Spatial and temporal structures of pheromone plumes in fields and forests

Wind-borne odour stimuli from a small point-source of pheromone are intermittent owing to the effects of atmospheric turbulence on the odour plume. The work reported here measures the characteristics of the intermittent stimulus in open fields and forests during typical daytime atmospheric conditions. To model the pheromone plume, negatively charged atmospheric ions were used as a tracer. They were released from an ion generator, and ion detectors measured the fluctuating flux of ions at positions up to 20 m downwind in the open field case and 10 m in the forest. In both the open field and in the forest, ion signals were highly intermittent, with a signal present only 20% of the time. Ion signals recorded in the forest consisted of bursts with gaps between them of at least three-fold greater duration than those from the open field. In both environments, bursts generally each comprised a series of 'spikes', on average three in the field and seven in the forest. To validate the use of ionized air plumes as models of pheromone plumes, the antennae of male Lymantria dispar (gypsy moth) were used as detectors to quantify the plume of synthetic (+)-disparlure emanating from a 2000 ng point source placed approximate to 10 cm from the ion source. A comparison of ion signals and EAGs (electronantennograms) suggests that the antennae respond to the main spikes within a burst, but no consistent relationship between the strength of the spikes and the magnitude of the EAG response was found. The average strengths of bursts in the ion detector signal decreased systematically as the distance from the ion generator to the ion detector increased. A similar trend, however, was not detected in the EAG response.