SEED PREDATION BY THE BEAN WEEVIL ACANTHOSCELIDES-OBTECTUS ON PHASEOLUS SPECIES - CONSEQUENCES FOR SEED SIZE, EARLY GROWTH AND REPRODUCTION

We examined potential implications of seed predation by the common bean weevil, Acanthoscelides obtectus for seed size variation in cultivated beans (Phaseolus vulgaris L., P. coccineus L. and P. lunatus L.) grown in common gardens. Significant seed size variation existed among different plants, among pods of differing seed number within plants, among successive seed crops within plants, and among seeds within pods. Within-plant sources of seed size variation were equally or more important than among-plant sources. Initial seed size strongly influenced early growth under laboratory and field conditions, and the advantage in early growth due to large seed size carried over to enhance subsequent plant reproduction in the field. Predation-related shifts in seed and seedling size distributions apparently occurred without active selection by the bruchid. Oviposition that was evenly distributed among pods, but clumped within one or a few seeds within pods, allowed a greater proportion of seeds to escape from many-seeded pods. This, coupled with negative size/number relationships within and among plants, resulted in shifts toward smaller mean seed size. Due to nearly ubiquitous embryo damage by bruchid larvae, seed size of host plants was not related to tolerance to predation. We conclude that indirect size-selective seed predation can have significant effects on maternal plant fitness both by altering the number of progeny produced, and by altering the fitness of surviving progeny.