Arborvitae Bagworms

Bagworms (Thyridopteryx ephemeraeformis) are native North American caterpillars that feed on foliage of Arborvitae (Thuja spp.), juniper, and other conifers across 4-9. Check your local placement within cold-hardiness zones 4-9 to confirm timing and risk. They build distinctive silken bags covered with bits of foliage for camouflage; females never leave the bag and eggs remain there to overwinter.

Seasonal timing is critical: eggs overwinter inside old bags, and larvae hatch in spring as soon as daytime temperatures consistently warm. Young caterpillars are small and mobile in late spring, feed heavily through early to mid summer, then grow larger and cause the worst defoliation before pupating. If you prefer lower-spray timing, align bag checks with natural garden pest control instead of waiting for hedge sections to turn brown.

You'll usually notice bagworms first by scattered bags low in the canopy or by small areas of browning foliage. Severe infestations cause branch dieback, whole-plant browning, and in repeated years can kill an Arborvitae. Quick action in spring and early summer makes most treatments effective and limits long-term damage.