Secondary Compounds in Milkweed Nectar Negatively Impact Thermal Tolerance in Bumble Bees

Bumble bees play a critical role in ecosystem health and pollination, but recently, populations have been in decline worldwide. One way to improve outcomes is to increase floral resource availability, which improves nutrition while alleviating the effects of other stressors, such as climate. In addition to macronutrients, floral nectar can contain both beneficial as well as toxic secondary compounds, including cardenolides and alkaloids, that may impact these bees. Given that pesticides can affect heat shock pathways and thus impact heat tolerance, natural toxic compounds in nectar could also impact heat response. In this study, we investigate how the consumption of toxic secondary compounds found in nectar impacts a bee's ability to thermoregulate during heat stress events. We studied thermal tolerance in bumble bees fed a variety of concentrations of ouabain, a commercially available cardenolide (a class of compounds common in milkweeds (Asclepias spp.)) as well as a variety of honeys and milkweed nectar. We found that increasing concentrations of ouabain resulted in a diminished ability to thermoregulate during heat stress events. Ouabain had a significant impact on thermal tolerance at concentrations both at and below both field-realistic doses and levels that impair bee behavior, with effects matched by milkweed nectar. This effect was found both in a species that tends to avoid milkweeds (B. impatiens) and a species that commonly visits milkweeds (B. griseocollis). Nevertheless, with the exception of milkweed honey, multiple honey sources showed no marked effect on heat tolerance in these bees, suggesting most floral diets may not impact thermal response. These results reveal that plant defense toxins consumed in the diet may have sublethal impacts on stress responses in these bees.