The evolution of larval competition and oviposition behaviour in the cowpea weevil Callosobruchus maculatus (Fab.) (Coleoptera: Bruchidae).

Callosobruchus maculatus (Fab.) is a beetle belonging to the family Bruchidae. The beetle is a serious economic pest of stored legumes in the tropics and subtropics. C. maculatus has been chosen for this study because it shows contrasting larval competition and oviposition strategies. Competition has been described in terms of the Avoid-Attack dichotomy (Smith and Lessells 1985). Research has been carried out to determine the Mendelian inheritance of larval competition and oviposition in the black Brazil and Yemen strains of Callosobruchus maculatus. The initial results were inconclusive because there was no significant (p 0.05) difference in the pattern of emergence between the Yemen and Brazil stains. Therefore, the experiments were repeated using the South India and Brazil strains. The results of the F1 generation were similar to results obtained by other workers (Messina 1991 and Toquenaga et al 1994), but the results of the F2 generation resembled the expected results for a single gene model in which Avoid behaviour was dominant, though the results were not clear cut. It was desirable to determine the inheritance of competition and oviposition to determine if evolution was acting on autosomal loci or a suite of genes. The cost of exploitation competition (E) was also estimated, using a method described by Colegrave (1995). The cost of passive exploitation has been described by Smith and Lessells (1985) and Smith (1990) and is a major determinant of the evolution of competition strategies in Callosobruchus maculatus. The Yemen, Brazil and South India strains were used in these experiments and experiments were conducted on three seed types. E 0.5 for all stains on the cowpea and adzuki beans but for the South India strain on mung E 0.5 and this may explain why Attack behaviour exists in the South India strain Finally the theoretical prediction of Smith and Lessells (1985), that competition evolves under frequency dependent selection, was tested by ratio experiments. These experiments demonstrated that selection was dependent upon the initial frequencies of the phenotypes in the population because populations that had an initial frequency of 30% Brazil or greater appeared to moving towards fixation of Avoid.