Adaptation to a novel host modifies host discrimination by the seed beetle Callosobruchus maculatus

Females of the seed beetle Callosobruchus maculatus avoid adding eggs to seeds that already bear eggs. Geographical variation in this behaviour has been thought to depend on differences in host size. In populations that attack small-seeded legumes, only one or two larvae can develop within a seed, and females are especially adept at detecting and rejecting occupied (egg-laden) seeds. We performed a mass-selection experiment in which replicate lines of a population associated with a small host (mung bean, Vigna radiata) were either maintained on this host or were transferred to a larger host (cowpea, Vigna unguiculata) that can support several larvae per seed. After more than 40 generations, we estimated the strength of host discrimination by presenting females a choice of egg-free and egg-laden seeds, and by quantifying how uniformly females spread their eggs among egg-free seeds. Compared to females maintained on mung bean, females from cowpea lines were more likely to accept occupied seeds in choice tests. They also distributed their eggs less uniformly, especially when cowpea served as the test host. Cowpea lines thus evolved to resemble populations that have long been associated with the larger host. A separate study showed that weaker host discrimination in the cowpea lines was accompanied by a decline in larval competitiveness, which may have further relaxed selection for avoidance of occupied hosts. Our results demonstrate that switching to a novel resource can produce rapid and predictable changes in a fitness-related insect behaviour. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour