For parasitoids of herbivores, the two most important biotic factors that will influence their fitness are the host species that they attack and the plant species that the host feeds on. Variation in these two trophic levels because of different habitat characteristics may largely drive the evolution of the interaction between parasitoids and their hosts. Through transplant experiments with three plant-insect populations in Mexico, we examined the consequences of plant variation for the interaction between a bruchid beetle, Zabrotes subfasciatus (Boheman) (Coleoptera: Bruchidae), that feeds on bean seeds of the genus Phaseolus L. (Leguminosae: Phaseolinae) and one of its main parasitoids, Stenocorse bruchivora (Crawford) (Hymenoptera: Braconidae). Results revealed great variation in performance among parasitoid populations. Both the population of origin of the parasitoid and of the host plant influenced the performance of developing parasitoids as well as adult oviposition behavior. Wasps from the Atila population were more likely to parasitize the herbivore and developed faster than wasps from the other two populations, Malinalco and Tepoztlan. The results call be explained in part by the spatial distribution of the host plant, host availability, and seed quality in this population. Variation in performance among parasitoid populations decreased when wasps were exposed to their host in cultivated seeds. This could be caused by the better and less variable quality of this novel resource, and consequently, of the bruchid host. The results from this study indicate that both host plant quality and genetic variation among populations are crucial factors in determining the nature and evolution of the interaction between parasitoids and their herbivorous hosts.
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