Bagworm Infestation on Spruce: Identification & Control

Question: Nebraska silver spruce infested with cocoons made of bark and needles. What are they and how to eliminate?

Answer: Common Bagworm (Thyridopteryx ephemeraeformis)

Spindle-shaped silk bags camouflaged with plant debris (1½–2½ inches). Larvae feed from inside; females never leave bag.

Life Cycle (One Generation/Year)

• Overwinter: Eggs (300–1000) inside female bag
• Hatch (late May–early June): Larvae balloon on silk to new hosts
• Summer: Build & enlarge bags; feed on foliage
• Mid-August: Attach bags firmly; pupate head-down
• Fall: Males emerge, fly to females; females lay eggs, mummify over them

Control Options

Mechanical (Small Infestations)

• Hand-pick bags in fall/winter (easy to spot on evergreens)
• Cut silk attachment band to prevent girdling
• Crush or soak in soapy water

Chemical (Large Infestations)

• Timing critical: Spray when larvae are young (late June, after ballooning)
• Thorough foliage coverage essential (bags protect larvae)
• Registered products: acephate (Orthene), bifenthrin (Talstar), carbaryl (Sevin), diazinon, malathion, pyrethrum, rotenone
• Bt (Bacillus thuringiensis) effective on young larvae only

Key Takeaway

Early detection & treatment (June) works best. Late sprays on mature bags are largely ineffective.