River Birch Sawfly Larvae: Not Caterpillars, Require Different Control

Those “caterpillars” defoliating your river birch are actually sawfly larvae—primitive wasp relatives that look like caterpillars but have more prolegs and a different lifecycle. This distinction matters because Bt (Bacillus thuringiensis), a common organic caterpillar control, does not affect sawfly larvae.

Effective Control Options

• Carbaryl (contact insecticide): Spray entire tree for good control.
• Imidacloprid (systemic): Apply to soil around tree before feeding activity begins.
• Insecticidal soap: Safer near water features; spray every 7 days during larval activity.

Critical Warning for Pond Owners

Carbaryl and imidacloprid are moderately toxic to fish. If your river birch is near a goldfish pond, insecticidal soap is the preferred option to protect aquatic life.

Identification Tip

Sawfly larvae have 6 or more pairs of fleshy prolegs; true caterpillars have 5 or fewer. Check the underside of the larvae to confirm.