Orientation mechanisms underlying behavioral response of Reticulitermes flavipes (Kollar) to CO2 concentration

Reticulitermes flavipes, the eastern subterranean termite, constructs tunnels to forage, which requires significant energy expenditures for resource discovery. Search efficiency can be increased by orientation to volatile cues emanating from resources, yet the cues and orientation mechanisms to volatiles mediating resource discovery are still poorly understood. CO₂ is known to play a role in resource finding by subterranean termites, and our previous bioassays have determined that CO₂ is both necessary and sufficient to induce termite response to food sources. To identify the orientation mechanisms of subterranean termite response to CO₂, tunneling and walking behaviors were observed over a range of CO₂ levels, presented either in a spatially homogeneous atmosphere or as a concentration gradient. Tunnel abundance, length and turn angles were not influenced by CO₂ concentration when presented as a homogeneous cloud. Response to CO₂ was only observed under gradient conditions, with termites focusing their movements within areas closer to the point source of CO₂. Analysis of termite walking tracks showed no change in either linear or angular velocity related to CO₂ concentration, indicating no involvement of either orthokinesis or klinokinesis in locating the CO₂ source, respectively. Rather, locomotory behavior was best characterized solely by a directed orientation to CO₂, i.e., positive chemotaxis. It is noteworthy that termites did not display arrestment even after locating the CO₂ source, suggesting a possible role for other short-range cues.