Pest profile

Poplar Twiggall Fly

Hexomyza schineri · Diptera: Agromyzidae[2]

Poplar twiggall fly larvae cause smooth, knot-like galls on aspen twigs that keep growing for years.[1]

Poplar twiggall, a smooth brown knot-like swelling on an aspen twig
Poplar twiggall, a smooth brown knot-like swelling on an aspen twig. Photo: Jonny Tostenson, public domain (CC0), via iNaturalist

How to identify it

  • Galls are smooth swellings on the current season's twigs, usually noticed only after leaves fall.[1]
  • Adults are stout, shiny, dark flies about 1/6 inch long; the larva is a yellow-green maggot.[1]

Where it lives

A native insect, it became much more common from the 1980s and has spread through Colorado and much of the Rocky Mountain region.[1]

US map: public records from 15 states since 1990 (GBIF).

Biology and life cycle

Full-grown maggots overwinter in the gall and pupate in late winter or early spring; most pupae drop to the ground.[1]

Adults emerge as new growth forms, and females insert eggs into developing twigs.[1]

Damage and signs

  • Old galls become swollen bands on branches; they disfigure trees but don't seem to threaten tree health.[1]
  • Serious galling is limited to aspen, mostly younger trees with lots of succulent growth.[1]

Health risk

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

Control

Prevention

Don't overwater or overfertilize aspen; slowing succulent growth slows gall formation.[1]

Sanitation

Pruning galls has limited value and, done late, removes the galls holding parasitic wasps.[1]

Biological control

A small parasitic wasp often kills 20 to 30 percent of the flies, sometimes over 80 percent, and chickadees eat pupae.[1]

Common questions

What are the round knots on my aspen twigs?
They're likely galls made by poplar twiggall fly larvae.[1]
Will twiggalls kill my aspen?
CSU Extension says old galls disfigure trees but don't seem to threaten their health.[1]

Sources

  1. Colorado State University Extension, “Poplar Twiggall Fly (Fact Sheet 5.579).” extension.colostate.edu/resource/poplar-twiggall-fly/ (accessed 2026-10-06)
  2. iNaturalist, “Euhexomyza schineri taxon page (order and family).” www.inaturalist.org/taxa/1006298 (accessed 2026-10-06)

Updated 2026-10-14. Built from sourced records. Spot an error? Tell us.