Reproductive interference explains the competitive disparity between congeneric beanweevils Callosobruchus maculatus and Callosobruchus chinensis

The system including the congeners C. maculatus and C. chinensis has long been a model for competition studies. Members of these two pest species often cohabit facilities designed for the storage of dried legumes, a crucial nutritional resource for the larvae of both species. In both natural and laboratory settings it has been observed that C. chinensis routinely drives C. maculatus to extinction. These observations convinced many ecologists that the trophic relationship between the two species presents an unequivocal case of competitive exclusion. Several predictively successful mathematical models based on resource competition alone accordingly estimated the competitive disparity between these two bean beetle species, suggesting that individual C. chinensis are approximately four times better competitors for a shared resource than their average C. maculatus counterparts. Recent studies (Kishi et al. 2009; Kyogoku and Nishida 2012, 2013; Kishi and Nakazawa 2013; Kishi and Tsubaki 2014) have called into question this apparently straightforward conclusion since there are few if any characteristic life history traits or behavioral mechanisms associated with exploitative or interference competition that would explain the competitive disparity. The authors of these studies contend that reproductive interference---a negative interspecific sexual interaction that reduces the fitness of at least one of the species involved---is a largely ignored but equally consequential factor when it comes to explaining interspecific population dynamics. This study tests for the presence and explanatory importance of reproductive interference on per capita female fitness by simultaneously manipulating conspecific density and operational sex ratio in the presence (and absence) of heterospecific males. The experimental design also reveals the operative mechanism(s) behind reproductive interference. Results of this study clearly support outstanding claims for the causal significance of reproductive interference in this system.