Successful animal-mediated pollination reflects how many times flowers are visited, and also how visitors interact with flowers. When animals “rob” nectar by feeding through holes at the base of flowers, they often provide no pollination services. Despite expectations that robbing should reduce plant fitness, nectar robbing sometimes inflicts no measurable plant-fitness cost. One possibility is that individual flower visitors exhibit constancy to either feeding through the flower’s opening (“legitimate” visitation) or robbing nectar. Such tactic constancy may ensure that at least some individuals of a species act as pollinators, securing successful pollination even in the face of nectar robbing. Pollinator constancy to specific species is well-documented, but little is known about if or when pollinators show constancy to specific food handling tactics. We quantified individuals’ constancy to robbing and legitimate visitation both within and across foraging bouts in the field. We observed the degree to which four focal bumble bee species exhibited constancy to either legitimate visitation or robbing of three plant species. We then conducted a controlled foraging experiment in an enclosure to test the hypothesis that switching tactics, like switching plant species, imposes a cost to bees by increasing their handling time and reducing their foraging efficiency.
Results/Conclusions
All focal bumble bee species showed high tactic constancy when observed in the field. Tactic constancy was high when we observed bees visiting flowers within a foraging bout, as well as across foraging bouts, for four Bombus species (Bombus bifarius, B. flavifrons, B. mixtus, B. occidentalis) visiting three different plant species (Corydalis caseana, Mertensia ciliata, Linaria vulgaris). Surprisingly, however, our experiment did not explain why this constancy occurred. Preliminary analyses indicate that switching tactics did not increase handling time. When we allowed B. mixtus workers to use one tactic while visiting M. ciliata flowers and then forced them mid-bout to use a new tactic, their flower handling times remained unchanged after the tactic switch. Thus, bees switching between robbing and legitimate visitation did not exhibit the same constraints that are hypothesized to underlie reduced foraging efficiency when switching among plant species. A high degree of tactic constancy suggests that once bees begin to visit flowers legitimately, they may continue to act mutualistically. The reasons why remain to be discovered.