Bagworms

Thyridopteryx ephemeraeformis

Insectalso: Evergreen bagworm, Bagworm moth

The caterpillars of a moth that build and live inside a tough, spindle-shaped bag woven from silk and bits of the plant they feed on. They attack many trees and shrubs but are most damaging on evergreens such as juniper, arborvitae, cedar, and spruce, where heavy feeding can defoliate and kill the plant.

🔎 How to spot it

The unmistakable sign is the bag itself: a one-to-two-inch, cone- or spindle-shaped case covered in pieces of needles, leaves, or bark, hanging from twigs and often mistaken for a cone or a bit of the plant. The caterpillar lives inside and pokes its head out to feed, dragging the bag along. Adult males are small, clear-winged black moths, while the females are wingless and never leave the bag.

🥀 Damage it causes

The caterpillars chew foliage, and on evergreens, which cannot quickly replace lost needles, heavy feeding strips branches bare and can kill the whole plant. On deciduous trees and shrubs the damage is usually less serious because they leaf out again. A single plant can carry hundreds of bags, each holding a feeding caterpillar, so populations build quickly.

🛡️ Prevent it

On small trees and shrubs, the simplest and most effective control is to handpick the bags from fall through early spring, when each bag holds the eggs that would start next year infestation, and destroy them rather than dropping them beneath the plant. Inspect susceptible evergreens every year. Encourage the parasitic wasps that attack bagworms by avoiding unnecessary broad-spectrum sprays.

🧯 If it is already here

When eggs hatch in late spring and the young caterpillars are small and exposed, Bacillus thuringiensis (Bt) or a labeled insecticide applied every one to two weeks during the hatch is effective. Sprays do not work well later, once the caterpillars are large and sealed inside their thick bags. For light infestations, continued handpicking is enough.

💡 Good to know

Because the female cannot fly and lays her eggs inside her own bag, removing bags by hand in winter directly removes next year population. Timing any spray to the early hatch is critical, since the bag protects the growing caterpillar from insecticides. Repeated defoliation is far more deadly to evergreens than to deciduous plants.