4.2 Autogamy and flower visitation by pollinators in different varieties of flowering crops

Thomas van Loo, Inagro

In this study, Inagro investigated to what extent a series of crops are attractive for pollinators and thus play a role in supporting these insects in the landscape. The study took place on a small-scale trial field at the premisses of Inagro in Beitem, Belgium in 2022. Additionally, a trial was set up to investigate to what extend several varieties of these crops benefit from insect pollination. The crops in this study were lupins, summer field beans, gold-of-pleasure, soybeans, red Phaseolus beans and chickpeas.

Pollinators and crops: bees and flowers need each other

Pollinators need plants to provide them with food to survive and to reproduce. Many plant species in their turn need pollinators for their own reproduction. Some flowering plants produce flowers but are also able to produce seeds without pollinators. Although being able to produce seeds without pollinators, yield in these plants often still is higher if they are supplementary pollinated by insects.

Flower visitors: (un)faithful?

Although Inagro counted quite some pollinators in the earliest flowering crops (summer field beans, gold-of-pleasure and lupins), the pollinator activity dropped dramatically from mid-July onwards. Furthermore, many flower visits were registered as “nectar robbing”, which is a behaviour where pollinators surpass the pollination mechanism of the flowers by gnawing a hole at the base of the flower. Therefore, there was little activity that resulted in effective pollination in this trial study.

• Only Buff-tailed bumblebees (B. terrestris) has been observed making these holes, both in this study as well as in previous studies.

• Honeybees themselves cannot make the holes but they to use the premade holes as well.

• From other pollinator species it is known that they only access the flowers via the regular way: these species have longer tongues with which they can reach the nectar glands at the base of the flower through a regular flower visit.

In previous, more large-scale studies, we found that both B. terrestris and honeybees show a mixed behaviour of robbing nectar combined with regular flower visits, so they both do act as true pollinators as well. When performing a regular visit, they probably collect pollen to feed to their larvae, instead of nectar.

Buff-tailed bumblebees and honeybees: dominant, but not in all crops

• In summer field beans, the highest densities of pollinators were observed (B. terrestris and honeybees), and here almost exclusively nectar robbing was observed as a type of flower visit.

• In gold-of-pleasure, many hoverflies were observed in addition to honeybees.

• In lupins, only regular flower visitation was observed. Possibly the flower tube of lupins is shorter, or the nectar glands are higher in the flowers than in field beans.

• Flower visitation in red beans was dominated by B. terrestris, which exclusively robbed nectar. The honeybees showed mainly regulatory behaviour.

• In chickpeas, almost all flower-visiting behaviour was regulatory (no individuals of B. terrestris were observed in the chickpea fields during monitoring).

• Very few individuals were observed visiting soybean (B. terrestris and honeybees), but then this crop only started flowering at a time when pollinator activity was already declining.

Of all the observations in this study, B. terrestris and honeybees were the dominant species.

Conclusion

Since true pollinator activity was low in almost all of the crops, the present study cannot draw conclusions on the effect of insectpollination in terms of nutritional value and yield parameters for most of the crops.

It would be valuable to revisit this kind of study, under a higher true pollination pressure (more bees, and/or a higher diversity of bees including long tongued species). However, what the present study might point to regarding crop pollination is on the one hand the need to support pollinators throughout the whole year to avoid drops in pollinator activity, and on the other hand the need to support bumblebee species which have longer tongues in the agricultural landscape, since these are the species which generally only exhibit regular flower visitation behaviour. These species however are the ones which have declined the most in the recent past. It is unclear to what extent the crops themselves also provide valuable pollen in terms of nutritional value for pollinators: this is needed to feed their larvae throughout the season in their colonies. In any case, flowering crops alone are probably not sufficient to meet all pollinator needs throughout the year. In addition, bumblebees also need nesting facilities in the near vicinity.