Acute ozone exposure impairs detection of floral odor, learning, and memory of honey bees, through olfactory generalization

Air pollution stemming from human activities affects the environment in which plant and animal species live and interact. Similar to primary air pollutants which are emitted, secondary air pollutants, such as tropospheric ozone (O3) formed from nitrogen oxides, are also harmful to human health and plant physiology. Yet, few reports studied the effects of O3 on pollinators' physiology, despite that this pollutant, with its high oxidative potential, likely affects pollinators behaviors, especially the perception of signals they rely on to navigate their environment. Volatile Organic Compounds (VOCs) released by plants are used as signals by different animals. For pollination services, VOCs attract different insects to the flowers and strengthen these interactions. Here, we used the honey bee Apis mellifera as a model to characterize the effects of acute exposure to different realistic mixing ratios of O3 (80-, 120-, and 200-ppb) on two crucial aspects: first, how exposed honey bees detect VOCs; and second, how O3 affects these pollinators' learning and memory processes. With electroantennogram (EAG) recordings, we showed that increasing O3 mixing ratios had a biphasic effect: an initial 25% decrease of the antennal activity when bees were tested directly after exposure (O3 direct effect), followed by a 25% increase in activity and response when bees were allowed a two-hour rest after exposure (O3 delayed effect). In parallel, during olfactory conditioning, increasing O3 mixing ratios in both exposure protocols scarcely affected olfactory learning, followed by a decrease in recall of learned odors and an increase of response to new odors, leading to a higher generalization rate (i.e., discrimination impairment). These results suggest a link between O3-related oxidative stress and olfactory coding disturbance in the honey bee brain. If ozone affects the pollinators' olfaction, foraging behaviors may be modified, in addition with a possible long-term harmful effect on pollination services.