Pollinators, critical keystone species, are declining in abundance, distribution, and diversity at alarming rates world-wide. The main cause is thought to be human-induced environmental change, with increased exposure to pathogens playing a significant role. In vertebrate systems, it’s well established that stimulation of the immune system produces molecules called cytokines. These help fight off infection, but in the process can generate neurological and cognitive impairments. Bees and other insect pollinators produce cytokine-like molecules called antimicrobial peptides (AMPs) upon immune stimulation, thereby providing the potential for similar ‘immune-brain crosstalk.’ To explore this possibility, we combined behavioral and molecular approaches to test the effects of immune stimulation on adaptive decision making processes in bumblebee foragers. We stimulated the immune system of bees by abdominally injecting either lipopolysaccharide (LPS; humoral pathway), elastomer (cellular pathway), or Ringer’s solution (vehicle control), and then assayed the ability of individuals to flexibly switch between visual and olfactory floral discrimination tasks similar to those experienced under natural foraging conditions. The degree of cognitive flexibility was assessed based on choice accuracy and switch frequency over 120 rewarding flower visits. To gain further insight, we used qPCR to measure AMP gene expression in each treatment and look for behavioral-genetic correlation.
Results/Conclusions
Across treatments, bumblebee foragers successfully selected rewarding target flowers with high accuracy and chose both color and odor tasks with similar frequency. Cellular pathway stimulation (elastomer), leading to hemocyte proliferation and encapsulation, did not affect cognitive flexibility, nor did the vehicle. However, humoral stimulation (LPS), leading to AMP production, dramatically reduced cognitive flexibility levels in foragers. In fact, bees became extreme task specialists with a switch frequency <0.05. Such impairments to behavioral flexibility would have significant fitness consequences for bees foraging in rapidly changing, highly competitive floral environments. We next found correlation of LPS-induced impairments to cognitive flexibility with significantly increased expression of 3 AMPs, one showing >75-fold upregulation. As expected, stimulation of the cellular pathway did not lead to prolonged increase of any AMPs tested. Taken together, our results suggest infectious agents stimulating the humoral pathway of the bumblebee immune system, reduce foraging proficiency through specific AMP-induced impairments to cognitive function. As foraging proficiency is a robust measure of fitness in social bees, pathogens could have a stronger deleterious effect, and thus play a larger role in pollinator decline than previously assumed. Our poster discusses the implications of these findings for pollinator conservation and the dynamics of plant-pollinator relationships.