Pestic. Sci. 1976, 7 , 633

Behaviour of Flying Male Moths Approaching a Source of the Female Sex Pheromone“

David Marsh

ARC Insect Physiology Group, Department of Zoology and Applied Entomology, Imperial College, London, S W7

Insects have frequently been observed to fly upwind along a windborne odour plume in a series of irregular zig-zags. Experimental evidence suggests that this behaviour is dependent upon opto- motor anemotaxis, but the reactions involved have not been studied in detail. The direction and speed of the insect’s displacement over the ground (its track and groundspeed respectively) are the resultants of its own velocity in the air (its orientation or course and its airspeed) and the wind velocity. Tracks of male Plodia interpunctella (Hubner) flying along a plume of the female sex pheromone in a laboratory wind tunnel were recorded on videotape and the moths’ groundspeeds determined. From the tracks, groundspeeds, and wind speeds the moths’ courses and airspeeds were estimated. Visual stimulation from the apparent movement of the ground pattern beneath the moths was then related to the tracks. Since insects orient to distant odour sources in a range of wind speeds, tracks of the moths were recorded in winds of three different speeds, namely 0.1, 0.2 and 0.3 m/s. The males zig-zagged irregularly upwind towards the pheromone source in each case. Comparisons of measurements made on sections of track between successive points of reversal relative to the windline, showed the moths’ tracks to be remarkably similar at all three wind speeds. Further analyses were based on straight sections of track along which the insects’ airspeed and course remained stable. It was found that groundspeed varied with the angle of the track to the wind, and that this relationship was consistent in the three wind speeds. Thus the moths compen- sated very efficiently for the differences in wind speed and flew along similar tracks at similar groundspeeds. Yet in doing so they must have made substantial changes in their courses and air- speeds. In maintaining a given angle of track to wind, they turned more into wind and flew faster at the higher wind speeds. Consequently the optomotor input, from the apparent movement of the ground pattern beneath them, varied with wind speed. This optomotor input has two compo- nents, direction and speed over the eyes. The direction of image movement depends on the angular difference between the insect’s course and its track, and this was evidently different in the different wind speeds. In contrast, the speed of image movement, being proportional to the groundspeed, was evidently constant in the different wind speeds. It is concluded that the moths could not have controlled their observed tracks by responding only to the optomotor stimulation from the ap- parent movement of the ground pattern, and were presumably integrating this information with information about their airspeeds.

a Summary of a contribution presented at a meeting Insect pheromones: aspects of their use in pest control on 6 April 1976 organised jointly by the Physicochemical and Biophysical Panel (Pesticides Group) Society of Chemical Industry and the Association of Applied Biologists. The full text of this paper will be published else- where.

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