Defoliating Caterpillars on Willow Oak: Identification and Control

A homeowner asks what worm is defoliating their willow oak and how to eliminate it. Several caterpillar species feed on oaks, most controllable with the same approach.

Common Culprits

• Orange-striped oakworm
• Variable oakleaf caterpillar
• Red-humped caterpillar
• Fall cankerworm (spring)
• Sawfly larvae (resemble caterpillars but have 6+ proleg pairs)

Control Strategy

Foliar Spray: Acephate (Orthene)

Spray all foliage thoroughly. Caterpillars die when feeding or crawling on treated surfaces. Spray trunk as well—many species descend to pupate and will contact the insecticide.

Sawfly Exception

Sawflies feed gregariously (in groups) and may require different timing or products. Confirm identification if Orthene fails.

Perspective on Late-Season Defoliation

Hardwoods tolerate late-season defoliation well. Trees have already stored carbohydrates for next spring. Even complete defoliation in August/September rarely kills healthy oaks—they'll leaf out normally next year.

Practical Advice

If the tree is too large to spray effectively, treat the trunk to catch descending larvae. Don't panic—your tree will survive. Focus on overall health: water during drought, avoid root zone compaction, and monitor for repeat infestations next spring.