Small green worms are eating my tree leaves. What are they and how do I stop them?

Small green caterpillars (often called "worms") feeding on tree leaves could be several species, but the most common culprits are cankerworms, inchworms, fall webworms, or sawfly larvae. Identification matters because treatment differs.

Quick identification guide:

• Cankerworms / inchworms: Move in a characteristic "looping" motion. Very common on oaks, maples, elms, and lindens in spring. About 1 inch long.
• Fall webworms: Build silky white web nests on branch tips. Yellowish-green with black spots. Feed communally inside the web.
• Sawfly larvae: Look like caterpillars but have more than 5 pairs of prolegs (fleshy stub legs). Common on pines, willows, and roses.
• Gypsy moth (spongy moth) caterpillars: Hairy with blue and red dots. Major defoliators of oaks and other hardwoods.

Treatment approach:

1. For light infestations, healthy trees can tolerate losing up to 30-50% of their leaves in a single season and will re-leaf. No treatment needed.
2. Bt (Bacillus thuringiensis var. kurstaki) is an organic option that kills caterpillars but is harmless to beneficial insects, birds, and pets. Apply when caterpillars are small.
3. Spinosad is another organic option effective against caterpillars and sawfly larvae.
4. Horticultural oil sprays can smother eggs before they hatch.
5. Banding trees with sticky bands (like Tanglefoot) in early spring traps wingless female cankerworm moths climbing to lay eggs.

Repeated years of heavy defoliation can weaken and eventually kill trees. See Common Tree Pests for more detailed pest profiles.