by Capstick et al.
A large proportion of crops are reliant, to some extent, on insects to pollinate them and produce seeds and/or fruit. The global decline in pollinators has implications for crop yield and quality, and it is important to understand how the number and species of pollinators present relates to crop yield.
Previous work has shown that field bean yield can be affected by the level of insect pollination, but yield is not always positively related to number or visitation of pollinators. We wanted to explore what factors affected insect pollination of field beans and how this related to yield. This could inform management to increase pollinators and pollination of field beans.
In this study we observed pollinators foraging in 13 field bean fields and measured the crop yield in these fields. We explored how these pollinators were affected by infield factors (sowing time and plant density) and the wider landscape (area of flower-rich habitat or area of field beans surrounding the focal field).
The pollinators we saw in the fields were mainly bumblebees and honeybees. The bees were either legitimately foraging or robbing nectar from the bean flowers. Foraging bees entered from the front of the flower taking both pollen and nectar, robbing bees made or found a hole in the back of flower and only took nectar. The former behaviour is much more effective at pollinating the field beans and can facilitate cross-pollination.
We found that bees differed in their behaviour depending on the type of bee. Long-tongued bumblebees were more likely to legitimately forage from the deep bell-shaped bean flowers whereas short-tongued bumblebees were more likely to rob nectar. Short-tongued bumblebees were also more likely to rob in winter sown crops.
The number of bees in the field bean fields was higher in the spring sown crops and in fields where there was a smaller area of field beans in the surrounding landscape. We didn’t find a relationship between the number of bees and the yield of the field beans. Contrary to our expectations we did find that when more bees were robbing in the field bean fields the yield was higher.
Our results indicate that the effect of pollinators on field beans are variable and depend on wider context. Further research is needed to see how we can manage habitat and bean crops to increase the overall number of pollinators and the number legitimately foraging specifically.