Gall Morphology and Community Composition in Asphondylia flocossa (Cecidomyiidae) Galls on Atriplex polycarpa (Chenopodiaceae)

Asphondylia flocossa Hawkins (Diptera: Cecidomyiidae) and its associated community of natural enemies were sampled in galls on Atriplex polycarpa (Torrey) Watson (Chenopodiacedae) at 3 locations north of Phoenix, AZ, between October 1994 and March 1995. Galls collected in the autumn had dramatically smaller diameters and shorter trichomes than galls collected in the spring. Asphondylia flocossa were more likely to emerge from larger than average galls with a thick trichome layer in both the autumn and winter samples. The different species of natural enemies each emerged from galls with different characteristics. There was significant variation among taxa of natural enemies for both gall diameter and trichome depth in both the spring and autumn samples. The community of natural enemies also varied significantly among populations and more markedly between seasons. Both Asphondylia flocossa and Rileya tegularis Gahan (Hymenoptera: Eurytomidae) were much more abundant in spring samples than in autumn samples. In contrast, Torymus cappilaceus (Huber) (Hymenoptera: Torymidae) and an undescribed species of Galeopsomyia (Hymenoptera: Eulophidae) were abundant in the autumn samples but almost non-existent in the spring samples. The first 2 species tended to emerge from relatively large galls while the latter 2 species tended to emerge from small galls. Torymus umbilicatus (Gahan) (Hymenoptera: Torymidae) was the only parasitoid abundant in samples from both seasons. This species emerged from galls with the widest range of diameters and trichome depths of any species, both within and between seasons. The feeding biology of the individual species coupled with seasonal and temporal covariation in gall morphology makes natural enemy control of gall size unlikely. The data is consistent with the hypothesis that gall morphology influences insect community composition.