The Control of Tsetse Flies in Relation to Fly Movement and Trapping Efficiency

1. The control of tsetse fly populations using traps or targets depends on the movement patterns of the flies, which determines how many flies find the traps, and on the efficiency of the traps, which determines the proportion of these flies that are killed. In this paper we develop models to predict population loss rates under various trapping regimes. The parameters in our models are the range of attraction of the traps, the mortality rate imposed by the traps, the rate at which the flies diffuse through an area, the fly population growth rate, and the distribution of the traps or targets. 2. We derive analytical results for two limiting cases: very mobile flies and inefficient traps; relatively immobile flies and very efficient traps. We show that if the flies are very mobile and the traps relatively inefficient, the rate at which the fly population is reduced is limited by the range of attraction, the trapping mortality rate and the population growth rate; if the flies are relatively immobile and the traps very efficient, the rate of reduction is limited by the mobility of the flies and the population growth rate. The actual situation will lie within these limits. Numerical simulations are used to test the validity of the analytical results. Data from field studies in Africa are used to test the predictions of the models and to confirm their validity. 3. We show how the efficiency of barriers constructed from lines of traps or targets depends on the width of the barrier, the mobility of the flies and the mortality rate within the barrier. 4. We calculate the distance beyond the range of attraction of a trap over which the trap will reduce the fly population density significantly. 5. We investigate the relationship between trap catches and population densities and determine the factors that effect the calibration of traps as sampling devices for the two limiting cases. 6. We investigate the rate at which a fly front will advance into country cleared of or previously unoccupied by flies and provide an explanation for observations regarding the relatively slow rate at which fly fronts advance. 7. Extending our models to inhomogeneous habitats and combining them with our knowledge of tsetse biology and information on climate and vegetation should make it possible to predict spatial and seasonal changes in tsetse fly densities and so to provide a sound basis for planning tsetse control operations.