A guide for thwarting robbers? The possible benefits of nectar guides to plants and bees

Background/Question/Methods

Many floral displays transmit patterns thought to direct pollinators to nectar. These “nectar guides” are often assumed to be mutually beneficial, as they may reduce pollinators’ handling time, increasing both the rate of nectar collection and pollen transfer. Yet despite a rich history of study, many details of how floral patterns influence foraging efficiency are relatively unexplored. For example, beyond speeding nectar discovery, nectar guides might also affect pollinators’ flower handling strategy. Bees sometimes extract nectar from a hole in the side of the flower that they or other floral visitors create, often without transporting pollen (“nectar robbing”). Does a nectar guide pattern influence bees’ propensity to rob nectar? To explore the relationship between nectar guides and flower handling, as well as the potential for learning to alter this relationship, we compared the responses of bumble bees (Bombus impatiens) to artificial flowers that either possessed or lacked patterns. We asked (1) whether the presence of a nectar guide affected bees’ propensity to rob nectar, and (2) whether previous experience using nectar guides affected bees’ long term landing preferences.

Results/Conclusions

The presence of a floral pattern affected bumble bees’ foraging behavior in several ways. First, we noted that our floral patterns clearly functioned as nectar guides: bees discovered nectar more quickly on patterned flowers, regardless of corolla shape. This effect was immediate (innate), but also had a learned component; additionally, differences in discovery time between patterned and unpatterned flowers persisted with experience. We discovered that nectar guides made legitimate visits more efficient for bees than nectar robbing, and increased the relative frequency of legitimate visits, compared to plain flowers. Thus, beyond speeding nectar discovery, a nectar guide can influence bees’ flower handling in a way that benefits the plant. However, for bees, the benefit of a nectar guide may depend on ecological context: when conditions over successive days changed such that flowers no longer provided a reward, bees visited the now-unrewarding flowers more persistently when they were patterned. Bees’ persistent response to patterned flowers even after rewards ceased suggests that rather than always being mutually beneficial, in fact nectar guides can at times promote plants' pollen transfer at the expense of bees’ foraging success.