Dispersal Polymorphism of Callosobruchus maculatus (Coleoptera: Bruchidae): Variation among Populations in Response to Crowding

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Abstract
Adults of a seed beetle, Callosobruchus maculatus (F.), are polymorphic. Low-density populations consist of relatively sedentary, “normalâ” adults; larval crowding within host seeds triggers the development of morphologically distinct, dispersing or “active” adults. Three geographically separate strains reared at four densities showed significant variation in the propensity to produce “active” beetles. Variation among strains could not be explained by differences in the threshold at which larvae respond to crowding. For all strains production of the active morph did not simply increase with increased crowding, as the percentage of progeny that were active declined at the highest densities. In addition to stimulating the production of active beetles, crowding delayed the emergence and reduced the mean weight of normal progeny. Compared to normal beetles, active beetles emerged later and lived 3- to 4-fold longer, but most failed to reproduce in the laboratory.