We compared the within-clone distribution and survival patterns of a tenthredinid sawfly,Euura exiguaeSmith, with those of a chrysomelid leaf beetle,Disonycha pluriligata(Leconte). Both of these insects are closely associated with the same host species, coyote willow (Salix exiguaNuttall), but the first is noneruptive and the second is eruptive in nature, and we have not discovered coexisting populations. Sawfly galls were significantly more abundant and larvae survived significantly better on young willow ramets than on older ones. However,Disonychaadults were more evenly distributed throughout willow clones and larvae survived relatively well on all ramet age classes tested. We describe essential differences between eruptive and noneruptive insect species based on preference and performance characteristics, and basic differences in life history. The key features in the noneruptive species probably are a tight relationship between oviposition preference for long shoots on younger plants, coupled with high larval performance on such shoots. Vigorous shoots provided a limited resource, constrainingEuurato show latent population dynamics. For the eruptive species, oviposition preference and larval performance were dissociated because females laid eggs in the ground, and could not evaluate plant quality relevant to larval feeding. Larvae then fed generally on almost any plant they climbed, and they defoliated many plants. With the high carrying capacity provided to larvae by all foliage in a willow stand, populations were not constrained by plant resources to remain at low densities, and they erupted.