Bagworms

What Are Bagworms?

Bagworms are actually caterpillars—the larvae of a moth. But instead of crawling around in the open, they live inside tiny silken bags covered in pieces of leaves and twigs from the plants they’re feeding on. The camouflage makes them hard to spot until the damage is already done.

They feed on both evergreen and broadleaf plants, but evergreens like junipers, cedars, arborvitae, and spruce are their favorite targets—and the most at risk. A heavy infestation can strip an entire plant of foliage and even lead to death.

Bagworm Life Cycle

Bagworms go through just one generation per year:

Fall/Winter – Eggs overwinter inside bags left from the previous season.
Mid–Late May through June – Eggs hatch, and tiny caterpillars emerge and begin feeding. This is the ideal time to treat.
June–July – Larvae continue feeding and growing. As they grow, their bags thicken and protect them from many treatments.
August – Larvae pupate in their bags.
Late Summer – Adult males emerge to mate; females remain wingless inside the bag and lay eggs for the next season.

What to Look For

Tiny cone-shaped bags (as small as ¼″) hanging from twigs or leaves.
Browning tips or thinning growth, especially on evergreens.
Scattered bags on fences, gutters, or unrelated plants—young caterpillars can “balloon” on silk threads to spread.

Act Quickly

Once caterpillars reach about ¼″ long, they’re already harder to kill. As they grow, their silken bags become thicker, more waterproof, and resistant to insecticides. Delayed treatment often leads to larger infestations, more damage, and more expensive solutions.

How to Treat for Bagworms

Your best window to treat is in early June, as soon as hatch begins.

Step 1: Manual Removal

Remove and destroy any visible bags from last season (especially in winter or early spring). These contain hundreds of eggs.

Step 2: Choose the Right Spray

For young bagworms (<¼″):

Use a product with Bacillus thuringiensis (Bt) or Spinosad.
Apply with a spreader-sticker for better coverage.
Example products: Fertilome® Borer, Bagworm, Leafminer & Tent Caterpillar Spray; Natural Guard® Spinosad.

For larger bagworms (>1.5″):

Switch to a stronger insecticide like Hi-Yield® 38 Plus Permethrin.
Apply two treatments, 10–14 days apart.

Be sure to spray thoroughly—bagworms often hide deep inside plant canopies.

Pro-Tip: Make Bagworm Control a Yearly Habit

If you’ve dealt with bagworms in the past, set a reminder each spring to inspect your plants around late May to early June. Treat proactively before they cause visible damage.

Want More Detailed Guidance?

Check out a printable info sheet here: Bagworm Info Sheet