Acanthoscelides obtectus (Say) and Acanthoscelides obvelatus (Bridwell) (Coleoptera: Bruchidae) are two sympatric beetle species that infest bean seeds (Phaseolus vulgaris L.; Fabaceae). Using field-sampled wild P. vulgaris pods and data of population density, body size, and parasitoid pressure for both species across elevation and temperature gradients in the Mexican Altiplano, we explored whether interspecific competition occurs between the two bruchids. We expected that population density, body size, and parasitoid pressure of A. obtectus and A. obvelatus to be inversely related to one another. We found that population densities of the two species differed among the elevations, but their body sizes were independent of expected density patterns. Moreover, the parasitoid emergence rate was correlated to A. obvelatus abundance but not A. obtectus abundance. Our data suggest that niches of A. obtectus and A. obvelatus overlap only to such extent that interspecific competition seems unlikely due to a) alternative hosts are available for A. obtectus, and b) P. vulgaris seeds and alternative legumes are a fairly common resource in the studied area.
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