No decoy effect in bees: rewardless flowers do not increase bumblebees' preference for neighbouring flowers

Nectarless flowers are common among flowering plants, which often retain colour-changed, rewardless flowers instead of shedding them. Yet, how these flowers influence pollinators foraging choices within an inflorescence remains unclear. We hypothesised that rewardless flowers in an inflorescence may act as "decoys", causing the rewarding flowers in the inflorescence to be perceived as more valuable by contrast. Using artificial inflorescences, we presented individual bumblebees (Bombus terrestris) with a binary choice between two equally rewarding inflorescences, one of which included additional unrewarded, differently coloured flowers. We found that the presence of rewardless flowers did not increase bees preference for neighbouring flowers, nor did it affect their overall choice between inflorescences. However, bees quickly learned to avoid the unrewarded flowers, drastically reducing visits and probing within a few foraging bouts. We review research on decoy effects in bees, and find very little support for their presence. Our findings contribute to the growing body of evidence that rewardless flowers do not induce decoy effects in bees, and highlight the need for further research into the ecological role of nectarless flowers within floral patches. It may be time to abandon the search for classical decoy effects in pollinators.