Notes on the biology and control of the poplar twiggall fly: Hexomyza schineri (Giraud) (Diptera: Agromyzidae), an emerging pest of aspen in Colorado

Within the past decade, populations of the poplar twiggall fly, Hexomyza schineri (Giraud) (Diptera: Agromyzidae), have greatly increased along the Front Range of Colorado. The insect produces smooth, knot-like galls on the current year twigs of Populus spp., particularly aspen. These swellings continue to expand even after the insect emerges from the gall the following spring. The average length of galls produced in aspen was 12.91 mm with an average diameter of 8.48 mm. The average number of live H. schineri larvae and pupae was 1.4/gall. However, number of insects per gall was not correlated to gall size. Only one species of parasitoid wasp, Eurytoma contractura (Bugbee) (Hymenoptera: Eurytomidae), was reared from the galls and percent parasitism averaged 21.5 percent. Additionally, predation by chickadees, particularly of the pupal stage, was observed. A wide range of galling was observed in an established planting of nine aspen clones, suggesting that development of resistant cultivars may be a useful control approach. The color fluorescent green was most attractive to adult flies and could be used to monitor adult insects but was not sufficiently attractive to be used in trap out strategies.