Bagworm on arborvitae and evergreens

Bagworms are one of those pests that are easy to overlook until the damage is severe -- the bags blend into arborvitae foliage so effectively that heavily infested plants can appear normal from a distance while dozens of caterpillars are stripping interior foliage. By the time the bags are obvious.

Bagworms are one of those pests that are easy to overlook until the damage is severe — the bags blend into arborvitae foliage so effectively that heavily infested plants can appear normal from a distance while dozens of caterpillars are stripping interior foliage. By the time the bags are obvious and visible, many of the caterpillars inside have already completed their feeding for the season. The difference between a plant that recovers and one that dies is often catching the infestation at the right time.

I don't grow arborvitae at my Long Island property, so this guide draws on Penn State Extension, Clemson HGIC, and NC State Extension research.

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The pest

The bagworm (Thyridopteryx ephemeraeformis) is a moth in the family Psychidae. Per Penn State Extension, it infests a wide range of trees and shrubs:

• Preferred hosts: arborvitae (Thuja spp.), Eastern red cedar (Juniperus virginiana), junipers, spruce, pine
• Also infests: oak, maple, locust, sycamore, willow, rose, and many other deciduous trees and shrubs

The bags are constructed by the larvae from silk and plant debris specific to the host plant, making them effectively camouflaged. On arborvitae, the bags look like small, spindle-shaped bunches of arborvitae foliage.

Biology

Per Clemson HGIC:

• Overwintering: eggs overwinter inside the bag made by the female moth; each bag may contain 500–1,000 eggs
• Egg hatch: late May to mid-June in most of the mid-Atlantic and Northeast
• Larval development: caterpillars feed from June through August–September; bags enlarge as the caterpillar grows inside; bags are 1–2 inches at maturity
• Pupation: September; males emerge as adult moths; females remain in the bag, mate, lay eggs, and die inside the bag over winter
• One generation per year