Understanding the consequences of pesticide exposure is a major focal point in research on pollination ecology under global change. Neonicotinoid pesticides are commonly found in floral nectar and pollen, where they can have well-documented negative effects on bee health and foraging behavior. Nearly all of this research relies on simplified foraging scenarios in which bees are offered a sucrose solution in place of nectar. Yet, real floral nectar is chemically complex, often containing secondary metabolites that can positively and negatively affect pollinator physiology and behavior. Given the possibility of additive or synergistic effects of nectar phytochemicals and pesticides, it is an open question whether research to date has accurately estimated the impacts of neonicotinoids on bees. For the first time, we examine the combined effects of consuming nectar secondary metabolites and neonicotinoid pesticides on the bumble bee species Bombus impatiens, asking how a diet containing secondary metabolites mediates how bees respond to an acute dose of a pesticide. In a laboratory experiment (n = 960 bees), we quantify worker longevity, activity level, and immune system performance (activation of phenoloxidase enzyme) under different diet and pesticide exposure regimes. We test three well-studied secondary metabolite compounds: caffeine (an alkaloid found in Citrus and Tilian nectar), thymol (a terpenoid found in Thymus nectar) and digoxin (a cardiac glycoside found in Digitalis nectar) in combination with an ecologically-realistic dose of imidacloprid, a neonicotinoid.
Results/Conclusions
We found compound-specific effects in all three responses (longevity, activity level, and immune functioning). Consumption of digoxin reduced longevity in workers compared to a control, with little effect of pesticide exposure. Pesticide-exposed workers were less active than non-exposed workers 24 hours post-exposure, however most bees regained activity 72 hours post-exposure. Bees that consumed digoxin were unable to regain activity, indicating that digoxin may exacerbate the negative effects of imidacloprid on activity. Conversely, bees that consumed caffeine showed higher immune system function compared to a control, regardless of pesticide exposure. These results indicate that consumption of caffeine may temper the negative effects of pesticide exposure. Overall, we report how nectar secondary metabolites can complicate our understanding of the effects of pesticides on pollinator populations and suggest nectar chemistry help inform future toxicity studies.