Pest description and damage Adults are stout, shiny, dark flies about 0.16 inch in length. The developing gall fly is a greenish-yellow maggot that causes galls to form on the current season's twigs. Obscured by leaves, the original galls rarely are noticed until leaves fall in autumn. However, the galled tissues continue to grow and swell. Ultimately, galls become large knots on trunks and larger branches, giving the plants a gnarled, bonsai-like appearance. During subsequent years, the galled area is incorporated into the growing twigs and branches and ultimately may appear as large swollen bands on trunks and branches. Although these old injuries produce a permanent disfigurement, they do not seem to threaten tree health. Serious galling has been limited to aspen. However, small numbers of galls sometimes are seen on other Populus species. Galling is most common on younger trees that produce a lot of succulent new growth.
Biology and life history 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. Most of the pupae then drop to the ground. As new leaf growth begins, the adult flies emerge from the pupae and become active. 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 are laid becomes increasingly swollen. At first, the swelling involves an indistinct enlargement. However, within two months, the full-sized gall is usually present. The developing gall fly maggot grows slowly within the gall all summer. It is difficult to find until late summer and fall, when it grows rapidly, filling a small cavity within the swollen area of the twig. Individual galls typically contain two to three larvae. There is one generation per year.
Removal of galls has limited potential for control. Pruning often requires substantial branch destruction and creates wounds that can allow pathogens to enter. Furthermore, this practice can be counterproductive if done after flies emerge in late winter or early spring. Late pruning may remove only those galls that contain the natural enemies (including parasitic wasps) of the poplar twiggall fly. Because problems with poplar twiggall fly are most severe in succulent aspen, do not over water or fertilize plantings. If the vigor of aspen growth can be moderated, then gall production can be slowed.
Parasitic wasps may give some control. In addition, birds feed on the larvae.
No effective chemical controls have been developed.
For more information
See "Gallmakers" in:
Cranshaw, W.S. 2011. Poplar twiggall fly. Colorado State Univ. No. 5.579 (https://extension.colostate.edu/docs/pubs/insect/05579.pdf)