Poplar Twiggall Fly

Naming

Euhexomyza schineri
Agromyza schineri Giraud, 1861
Hexomyza schineri

Distribution

Over most of the range where Quaking Aspen (Populus tremuloides) and other Poplars grow.

Reproduction

The female deposits eggs on developing aspen shoots, usually near a bud. The feeding of the maggot causes the twig to form a gall that’s noticeable by fall. The larvae feed all summer and overwinter as a maggot inside the gall and pupate either late winter or early spring.

The poplar twiggall fly overwinters within the gall as a full-grown, yellow-green maggot. Pupation occurs within the gall in late winter or early spring. The majority of the pupae then drop to the ground.

At the time that new growth forms, the adult flies emerge from the pupae and become active. Adults are stout-bodied, shiny, dark flies about 1/6 inch long. During the day, they rest and sun themselves on leaves. After mating, females move to developing twigs and insert eggs into the stems. The larvae hatch from these eggs and produce the distinctive swelling in response to their feeding.

Areas below buds appear to be particularly favored sites for galls. As the stems continue to grow, the area where eggs were laid becomes increasingly swollen. At first, the swelling involves a fairly indistinct enlargement. However, within two months the full sized gall is usually present.

The developing gall fly is a greenish-yellow maggot that grows slowly within the gall all summer. It is difficult to find until late summer and fall, when it becomes full-grown, filling a small cavity within the swollen area of the twig. Individual galls typically contain two to three larvae.

Food

Quaking Aspen (Populus tremuloides) and other Poplars.

Predators

A small parasitic wasp (Eurytoma contractura) parasitizes and commonly kills large numbers of the poplar twiggall fly. Observed parasitism typically ranges from 20 to 30 percent but has exceeded 80 percent in some years. The adult wasps emerge from the galls about two weeks after the adult flies first appear.

Predation of the pupae by chickadees and other birds also occurs in spring. Their activity is evident by some tearing around the exit hole by the beaks of the feeding birds.