The poplar twiggall fly has become an increasingly common problem throughout Colorado’s Front Range. The gall, or “swelling,” is located on the twigs and branches of Populus tree species, especially aspen. The fly overwinters in the gall as a fully developed maggot. In late winter or early spring, the maggot drops to the ground and pupates. Adult flies emerge in spring when new leaves and shoots develop on the host plant. After mating, female flies lay eggs by inserting them into plant tissue. They produce one generation of flies per year.
The combination of egg laying and larval feeding causes the gall to form. The damage caused by the twiggall fly is mainly aesthetic, although the area where the gall forms may become weak, causing those twigs to break during storms and high winds.
Natural controls, including parasitic wasps and some birds, help keep poplar twiggall fly populations in check. Control with insecticides has proven unsuccessful or marginal. The best approach is simply to consider the gall a part of the tree’s physical appearance.
For more information, see the following Colorado State University Extension fact sheet(s).