Evaluating the molecular, physiological and behavioral impacts of CO2 narcosis in bumble bees (Bombus impatiens).

Exposure to carbon dioxide (CO2) has pleiotropic effects in many insect species, ranging from eliciting rapid behavioral responses such as attraction, to dramatic physiological changes, including ovary activation. In bumble bees, CO2 narcosis causes queens to bypass diapause and initiate egg laying, but its mode of action is not well-understood. Here, we evaluated the effects of CO2 narcosis on the behavior, physiology and immune function of virgin bumble bee queens (Bombus impatiens). We tested the hypothesis that CO2 induces these changes by stimulating oxidative stress response pathways. We found that CO2 stimulates ovarian activation and egg production and suppresses lipid (but not glycogen) accumulation in virgin queens. Additionally, CO2 treated queens were more active (particularly in terms of flight) and performed, but did not receive, more aggressive behaviors compared to controls. Moreover, CO2 positively affected immune function in queens, reduced transcript levels of 5/6 antioxidant enzyme genes and had no effect on longevity. Thus, although CO2 treatment stimulated reproduction, we did not observe any evidence of a trade-off in queen health parameters, aside from a reduction in lipids. Overall CO2 narcosis does not appear to stimulate a typical stress response in virgin bumble bee queens. On the contrary, CO2 narcosis appears to stimulate changes that prepare queens to cope with the nutritional, metabolic and behavioral challenges associated with reproduction and colony-founding.