In order to estimate the possible effect of sterile-male releases alone on tsetse fly 

 population trends, a mathematical model has been established on the basis of the various 

 assumptions made in the foregoing discussion. A low-level population averaging 200 

 flies per square mile has been chosen as the natural population density for the model. 

 The calculations are shown in model 11. If the natural male population is outnumbered 

 3:1 during the first 3-month period (or generation), a downward trend in the population 

 would theoretically result, permitting a reduction in the number released each subse- 

 quent period until elimination is achieved by the end of a year. It is the assumption in 

 this model that the releases are reduced by one-half each period. Since the theoretical 

 rate of decline in the natural population is greater than the reduction in sterile males 

 released, the ratio of sterile to fertile males will gradually increase. 



On the basis of the population dynamics of tsetse flies in general, we could expect 

 an initial release rate as low as 1:1 to start a downward trend in the natural population. 

 However, the 3:1 ratio was selected so that the estimates would be as conservative as 

 possible. A point that should be mentioned is that older females, already mated before 

 sterile males are released, would not be affected if monogamous mating by females is 

 the rule. Therefore, male releases could only begin exerting effects on the overall 

 reproductive potential of the population as virgin females emerge from pupae. Thus, 

 the full impact of the releases will not begin to be felt until older, previously mated 

 females decline because of normal environmental resistance factors. In actual practice, 

 therefore, the effect on population trends will be delayed in the beginning. Theoretically, 

 however, the effects of the releases should extend beyond the period indicated by virtue 

 of some survival of released males beyond the time indicated. From a practical stand- 

 point, these should be compensating factors so far as the number of insects and time 

 required to achieve elimination of a population are concerned. The figures in model 11 

 indicate that the release of about 1, 700 male tsetse flies during the course of a year 

 should eliminate a low natural population. This in itself is encouraging, since there are 

 no doubt many situations where such low levels on the average would occur in large 

 infested areas. ^However, there would appear to be a number of advantages to the use of 

 at least one insecticide mist spray prior to the use of sterile insects. Therefore, 

 integrated programs of sterile insects and insecticides will be considered next. 



The Use of Sterile Males Integrated with Other Control Methods 



Under conditions where the average population density of tsetse flies is high 

 (1, 000 or more per square mile), the sterile-male-release method should prove useful 

 as an adjunct to chemical control procedures, or chemical control might be regarded as 

 the adjunct to the sterility method. The research by K. S. Hocking and associates of 

 the Tropical Pesticide Research Institute, Arusha, Tanganyika, has shown that two or 

 three airplane sprays will drastically reduce natural population densities, even though 

 6 to 8 treatments may be required for eradication. Under circumstances where the 

 natural population of tsetse flies in an area is too high to manage economically with 

 sterile-male releases alone, the prior application of several airplane mist sprays 



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