No abstract

Host specialization among closely related herbivores is common and is therefore a major theme in ecology. Despite its ecological importance, no general explanatory framework for host specialization is currently available. We introduce a new model of the evolution of host specialization in herbivorous insects. We use a twodimensional lattice comprising 32 9 32 cells. Moreover, the model incorporates reproductive interference, defined as any negative outcomes resulting from interspecific mating, between two species of herbivores feeding on two different host plants as well as ordinary resource competition and differences in host suitability. Our simulation showed that reproductive interference, together with other factors such as host-related performance and resource competition, can actually drive the evolution of host specialization and thereby host partitioning between herbivorous insects during secondary contact. Host specialization arises particularly when levels of both reproductive interference and resource competition are intermediate. In contrast, host specialization seldom occurs if reproductive interference is absent. Thus, reproductive interference is as key to specialization as is spatial partitioning, which was not regarded as a realistic outcome when only resource competition was considered.

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Intra-and interspecific larval interactions that take place in a host body were investigated for two tachinid flies Epicampocera succincta and Compsilura concinnata (Diptera: Tachinidae) parasitizing Pieris butterfly larvae. E. succincta, a specialist on Pieris butterflies, showed contest-type intraspecific competition, eliminating all the other conspecific larvae. On the other hand, an extreme generalist parasitoid C. concinnata exhibited scramble-type competition, sharing the host with other conspecifics and suffering reduced body size as a result. However, when these two species occurred together in a single host, C. concinnata had a much higher chance of survival. Moreover, C. concinnata could often survive in the presence of a parasitoid wasp Cotesia glomerata (Hymenoptera: Braconidae) while E. succincta could not. The high tolerance of C. concinnata could be attributable to its being an extreme generalist" To attack and survive on many different hosts, one has to be able to deal with various competitors. The competitive inferiority of the specialist E. succincta, on the other hand, may be a result of relatively recent encounter with those competitors.