Priming bumble bees with caffeine, odour of the target crop, and a food reward, has minimal effects on fruit crop pollination and yield in a semi-commercial setup

Caffeine is a widely occurring plant defence chemical that occurs in the nectar of some plants, e.g. Coffea or Citrus spp., where it may influence pollinator behaviour to enhance pollination. Previous laboratory work found inexperienced bumblebees (Bombus terrestris) located new food sources emitting a learned floral odour more consistently if they had been fed caffeine in association with the odour and a food reward. Inexperienced bees primed with caffeine also made more initial visits to target robotic flowers emitting the target odour, compared to control bees or those primed with odour alone. We tested whether these behaviours could be replicated under semi-field conditions in strawberry crops, to improve crop pollination and hence marketable yield. In three trials in mini-polytunnels, odour/caffeine-primed and control bumblebee colonies were allowed to forage on strawberry crops with nectar-rich distractor flowers present. Some small effects of caffeine priming were observed (a slight increase in the proportion of visits to the target flowers in one trial), but after controlling for polytunnel identity, the priming treatment did not influence crop yield and quality. While caffeine priming of commercial bumblebee colonies may have potential to improve pollination in crops, further research is needed to optimise the system for in-field use.