Occurrence of the oak rough bulletgall wasp, Disholcaspis quercusmamma (Walsh) (Hymenoptera: Cynipidae), as a street tree pest in Colorado

The oak rough bulletgall wasp, Disholcaspis quercusmamma (Walsh) (Hymenoptera: Cynipidae), has become an increasingly important shade tree pest in Colorado with the expanded planting of its hosts, bur oak and swamp white oak. The insect produces woody galls on the twigs of the plant that are disfiguring and also produce honeydew that is attractive to nuisance bees and wasps. Emergence of the wasp from the galls began 20 October in 1992, but not until 29 November in 1993. Although oviposition occurs shortly after emergence, visible galling does not appear until the following July. The eurytomid Sycophila dubia (Walsh) was the only parasitoid observed to be associated with the insect, with parasitism rates between 14.5 and 68.1 percent recorded at three Fort Collins study sites. Diameter of normal, nonparasitized galls averaged 11.1 mm, significantly larger than for parasitized galls (9.1 mm). Great differences in amount of annual gall production occur among trees, suggesting that resistance to galling may be widespread.