UC San Diego

The western honey bee (Apis mellifera) is a super-generalist in terms of floral visitation, but its effectiveness as a pollinator varies greatly. This variation has called attention to the importance of native insects as pollinators in agricultural systems and has raised concerns that floral visitation by honey bees can adversely impact some plant species. To clarify what factors underlie this variation, we use multiple approaches to compare the effectiveness of specialist (Peponapis pruinosa) and generalist (honey bees) pollinators of cultivated squash (Cucurbita pepo). Previous studies report conflicting results on the effectiveness of P. pruinosa and A. mellifera as pollinators of C. pepo but have not typically considered how specific foraging behaviors (e.g., degree of contact with the stigma or anthers) influence their effectiveness. To resolve this uncertainty, I conducted single and multiple visit trials on C. pepo plants at the UC San Diego Biology Field Station to examine how visitation and behavior of these two bee species affect pollinator effectiveness. Visitation by female squash bees was found to have a greater positive effect on pollen removal, pollen deposition, fruit set, and seed set (i.e. seed number) compared to honey bee visitation. These findings were consistent with the behavior of the pollinator species. Cumulative duration of stigmatic and anther contact by female squash bees had a greater effect on pollen deposition and pollen removal compared to the same behaviors in honey bees. Previous studies on cultivated squash conducted in agricultural settings have found that the effectiveness of honey bees and squash bees appears more similar than what we report here. These contrasting results could be explained by differences in resource availability in large scale agricultural versus small scale plots. At our study sites, for example, honey bees primarily collected nectar from cucurbits and foraged for pollen on other plant species in the immediate environment.