Bee visitation rates to cultivated sunflowers increase with the amount and accessibility of nectar sugars

Authors: Mallinger, Rachel; Prasifka, Jarrad

Submitted to: Journal of Applied Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2016
Publication Date: 1/28/2017
Citation: Mallinger, R.E., Prasifka, J.R. 2017. Bee visitation rates to cultivated sunflowers increase with the amount and accessibility of nectar sugars. Journal of Applied Entomology. 141(7):561-573.

Interpretive Summary: Bees and other pollinators show preferences for certain flowers based on traits such as flower size and shape, color, scent, and the quantity and quality of nectar or pollen. For crops that are pollinated by bees, such as sunflower, identifying and breeding for attractive flower traits could increase pollination rates and subsequent crop yields. In this study, over two years, we examined variation in floral traits among different inbred lines that are used in sunflower breeding programs. We furthermore explored how these flower traits affect bees by observing bee visits to the different sunflower inbreds in the field and greenhouse. We found differences among sunflowers in three important traits: amounts of nectar sugar per sunflower head, percent sucrose in nectar, and corolla lengths, which determine nectar accessibility to pollinators. We additionally found that bees preferred sunflowers with higher amounts of nectar sugar and shorter corollas. In a greenhouse experiment, sunflower inbreds showed similar differences in nectar volume compared to those found in the field, showing that this trait is consistent across different growing environments. Our results suggest that sunflowers with more nectar sugar and shorter corollas could be better pollinated by bees and therefore have higher crop yields.

Technical Abstract: Pollinators make foraging decisions based on numerous floral traits, including nectar and pollen rewards, and associated visual and olfactory cues. For insect-pollinated crops, identifying and breeding for attractive floral traits may increase yields. In this study, we examined floral trait variation within cultivated sunflowers and its effects on bee foraging behaviours. Over two years, we planted different sunflower inbred lines, including both male-fertile and male-sterile plants, and measured nectar volume, nectar sugar concentration and composition, and corolla length. During bloom, we recorded both managed honey bee and wild bee visits to these lines. Furthermore, we examined consistency in bee foraging preferences and patterns of nectar production by comparing field results to those from a greenhouse experiment. Sunflower inbred lines varied significantly in all floral traits, including the amount and composition of nectar sugars, and in corolla length. Both wild bee and honey bee visits significantly increased with nectar sugar amount, and decreased with corolla length, but appeared unaffected by nectar sugar composition. While wild bees made more visits to sunflowers providing pollen (male-fertile), honey bees preferred plants without pollen (male-sterile). Additionally, relative nectar production among inbreds in the greenhouse was similar to that in the field, and bee visitation rates in the greenhouse likewise increased with nectar quantity. Our results show that sunflowers with more nectar sugar and shorter corollas receive greater pollination services from both honey and wild bees. Selecting for these traits could increase sunflower crop yields and provide accessible, resource-rich flowers for bees.