Impacts and mechanisms of CO2 narcosis in bumble bees: Narcosis depends on dose, caste and mating status and is not induced by anoxia

Carbon dioxide (CO2) is commonly used to immobilize insects and to induce reproduction in bees. However, despite its wide use and potential off-target impacts, its underlying mechanisms are not fully understood. Here we used Bombus impatiens to examine whether CO2 impacts are mediated by anoxia and whether these mechanisms differ between female castes or following mating. We examined the behavior, physiology, and gene expression of workers, mated and virgin queens following exposure to anoxia, hypoxia, full and partial hypercapnia, and control. Hypercapnia and anoxia caused immobilization, but only hypercapnia resulted in behavioral, physiological, and molecular impacts in bees. Recovery from hypercapnia resulted in increased abdominal contractions and took longer in queens. Additionally, hypercapnia activated queens-but inhibited workers ovaries in a dose-dependent manner and caused a depletion of fat-body lipids in both. All responses of hypercapnia were weaker following mating in queens. Analysis of gene expression related to hypoxia and hypercapnia supported the physiological findings in queens, demonstrating that the overall impacts of CO2, excluding virgin queen ovaries, were unique and were not induced by anoxia. This study contributes to our understanding of the impacts and the mechanistic basis of CO2 narcosis in insects and its impacts on bees physiology.