Although biconical traps are now widely used for the sampling of tsetse populations, they provide only relative population estimates determined by the fly density, activity and trap response. Studies were therefore undertaken to convert such biconical trap indices into unbiased absolute population density estimates. The first step was to determine the range of attraction of such traps for the main trypanosome vector in Kenya, Glossina pallidipes, although some data were also obtained for G. brevipalpis. Two different techniques were used. Firstly traps were placed at varying distances apart, resulting in competition and consequent reduction of catch sizes per trap when ranges of attraction overlapped. Secondly, the catches in grassland at varying distances from a forest edge were compared with two predicted functions relating catch size to distance. Results from the competition experiments showed a marked decline in catch per trap when the intertrap distance was less than 30–40 m. The resultant estimate of maximum range of attraction of 15–20 m for G. pallidipes was compatible with the observed pattern of decline of catches at increasing distances from the forest edge. The range for G. brevipalpis was tentatively estimated as up to 10–15 m.
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