How to Treat for Bagworms | Timing & Methods That Work

The most effective treatment for bagworms combines hand-removing bags in fall through early spring with spraying Bt or spinosad on young larvae in late spring to early June.

You are reading this because bags are already on your trees or you want to stop them before they appear. The single biggest mistake people make is spraying too late — by late August the larvae have stopped feeding and sealed themselves inside their bags, and no chemical will reach them. Here is what works, and when to do it.

When to Treat Bagworms: Timing Is Everything

Bagworms spend the winter as eggs inside the female’s bag from the prior season. Eggs hatch in late spring, typically mid-May to mid-June depending on your region. The newly hatched larvae are tiny, crawl out, spin silk, and begin feeding on your arborvitae, Leyland cypress, or pine. This is the only window where chemical control is highly effective. By late summer the larvae are large and tough; by late August they pupate and control is essentially impossible.

Regional indicators for the spray window: in Iowa, wait for catalpa trees and Japanese tree lilacs to bloom — that confirms egg hatch. In Nebraska, spray when bags are smaller than half an inch. In Oklahoma and Georgia, aim for early to mid-June. The mechanical removal window is much wider: you can pick or cut off existing bags anytime from fall through early spring, before the new eggs hatch.

Handpicking Bags: The Mechanical Method

For small infestations on reachable trees and shrubs, hand removal is your first and cheapest line of defense. Inspect evergreens in late winter or early spring. Use sharp scissors or shears to cut the silk loop that holds each bag to the branch. Drop the bags into a bucket of soapy water — dish soap works — to kill any live caterpillars and unhatched eggs. Then seal the bags in a trash bag and dispose of them immediately. Do not throw bags on the ground; bagworms can chew through standard trash bags and crawl back to your trees.

Repeat this every fall, winter, and early spring to keep populations low. Mechanical removal alone cannot stop a heavy infestation, but it dramatically reduces the number of larvae that hatch in spring.

Chemical Control for Young Larvae

Spraying is only effective when larvae are small — under an inch long. The two best reduced-risk options are Bacillus thuringiensis (Bt) and spinosad. Both must be eaten by the caterpillar, so thorough canopy coverage is critical: hit the undersides of leaves and deep into the tree’s interior. Surface spraying does nothing. Bt is OMRI-approved for organic use and safe for humans and animals. Spinosad works similarly but may need two or three passes. Apply either every 7 to 14 days during the hatch window.

If you missed the early window and the larvae are already large in late June, synthetic pyrethroids such as bifenthrin or carbaryl may still work, but their effectiveness drops fast. These broad-spectrum products also kill beneficial insects that naturally control spider mites and scale, so use them only when necessary. Always follow the label’s safety instructions. For a tested lineup of what works best, check our treatment for bagworms product roundup.

One common mistake: spraying before eggs have hatched wastes your time and chemistry. Wait for the visual cues — catalpa bloom, or bags smaller than half an inch. Another mistake: relying on systemic insecticides like dinotefuran. These are professional-use only and only work on shrubs under 6 feet tall; homeowner products for tall trees simply do not exist.