RAPID evolution of host association is now occurring independently in two populations of the host-specialist butterfly Euphydryas editha, each of which has recently incorporated a novel host species into its diet. The reasons for these episodes of rapid evolution lie in human land use practices: logging in one case and cattle ranching in the other. In contrast to other insects that have used tolerance of human activities to expand their ranges into disturbed habitats1–3, these rare butterflies have remained at their original sites and evolved adaptations to the changes occurring at those sites. At both sites, the proportion of insects preferring the novel host has increased, in one case clearly because of genetic changes in the insect population. This process is now starting to generate insects that refuse to accept their ancestral host, foreshadowing a new problem in conservation biology. By adapting genetically to human-induced changes in their habitat, the insects risk becoming dependent on continuation of the same practices. This is a serious risk, because human cultural evolution can be even faster than the rapid genetic adaptation that the insects can evidently achieve.