FINE-SCALE STRUCTURE OF PHEROMONE PLUMES MODULATES UPWIND ORIENTATION OF FLYING MOTHS

IN studies of moths flying upwind to a pheromone source, attention has focused on the influence on flight orientation of the composition(1,2) and concentration(3,4) of the chemical message, and of changes in the visual environment(5,6) and in wind speeds(7-9). The chemical signal must be intermittent for moths to fly upwind(10-12), when they usually follow a zigzag track, the evident expression of a self-steered counterturning programme(13,14). The integration of counterturning and optomotor anemotaxis allows insects to polarize the zigzags upwind in odour plumes(10,15). Not all moths, however, zigzag along a plume(16,17). It has been suggested that the propensity to zigzag or to fly straight upwind is related to the frequency at which males encounter pheromone filaments that comprise the plume, as well as the male's latency of response, characteristic for each moth species, to both the onset and loss of contact with filaments(18). Here we present evidence that flight manoeuvres are dictated by the interactions of the male with individual odour pulses. We use Cadra cautella, the almond moth, to show how the structure of an odour plume(19,20) can greatly modify the flight track. Males following either turbulent or mechanically pulsed plumes fly faster and straighter upwind, and locate sources more frequently than males following continuous narrow plumes. Males also fly straighter upwind to fast-pulsed plumes than to slow-pulsed plumes. The temporally modulated interplay between counterturning and optomotor anemotaxis that is induced by the plume's structure therefore seems to explain the manoeuvres and resultant flight track shapes made by C. cautella males when flying upwind towards a pheromone source.