Chemo-orientation of maleTrogoderma variabile (Coleoptera, Dermestidae) in a simulated corridor of female sex pheromone

Abstract

Chemo-orientation of male granary beetles (Trogoderma variabile) to female sex pheromone [(Z)-14-methyl-8-hexadecenal] was investigated using a locomotion compensator (Fig. 1). 1. Beetles responded in less than 1 s to a puff of female sex pheromone in still air with an alert, antennae-raised posture, and local search orientation, with loops generated by a relatively constant turn bias (Fig. 2). 2. In wind experiments the locomotion compensator maintained the beetle in the center of a small wind tunnel above the sphere. Lateral boundaries of a pheromone corridor parallel to the wind were generated by real-time ‘closed-loop’ control of the pheromone, preserving the spatial relationships between the insect's movements and the stimulus pattern. Changing the odor concentration in the wind-tunnel simulated what the beetle would perceive if it walked across a boundary between clean air and air with pheromone. 3. Most of the beetles oriented upwind in a relatively straight path within the 10-cm-wide pheromone ‘corridor’, and reached the boundary of the pheromone corridor as a result of net sideways displacement across the wind (Fig. 3). As a beetle crossed the boundary of the corridor, pheromone was eliminated from the air stream, and the beetle decreased its locomotory rate and path straightness, and increased its absolute turning rate (Tables 1 and 2, Fig. 4). Sign-reversal turns ‘outside of the corridor’ headed the beetle up- or downwind back across the wind toward the pheromone corridor. As the beetle re-entered the pheromone corridor, pheromone was emitted into the airstream; the beetle increased its locomotory rate, and then turned, reversing its tack heading (Tables 1 and 2, Fig. 4). The responses to pheromone being turned on and off resulted in a zigzag path along the border of the corridor (Fig. 3). 4. In computer-timed, ‘open-loop’ experiments, the beetles set and maintained a preferred course relative to the wind direction when the air stream contained uniform and constant sex pheromone (Figs. 5, 6 and 7). Although each path had a consistent heading relative to the wind, numerous upwind zigzag turns occurred (Fig. 5). Beetles that continued to move when the pheromone concentration decreased, turned in either an upwind ‘zig’ or a downwind ‘loop’, but no beetle maintained an upwind angle or continued an upwind zigzag as observed in the closed-loop experiment. The frequency of turns neither decreased nor increased after the pheromone was turned off. 5. The chemotactic turning response, correlating with the abrupt change in pheromone stimulus around the beetle, is evidence that insects are capable of detecting purely temporal changes in odor stimuli in an air current. Because the distance between zigzag turns is larger in the open-loop than in the closed-loop experiments, and since the distance walked perpendicular to the center axis of the wind is larger in open-loop than in the closed loop experiments, the temporal stimulus change rather than internally-controlled turn generators, directs the zigzag turns at the ‘corridor edge’. Zigzag turns in the open-loop experiment seem to be ‘internally-controlled’ turns or correcting responses of the anemotactic guidance system rather than chemotactic responses, since the pheromone stimulus is constant.