Honey bee (Apis mellifera L.) foraging rewards in sunflowers: effect of floral traits on visitation and variation in pollen quality over two consecutive years

Abstract

Sunflower production depends on pollination by honey bees, Apis mellifera L., and wild bees. For wild sunflower oligoleges, preference for various sunflower lines seems to reflect the quantity (or accessibility) of floral rewards, with floret size (≈accessibility of sunflower nectar) appearing most important. However, it is less clear how honey bees, a key generalist, are affected by sunflower floral traits. The honey bee-sunflower interaction was explored to test if accessibility of nectar (≈floret size) and related floral traits explain foraging preferences and to assess potential nutritional variation in sunflower pollen. In the first two years, honey bee foraging preference increased with decreasing floret length similar to previous observations with wild pollinators. In the second year, no similar response was seen, though nectar rewards (by volume or volume × concentration) were remarkably low compared to the previous year. Pollen collected from bagged plants in the second year showed sunflower lines differed in concentrations (µg/mg ± SE) of total fatty acids and essential fatty acids. In general, it appears that honey bee responses to floral traits are similar to those of wild bees responsible for pollinating much of the sunflower crop in the central United States and that variation in the nutritional quality of sunflower pollen is greater than previously known. Because of the broad geographic distribution of the sunflower crop, additional research on how the environment influences floral rewards and sunflower-pollinator interactions may be needed.