The relationship between oviposition preference and offspring performance is critical to the evolutionary ecology of the interactions between herbivorous insects and plants, but is not well understood. To improve our understanding of this relationship we must critically evaluate the relative importance of the selective forces or causes of mortality that affect developing larvae. We documented a strong relationship between oviposition preference and larval performance for 2 species of galling tenthredinid sawflies, a petiole galler,Euurasp., and a leaf galler,Pontaniasp., nearP. pacificaon the arroyo willow,Salix lasiolepis. There was a correlation between attack and survival on different ramet ages for both sawfly species. Mortality caused by plant resistance by ramet age primarily explained the pattern of attack of both sawflies. The effect of plant resistance was strongest on willow shoots on very young ramets. This effect decreased rapidly as ramet age increased and then increased relatively slowly as ramets aged, becoming very strong on the oldest ramets. Mortality caused by plant resistance was 8-fold greater than mortality caused by natural enemies forEuuraand 3.2-fold greater for Porltania. Natural enemies may playa lesser role in the population dynamics of the sawflies that may be subsidiary to the plant-herbivore interaction. We reason that selection has resulted in ovipositing females flying primarily in the upper parts of willow clones and avoiding the highly resistant younger ramets common in the lower parts of clones. Within the upper parts of clones, the evolution of female behavior that discriminates against shorter, slower growing shoots on older ramets would be selected for by the decreasing survival of offspring on these shoots.