The black-legged tickIxodes scapularisdetects CO2without the Haller’s organ

Abstract

ABSTRACTBoth male and female ticks have a strong innate drive to find and blood-feed on hosts. Carbon dioxide (CO2) is considered a critical behavioral activator and attractant for ticks and an essential sensory cue to find hosts. Yet, how CO2activates and promotes host-seeking in ticks is poorly understood. We studied CO2responses in the black-legged tickIxodes scapularis, the primary vector for Lyme disease in North America. Adult males and females were exposed to 1, 2, 4, or 8% CO2, and changes in walking behavior and foreleg movement were analyzed. We find that CO2is a potent stimulant for adultIxodes scapularis, even at lower concentrations (1%). Behavioral reactions depend on the animal’s state: Walking ticks increase their walking speed, while stationary ticks start to wave their forelegs and begin to quest – both behaviors resembling aspects of host-seeking. Furthermore,Ixodes scapularishas no clear concentration preference and is not tuned more robust to breath-like CO2concentrations (∼4%) than to the other concentrations tested. As soon as the CO2level is above a certain threshold,Ixodes scapularisreact, indicating that CO2acts as a behavioral activator and can be used as a long-distance cue to detect approaching hosts. Moreover, we provide convincing evidence that the foreleg Haller’s organ is not necessary for CO2detection. Even with disabled or amputated Haller’s organ,Ixodes scapularisrespond robustly to CO2, signifying that there must be CO2-sensitive structures important for tick host-seeking that have not yet been identified.