The distributional patterns of galling sawflies (Hymenoptera: Tenthredinidae), on a single host plant, Salix lasiolepis, were tested for patterns of nestedness. Data were collected over 2 years from sites spanning the host-plant range: Arizona to Washington State. These sawflies are highly host-species-specific. Similarities in sawfly composition across sites were examined under such biogeographic themes of distance, latitude, drainage basin, and connectivity of habitats. We found a highly nested subset structure in this sawfly assemblage. Interestingly, these assemblages were not nested based on direct distance apart from each other or by latitude or drainage basin, but by connectivity of their habitat. Furthermore, the nested subset pattern seen in these sawflies was most likely extinction-dominated due to the low dispersal distance of species in this taxon and the highly fragmented habitat of the host plant. The predicted species extinction order was as follows: leaf-edge galler (Phyllocolpa sp.), leaf-lamina galler (Pontania spp.), stem galler (Euura lasiolepis), petiole galler (Euura sp.). Conservation planning based on our data would suggest that ‘area per se' may not be the only unit to consider because extinction risk in the isolated habitats or fragmented drainages was extreme, therefore, connected habitats or long coherent drainages are essential for sawfly persistence.