The interplay of experience and pre-existing bias in nectar-robbing behavior by the common eastern bumble bee

The ontogeny of nectar robbing by pollinators is not well understood. In this study, we investigated the interplay of pre-existing biases and experience in the expression of nectar-robbing behavior by common eastern bumble bee (Bombus impatiens) workers. Flower-naive bees were released individually into an arena containing six live Tecoma stans flowers in one of two treatments. One treatment consisted of open flowers bearing a single artificial slit, allowing the bee to rob or enter legitimately for nectar, while the other consisted of flowers bearing slits, whose corollas were plugged with cotton, requiring the bee to rob for nectar. A subset of bees was tested twice, once on each treatment; the order of treatments was balanced across bees. Results showed that first attempts by flower-naive bees were biased nearly absolutely towards legitimate visitation regardless of flower treatment. Most bees in both treatment groups robbed over the course of a trial, but plugged flowers were robbed significantly more. Previous experience affected robbing frequency on subsequent visits, suggestive of learning; bees induced to rob on plugged flowers tended to continue to rob even when legitimate access was restored. We speculate that the initial bias towards legitimate visits is due to the flower corolla's distinctive odor and nectar guides, to which bees respond innately. Bees nevertheless readily learn to rob, particularly when it is the only option for extracting nectar. Our findings support the view that studies of cognition are essential to understanding the pattern of cooperation and conflict in plant-pollinator mutualism.Significance statementNectar-robbing behavior is widely observed among pollinators in nature. Unlike visitation of flowers through the floral entrance, nectar robbing often does not transfer pollen. However, despite the importance of nectar robbing on plant fitness, little is known about the ontogeny of nectar-robbing behavior. In this study, we found that naive Bombus impatiens workers strongly preferred legitimate visits over nectar robbing on their first visits to live flowers. However, bees quickly switched to nectar robbing when nectar could not be accessed legitimately and continued robbing even when legitimate access is restored. Our results suggest that flowers may take advantage of their pollinators' innate responses for their own or mutual gain, but also that pollinators can overcome this manipulation through learning.