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Pollinator Effectiveness of Peponapis pruinosa and Apis mellifera on Cucurbita foetidissima


Differences between specialist and generalist pollinators have long been a topic of interest for biologists. Plants of the genus Cucurbita (Cucurbitaceae) are visited by generalist pollinators, such as honey bees (Apis mellifera), and by specialist pollinators, known as squash bees (e.g., Peponapis pruinosa). Previous studies have examined pollinator effectiveness between Apis and Peponapis on agricultural Cucurbita species, but few have investigated the effectiveness of these pollinators in a non-agricultural context. In the summer of 2017, I conducted single visit pollination trials on 22 buffalo gourd (Cucurbita foetidissima) plants at Los Peñasquitos Canyon Preserve, San Diego County, CA to measure pollinator effectiveness between honey bees and squash bees. The percentage of fruit set from single visits made by female P. pruinosa (57.9%) was higher than that of male P. pruinosa (23.5%). Single visits by Apis never resulted in successful fruit set. Control fruit, by comparison, set at a percentage of 85.7%. The average weight of seeds from fruit resulting from female P. pruinosa single visits was significantly higher (by 28.6%) than for single visits by males, but neither seed number nor fruit volume differed between male and female single visits. These results indicate that Apis and male P. pruinosa are less effective at pollinating buffalo gourd compared to female P. pruinosa. Differences in how these pollinators gather nectar from Cucurbita may be one explanation for differences in fruit set. These differences may be exaggerated in non-agricultural systems, where generalist pollinators like Apis have a variety of floral resources to choose from.

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