Associate Professor University of Nevada, Reno Reno, Nevada, United States
Abstract: Foraging bumble bees encounter a variety of naturally-occurring nectar chemicals, which can have direct effects on both bee foraging performance and plant reproductive success. For example, it could be in a plant’s interest to produce compounds that manipulate the bee’s perception of nectar quality. Non-protein amino acids (NPAAs) are a common component of nectar, including γ-Aminobutyric acid (GABA) and β-Alanine. In insects, GABA is a neurotransmitter that influences motor function, sensory processing, and cognition, and β-Alanine is an agonist for GABA receptors. Although dietary NPAAs can have longer term effects on bumble bee survival and movement, how they influence behavior on a shorter timescale is an open question. Furthermore, the combined effects of GABA and β-Alanine on bumble bee behavior have not been studied, though they can occur within and across co-flowering plant species. Our goal was to determine if GABA and β-Alanine, individually or together, affect the gustatory responsiveness of bumble bees. We used three commercial colonies of the common eastern bumble bee (Bombus impatiens) to address these questions in a lab setting, with bees housed in individual chambers. We dosed each bee (n=122) with either sucrose alone (control), or sucrose containing ecologically realistic concentrations of GABA (77.3 ppm), β-Alanine (204.9 ppm), or a combination (77.3 and 204.9 ppm, respectively). We then tested the gustatory responsiveness of each bee by touching their antenna with a strip of paper dipped in increasing concentrations of sucrose (1%, 5%, 10%, 20%, 30%, 50%), with water offered in between each sucrose presentation. We recorded whether or not their proboscis extended in response to each gustatory stimulus. We then compared the treatment groups to see if dosing had an effect on responsiveness to sucrose and/or water. As expected, we found that as sucrose concentration increased, the proportion of bees extending their proboscis increased (p< 0.001). However, we did not detect significant differences between treatments, suggesting that NPAA consumption does not alter sucrose perception specifically. When we compared the bees' responses to water, however, we found that treatment with any NPAA (GABA, β-Alanine, or their combination) increased responsiveness relative to control; this difference was significant for bees treated with GABA (Tukey post hoc test, p=0.04) or a combination (Tukey post hoc test, p=0.05). This suggests that nectar NPAAs may indeed alter bee foraging behavior in ways that could be relevant to pollination efficacy.
