Insect pollination of faba beans, Vicia faba : the abundance and foraging behaviour of different pollinators and their effect on crop yield

Abstract: Faba beans, Vicia faba is known as a self-pollinating crop, however significant yield increases have been found in plants visited by honeybees and bumblebees. The bees either make positive visits, entering the flower and transferring pollen between different flowers resulting in cross-pollination, or negative visits, piercing the flower to “steal” nectar or collect nectar from extrafloral nectaries. Negative visits do not contribute to cross-pollination, however the movement on the flower facilitates self-pollination, suggesting both positive and negative visits are beneficial to some degree. In this study the benefit of insect pollination was estimated by comparing the results from different experiments with pollination by honeybees and bumblebees or hand pollination. The mean benefit of insect pollination across a range of yield parameters was calculated by dividing the yield of cross-pollination treatments (insect pollination or hand pollination) with the yield of treatments without cross-pollination. The average benefit of cross-pollination was calculated as 42,6% (both insect- and hand pollination) and 51,8% (only insect pollination), however the range varied between -54% and 245%, due to differences in method, faba bean cultivar, climatic and agronomic factors such as soil conditions. Cross-pollination increased beans and pods per plant, beans per pod and pods per node and more beans reached maturity. However, in some experiments individual bean weight was greater in plants excluded from pollinators resulting in profitable bean yield despite inadequate pollination. The most efficient pollinating species was found to be the long tongued Bombus hortorum due to a relatively large proportion of positive visits, high flower constancy and optimal bodyweight for tripping the flower. However, the honeybee, Apis mellifera was most abundant, suggesting it is the most important pollinator in faba bean cultivation despite their relatively lower tendency to make positive visits, making them less efficient on a per visit basis.