Diet maintenance and evolution in Euphydryas editha populations

Insect herbivores exhibit narrow patterns of resource use compared to vertebrates and predaceous insects. This may arise because of the small size of insect herbivores, enabling them to develop to maturity on single host individuals. Use of only one resource in a particular locality may be considered normal for insect herbivore populations. Categories of factors are identified that may lead to use of more than one resource by herbivorous insect populations. In one Euphydryas editha (Lepidoptera: Nymphalidae) population, use of more than one resource may be assisted by genetic diversification within the butterfly population. Some individuals are both behaviorally and physiologically specialized to utilize one host species, while other individuals are specialized to use another host species. In a second population, spatial heterogeneity exists such that one host species is used in one area, and a different host species is used in a second area. Females varied in post-alighting oviposition preference and tended to remain in areas with preferred hosts but left areas possessing less preferred hosts. Non-random movements can increase the degree of dietary divergence between insects in adjacent habitats. A comparison of several E. editha populations revealed that some had incorporated a European weed into their diet and had undertaken an evolutionary change in oviposition preference. Different local conditions may lead to divergence between populations, each population adapting to local host plant availabilities and suitabilities.