FINE-SCALE STRUCTURE OF PHEROMONE PLUMES MODULATES UPWIND ORIENTATIONOF 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 th e 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 usuall y follow a zigzag track, the evident expression of a self-steered coun terturning programme(13,14). The integration of counterturning and opt omotor anemotaxis allows insects to polarize the zigzags upwind in odo ur 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 straigh t upwind is related to the frequency at which males encounter pheromon e 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 flig ht manoeuvres are dictated by the interactions of the male with indivi dual 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 t han males following continuous narrow plumes. Males also fly straighte r upwind to fast-pulsed plumes than to slow-pulsed plumes. The tempora lly modulated interplay between counterturning and optomotor anemotaxi s that is induced by the plume's structure therefore seems to explain the manoeuvres and resultant flight track shapes made by C. cautella m ales when flying upwind towards a pheromone source.