Measuring the performance of honey bee pollinators: Bagging effects, nectar volume, and pollen deposition

Recent declines in managed honey bee populations have spurred interest in alternative crop pollinators and in studies that compare the pollinator performance of honey bee and native bee species. Per visit pollen deposition is a common measure of pollinator performance that is usually obtained by bagging flowers with fine netting to prevent unwanted bee visits. In watermelon (Citrullus lanatus), we tested the hypothesis that bagging of female flowers induces atypically large volumes of nectar within flowers, which results in disproportionately high values of per visit pollen deposition by honey bees. In two seedless watermelon fields in Southern California, we measured floral nectar volumes, honey bee handling time, and per visit pollen deposition on bagged control flowers and on bagged flowers with nectar removed, either completely or partially.  Bagged flowers without nectar removal contained, on average, twenty times more nectar than either unbagged open-pollinated flowers or bagged flowers with complete nectar removal. At both sites, complete nectar removal had a highly significant effect on per visit pollen deposition and handling time. Two to three times more pollen was deposited on control flowers, on average, than on complete nectar removal flowers, for stigmatic and total pollen deposition. Similarly, complete nectar removal from flowers significantly reduced floral handling time compared to controls at both sites. For watermelon and perhaps other cucurbit crops, our findings suggest that overestimation of per visit pollen deposition by honey bees is a concern in comparative pollinator performance studies.