Cowpea is an important grain legume crop in Africa. Cowpea flowers are capable of self-fertilization, which might be the most frequent type of reproduction. Nevertheless, the rate of cross-pollination could vary between 1% and 10%, depending on the populations of bumblebees or domestic bees that are present, the climate and the cultivar. The aim of this study was to identify and assess the efficiency rate of the pollinators of the wild cowpea, Vigna unguiculata subsp. unguiculata var. spontanea, the progenitor of the domesticated cowpea Vigna unguiculata subsp. unguiculata. Pollination study was conducted by observing patches of flowers (average 10–20/m2) from dawn to midday, approximately 05:00–12:30 hr for three years. All insects visiting the flowers were noted and identified to species level, where possible, and their foraging behaviour was monitored. Pollen grain deposit and pod set were estimated using single bee visits on plants with strictly outcrossing flowers. Major flower visitors were large bees of the family Megachilidae and the genus Xylocopa (Apidae, Xylocopinae). Xylocopa caffra (Linnaeus) was the most abundant bee species, accounting for 59% of total bees observed and 58% of total flower visited. Pod set was higher in visits by megachilid bees compared to Xylocopa bees. Within Xylocopa, there is a good correlation between size and efficiency (r = .95, p = .0477), the larger insects being more efficient. Honeybees, small bees (Ceratina and Nomia species) and Lepidopterans also visited flowers but without any effect on pollination. Cowpea pollination system is specialized, and pollinators are restricted to Megachilidae and Xylocopa species. Variation in pollen load deposit and pod set reveals that pollination success depends on the insect species involved and their size. Megachilids are the most efficient pollinators, especially since they are able to promote outcrossing more efficiently than Xylocopa species.
