Contrasting the effectiveness of specialist and generalist pollinators of cultivated and wild cucurbits

Background/Question/Methods: 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 (squash bees) and generalist (honey bees) pollinators of cultivated squash (Cucurbita pepo) and wild buffalo gourd (C. foetidissima) in southern California. We studied these interactions at two study sites: an experimental plot with hand-planted C. pepo and a riparian woodland with wild C. foetidissima. At both sites, more than 80% of visitors to squash flowers were either native Peponapis pruinosa (eastern cucurbit bee), which specialize on plants in the Cucurbitaceae, or non-native honey bees. In addition, we compare our results with previous work on cucurbits in agricultural systems to examine how environmental context can influence estimates of pollinator effectiveness.

Results/Conclusions: In the Cucurbita pepo system, single-visit pollinator effectiveness trials revealed significantly higher fruit set for flowers visited by female squash bees compared to flowers visited by honey bees (85% versus 12%, respectively). Compared to honey bees, female squash bees spent more than seven times longer in contact with squash stigmas and deposited more than ten times more pollen per stigma. Trials based on multiple visits further illustrated that visitation by female squash bees was more effective in terms of stigmatic pollen deposition and the likelihood of fruit set compared to visitation by honey bees. In the C. foetidissima system, single-visit pollinator effectiveness trials again revealed significantly higher fruit set for flowers visited by female squash bees compared to flowers visited by honey bees (50% versus 19%, respectively). 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 agricultural versus non-managed systems. 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.