scout in late May: Look for tiny (¼") new bags on branch tips
• Spray Bt (Bacillus thuringiensis): When bags < ½", highly effective, safe for beneficials
• Spinosad: Organic, works on larger larvae than Bt
• Systemic (imidacloprid): Apply as soil drench in early May for season-long control

Host Range

Arborvitae, juniper, cedar, spruce, pine, and many deciduous trees. One infested plant spreads to neighbors rapidly.

Mark your calendar: "Bagworm spray = late May." Early treatment prevents the unsightly bags and defoliation entirely.