We examined the hypothesis that plant traits predictably affect community structure and diversity via effects on a keystone species. In cottonwoods, host plant genetic variation arises due to naturally occurring interspecific hybridization between Fremont (Populus fremontii) and narrowleaf cottonwood (P. angustifolia). Host plant selection by the poplar leaffolding sawfly (Phyllocolpa bozemani) was significantly affected by tree cross type (i.e., Fremont, F1, backcross hybrid, and narrowleaf cottonwood), and this selection in a common garden corresponded with the natural distribution of sawflies in the wild. We then experimentally examined the effect of sawflies on the surrounding arthropod community by choosing two branches on a tree and removing all sawfly galls from one of the branches. When we did this, we found that sawflies positively affected arthropod species richness and abundance. This positive effect of sawflies on the surrounding arthropod community was likely through their creation of habitat (i.e., an open gall that provides shelter). Also, when we performed NMDS ordination and ANOSIM analysis on arthropods found on sawfly and control branches, we found that arthropod community composition was significantly different. Therefore, sawflies not only increased arthropod species richness and abundance where they were present, but their presence altered community composition. These results show that host plant traits that affect the distribution of a keystone species extend to affect an entire community of arthropods, and argue for the preservation of host plant genetic diversity as a means of conserving arthropod diversity.