Caterpillar Infestation on Mature Liquidambar

Managing Caterpillars on a 25-Year-Old Sweetgum

A Bakersfield, CA homeowner reports severe caterpillar infestation on a mature liquidambar (sweetgum) pruned annually. They ask if the tree's health is at risk, whether pruning helps, and if winter cold will eliminate the pests.

Impact Assessment

Caterpillars (likely various loopers, tent caterpillars, or bagworms) defoliate but rarely kill a healthy mature tree. One season of heavy feeding reduces growth and energy reserves but the tree typically re-foliates. Repeated annual defoliation over 2–3+ years can cause decline.

Pruning and Winter Effects

• Pruning: Removing infested branches helps only if caterpillars are concentrated there. General pruning does not reduce populations.
• Winter cold: Many species overwinter as eggs on the tree or pupae in soil; cold alone won't eliminate them.

Management Strategy

First year: Do nothing. Natural predators (birds, wasps, diseases) often crash populations.

Recurring problem (2+ years): Apply a systemic soil drench in late spring (April–May) with imidacloprid (e.g., Bayer Advanced Tree & Shrub Insect Control). Roots absorb it; caterpillars die when feeding on treated foliage. One application lasts the season.

Cultural Practices

• Water deeply during drought to reduce stress.
• Mulch 3 inches out to drip line.
• Avoid broad-spectrum foliar sprays that kill beneficial insects.