1Max-Planck Institut Seewiesen, 82319 Starnberg, Germany.
2Group Sense Organs and Behaviour, Department of Animal Physiology, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands.
Targets as developed in Zimbabwe for the control of tsetse flies resemble blue-black-blue banners mounted on frames 170 cm wide and 100 cm high. The black center field is treated with insecticide. Attracted from up to 90 m by odour baits, the flies come to the targets and usually land on the black portion where they take up a dose of insecticide upon contact. An important parameter is the contact time between target and fly. Especially with non-granular insecticides, a fly must contact the target for a certain period of time to absorb a lethal dose. Earlier, we found that taste cells on the legs of tsetse flies respond to a number of the chemicals isolated from human sweat, including uric acid. Here we report contact times of tsetse flies on targets at the Rekomitjie Research Station in Zimbabwe with and without uric acid. Flies appear to stay longer on targets treated with uric acid than on untreated targets. Sequential fitting of decay-curves of the form y=Ae-Bx and y=Ae-Bx+Ce-Dx to plots of the percentage of flies remaining on targets against time suggests that the rate of leaving the target surface is best modelled with two exponential terms. The rate constants show that uric acid slows the rate of leaving the target surface within the first few seconds after landing but less so thereafter. Uric acid is a good candidate for prolonging the contact times, possibly allowing a decrease of insecticide density on the target surface without impairing the targets' effectiveness.