FEMALE-BIASED SEX-RATIOS IN RESPONSE TO INCREASED DENSITY IN A BRUCHID SEED PREDATOR - A CONSEQUENCE OF LOCAL MATE COMPETITION

Data were collected concerning the effects of clumped oviposition on growth, development and emergence patterns for the common bean weevil, Acanthoscelides obtectus Say (Coleopteria: Bruchidae). These data support previous evidence of significant negative effects of high density within seeds of host Phaseolus (Fabaceae) species on larval survivorship, emergence time, and size of adults. In two separate field experiments using P. coccineus L., sex ratios of emerging adults were observed to shift toward significant female-bias with increasing larval density within seeds. Because total (egg-to-adult) mortality was poorly related to sex ratio changes, differential mortality was most likely associated with overcrowding and competition among late instar larvae. Shifts in sex ratio via intraspecific competition and differential mortality are discussed in relation to oviposition and mating behaviors of this bruchid, and in relation to conditions of local mate competition that may favor female-biased ratios.