Wind speed effects on odour source location by tsetse flies (Glossina)

Tsetse flies (mainly Glossina pallidipes Aust.) were captured by various means at sources of artificial host odour in Zimbabwe and Kenya. Their rates of arrival and flight directions were compared with simultaneous data on the wind's speed and direction, on time-scales ranging from 1 s to 30 min. It was predicted that because increasing wind speed up to 1 m s(-1) straightens out the airflow (Brady et al., 1989) it will straighten out odour plumes, make them easier to navigate, and should therefore increase the rate of arrival of flies at an odour source. In the event, the relationship proved to be more complex, with both positive and negative correlations of arrival rate on wind speed. It seems there is a bimodal relationship: odour source finding is positively related to increasing wind speed in weak winds up to similar to 0.5 m s(-1) (presumably as the odour plume straightens out), but is negatively related to increasing wind speed in strong winds above similar to 1.0 m s(-1) (presumably due to increasing turbulence breaking up the odour plume).