Evidence is growing that host–associated genetic differentiation (HAD) is an important factor facilitating the extraordinary diversification of phytophagous insects. However, relatively little attention has been paid to how parasitoids of these insects respond to genetic and ecological diversification of their hosts, and whether HAD may “cascade” upward through trophic levels.  Here I describe evidence of cascading HAD in two parasitoids of galling insects on goldenrods and present a conceptual tri-trophic model for how ecological processes may drive this evolutionary divergence. I examine the consequences of HAD in parasitoids for the interpretation of observed ecological patterns, and I briefly discuss evidence for the role of this form of diversification in generating the great diversity of insect parasitoids.