UC San Diego

Differences between specialist and generalist pollinators provide insight into the evolution of specialization in plant-pollinator interactions. Plants of the genus Cucurbita (Cucurbitaceae) are visited by both generalist pollinators (e.g., Apis mellifera) and by specialist pollinators (e.g., Peponapis pruinosa). Previous studies have estimated pollinator effectiveness of Apis mellifera (honey bees), and Peponapis pruinosa (squash bees) in agricultural Cucurbita species, but none have investigated behavioral differences that underlie variation in effectiveness. In the summer of 2017, I conducted single visit pollinator effectiveness trials on 21 acorn squash (Cucurbita pepo) plants at the UC San Diego Biology Field Station in San Diego County, California to link interspecific behavioral differences of pollinators to their effectiveness. Female squash bees spent more than seven times longer per single visit in contact with squash stigmas and in turn deposited more than ten times more pollen compared to generalist honey bees. For all trials combined pollen deposition and fruit set increased with time bees spent on receptive stigmatic surfaces. Single visits by squash bees were more likely to result in fruit set compared to honey bees (85% vs. 12%). In terms of single visits, these results indicate that Apis mellifera are less effective at pollinating acorn squash compared to female Peponapis pruinosa. The results of this study differ from previous studies because Apis mellifera on our study site forages for pollen from a variety of different plant species in the surrounding landscape. In agricultural systems, in contrast, honey bees may have fewer options for pollen foraging.