Submitted to: Journal of Apicultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2008
Publication Date: N/A
Interpretive Summary: Wild squash bees have all advantages we ascribe to an effective crop pollinator. They are abundant, competitive, efficient, faithful and fast. Squash bees clearly benefited from both ancient and modern squash cultivation and helped in the success of early American Agriculture. Squash bees have colonized diverse habitats from Canada to Brazil. Yet pollinator diversity is inherently low for New World Cucurbita, as squash bees are the predominant pollinators. Because female squash bees gather pollen before sunup, an hour before any other bee species, they become strong competitors, keeping less specialized foragers from host flowers. Squash bees are also efficient pollinators of squash; six floral visits by both male and females set marketable fruits, but they often visit a flower more than 60 times in a day. Male squash bees do contribute to fruit quality; however, females are essential for sustained crop production and seed set. Of the 50 US farms we surveyed, ~75% had enough squash bees to set a full squash crop. Add in all other native pollinators, then wild bees as a whole worked the entire squash crop (=98% of farms). So when a crop shows signs of poor fruit set, too few bees might be a cause, but high crop stress might also be a problem.
Technical Abstract: Wild squash bees have all five traits ascribed to the most effective crop pollinators. They are abundant, competitive, efficient, faithful to a specific crop and fast. Shared pollinator surveys covering 2,700 ha of US squash and pumpkin (n = 50 farms) show strong parallels among Cucurbita’s bee guilds based on taxonomic composition (Apidae and Halictidae) and species richness (n = 15). Most noteworthy among these pollinators were squash bees—narrow pollen oligoleges that predominated at Cucurbita flowers. Because female squash bees gather pollen before sunup, 30 - 60 min before any other bee species, they render blooms less rewarding to competing honey bees and bumble bees. Squash bees are also efficient crop pollinators. Six or more Peponapis pruinosa (Say) visits (i.e. visitation threshold) by mostly male bees, set well-seeded C. pepo L. fruits over 200 g. Sole visitation by male squash bees boosts a squash’s size, but if a female squash bee also visits then squash weight, maturity and seed set all increase. Combined data for squash bee efficacy and native bee density show pollination rates as high 60 visits per bloom occurred at most US squash farms. Among the 50 survey sites, ~75% sustained squash bees densities > 100 per 1000 flowers, more than enough pollinators to set a full crop. Add bumble bees, this percentage jumps to 98% of farms. If one adds rarer bee species, then native bees as a whole pollinated the entire squash crop. When mature plants set fruit that are too few or imperfect, low native bee density is a possible cause, but chances are, crop stress is also problem.