Brownheaded Ash Sawfly

How to identify it

• Larvae are pale green worm-like caterpillars with some light banding.^[1]
• Young larvae leave small pinhole wounds inside the leaf; adults are small black wasps that don't sting.^[1]

Where it lives

Native to the eastern US, where it's rarely a problem, it appeared in Colorado's Arkansas Valley by the early 1980s and has since spread along the Front Range and onto the Western Slope.^[1]

US map: public records from 10 states since 1990 (GBIF). View the map

Biology and life cycle

Full-grown larvae overwinter in cocoons in soil under ash; adults emerge by late April and lay eggs in expanding leaf edges.^[1]

Larvae finish feeding within two to three weeks of hatching; there's one generation a year.^[1]

Damage and signs

• Repeated heavy defoliation stresses trees, and wandering larvae can be a nuisance.^[1]

Health risk

It feeds on plants; CSU Extension mentions no risk to people.^[1]

Control

Prevention

Watch for pinholes early; healthy trees benefit from control only if at least 25 percent of foliage is threatened.^[1]

Traps and physical control

A strong jet of water knocks many larvae off, and few climb back.^[1]

Pesticides

Insecticidal soap sprayed on the larvae works; Bt and neem don't appear effective.^[1]

Common questions

What are the green worms eating my ash tree in May?

Likely brownheaded ash sawfly larvae, which feed on ash leaves in May and early June.^[1]

Will Bt control ash sawflies?

No; CSU Extension says Bt and neem don't appear effective against it.^[1]

Sources

1. Colorado State University Extension, “Brownheaded Ash Sawfly (Fact Sheet 5.586).” extension.colostate.edu/resource/brownheaded-ash-sawfly/ (accessed 2026-10-06)
2. iNaturalist, “Tomostethus multicinctus taxon page (order and family).” www.inaturalist.org/taxa/851506 (accessed 2026-10-06)