ARE TSETSE-FLY POPULATIONS CLOSE TO EQUILIBRIUM

Glossina or tsetse flies, the vectors of sleeping sickness, form a uni que group of insects with remarkable characteristics. They are vivipar ous with a slow rhythm of reproduction (one larva approximately every 10 days) determined by the regular ovulation of alternate ovaries. Thi s unusual physiology enables the age of the females to be estimated by examining the ovaries. The resulting ovarian age structure of tsetse fly populations has been used to develop research into the demography of tsetse flies. Several authors have proposed methods of estimating p opulation growth rates from ovarian age distribution data. However, su ch methods are applicable only when the growth rate (lambda) is equal to 1 (i.e. the intrinsic rate of increase r is equal to 0). In fact, i n this type of estimation, the adult survival rate a (or equivalently the mortality rate) cannot be dissociated from the growth rate. Other independently determined demographic parameters must be used to remove this lack of identifiability. We have built a matrix model of the dyn amics of tsetse fly populations which enables the growth rate to be ca lculated from the pupal survival rate, the pupal period and the adult survival rate. Assuming that the age-groups of the population studied have reached a stable distribution, it is possible to calculate the pr obabilities for the observed sample of belonging to each of the age-gr oups, to construct a likelihood function and thus to obtain an estimat e of the 'apparent survival rate' beta = a/lambda If the pupal surviva l rate and the pupal period are known, a and lambda can then be calcul ated from beta. The application of this method to data collected for o ver two annual cycles in a savannah habitat (Burkina-Faso) showed a hi gh overall stability in the populations of Glossina palpalis gambiensi s. Seasonal fluctuations could be easily interpreted as being the resu lt of climatic changes between the dry and rainy seasons.