Nutritional resources for pollinators from mass-flowering crop cultivars

Abstract

The scarcity of flowers to provide dietary nectar and pollen is a key driver of recent declines in pollinators in agricultural areas, but the planting of mass-flowering crops enhances resources available to pollinators during parts of the year. This thesis investigates the nutritional resources provided for insect pollinators from various cultivars of two mass-flowering crops: short rotation coppice willow (Salix species) and oilseed rape (Brassica napus L.). Willow cultivars vary in the numbers of flowers produced per plant, as well as in the quantity of nectar sugar secreted by those flowers. There were neither qualitative nor quantitative differences in pollen production between the cultivars. Foraging insect pollinators showed preferences for cultivars with more rewarding flowers. Oilseed rape flowers of different cultivars produced a mass of nectar sugar that varied by up to three fold in mass when grown in a glasshouse. Cultivars differed in the size of their flowers, but neither flower size nor the seed yields they produced in industry trials were correlated with their nectar yields. When plants were grown in field conditions, differences between oilseed rape cultivars in nectar production were also present, although less pronounced. The weights of bumble bee (Bombus terrestris L.) colonies diverged after two weeks when restricted to foraging on plots containing either a high or a low nectar yielding cultivar. The findings indicate that efforts to breed and to plant more widely the more rewarding cultivars of mass-flowering crops would enhance the resources available to pollinators in spring. As this is a critical time for pollinators, the extra resources could aid their survival and lead to more robust populations.