Competition between the bean weevils Acanthoscelides obtectus and Zabrotes subfasciatus in common beans

Abstract

Bruchid pest species of common beans (Phaseolus vulgaris (L.)) in the Neotropics, the bean weevil Acanthoscelides obtectus and the Mexican bean weevil Zabrotes subfasciatus, exhibit niche and range overlaps, frequently co-occurring in bean storage facilities of south-central Brazil. Although both species are well-known and important pest species, the impact of their co-occurrence on their fitness has not been an area of interest. Here, we performed direct competition experiments between both of the species following an additive series. The competitive ability of each species in the presence of its heterospecific competitor was assessed using mixed (adult) insect infestations in common beans. The initial number of insects of one species was fixed at 50, whereas the other species had a number of insects ranging from 0 to 50. Therefore, each species with a variable number of insects started the competition at an initial proportion of 0, 0.17, 0.29, 0.38 and 0.50. The number of adults that emerged and the instantaneous rate of increase (ri) for each species were determined after one generation under direct competition. Our results indicate that the competitive ability of Z. subfasciatus was favored by the presence of its heterospecific competitor (A. obtectus), while the opposite pattern did not occur. Thus, colonization by A. obtectus was not significantly compromised by Z. subfasciatus, while Z. subfasciatus seemed to actually benefit from the presence of A. obtectus under the conditions of our study. These results indicate that these co-occurring species do not exhibit negative interactions in storage facilities, and the presence of A. obtectus may even favor Z. subfasciatus.