The black-legged tick Ixodes scapularis detects CO2 without the Haller's organ

Both 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 CO2 activates and promotes host-seeking in ticks is poorly understood. We studied CO2 responses in the black-legged tick Ixodes 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 CO2 is a potent stimulant for adult Ixodes scapularis, even at lower concentrations (1%). Behavioral reactions depend on the animals 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 scapularis has no clear concentration preference and is not tuned more robust to breath-like CO2 concentrations ([~]4%) than to the other concentrations tested. As soon as the CO2 level is above a certain threshold, Ixodes scapularis react, indicating that CO2 acts 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 Hallers organ is not necessary for CO2 detection. Even with disabled or amputated Hallers organ, Ixodes scapularis respond robustly to CO2, signifying that there must be CO2-sensitive structures important for tick host-seeking that have not yet been identified.