Local variation in the population dynamics of Glossina palpalis palpalis (Robineau-Desvoidy) (Diptera: Glossinidae). I. Natural population regulation

Based on data collected during regular, biconical trap sampling of Glossina palpalis palpalis (R.-D.) over two and a half years during and after two insecticide programmes in Ivory Coast, various analytical techniques were developed which allow the estimation of mortality rates and life-time fertility from ovarian age distributions of female flies. Data from trapping-out experiments were used to calculate the natural resilience of local populations. The results reveal that the dynamics of the populations showed considerable local variation, with two major types of sites. At those sites with large numbers of domestic pigs, the fly population was dense and showed a high degree of natural resilience and a low mean mortality rate, so that it could be selfsustaining through reproduction alone. At sites with few pigs, the fly population was scarce and showed a lower degree of natural resilience and a higher mean mortality rate such that the population couJd only be maintained by continuous input of immigrant flies. Studies of the natural dynamics of tsetse can be used to predict the likely effectiveness of alternative control schemes; fly mortality rates must be permanently increased in order to reduce fly numbers or infection rates to below some threshold for disease transmission. Introduction The suppression of insect pests by chemical and other means inevitably interferes with the natural regulation that populations experience. Only when natural control and regulation are fully understood can they be manipulated in pest control programmes to maximise benefits (a greater degree of suppression for a longer period of time) whilst at the same time minimising costs, both economic and environmental. Both benefits and costs therefore have ecological components, the former related to the dynamics of the target species, the latter to those of non-target organisms that may include the natural enemies of the target species. In general, ecological data collected for one purpose are often inadequate for other, even related purposes. During pest control programmes, however, sufficient long-term data collected to demonstrate the efficacy of the programmes themselves may also be analysed to provide insight into the natural processes referred to above. This and a second paper (Randolph et al., 1984) provide an example of this dual use of sample data for the tsetse fly Glossina palpalis palpalis (Robineau-Desvoidy) in West Africa. In 1978, the UNDP/World Bank/WHO Special Programme for Research and Training Dn Tropical Diseases (TDR) began an entomological investigation of a focus of human ' Present address: World Health Organization, 1211 Geneva 27, Switzerland. 404 DAVID J. ROGERS, SARAH E. RANDOLPH and FELIX A. S. KUZOE sleeping sickness in the Bouafle' region of Ivory Coast (project TDR 308). This is at the edge of the forest zone of West Africa, although natural forest remains only in small patches interdigitating with savanna and cultivation which is based on many small villages or seasonal 'campements'. An epidemiological study in the region is described by Haller et al. (1979), and two separate insecticidal campaigns were reported by Kuzoe et al. (1979) and S6k6t61i & Kuzoe (1981). Baldry (1980) provides an overview of the West African tsetse situation. During the entomological work of the project, the tsetse fly populations in a number of villages, or along major paths, were sampled by means of biconical traps (Chalh'er & Laveissiere, 1973). Much of the present paper concerns an analysis of these trapping results and experiments arising from them, and discusses their implications for the natural regulation of tsetse populations. The second paper is specifically concerned with the interpretation of the insecticide spray programme's results. Materials and methods Collection of regular samples Biconical traps were placed in the following villages (Fig. 1), in approximately increasing order of size (number of traps in brackets after each): Koffikro (4), Klebo I (4), Yaokro (9), Congo Aboisso II (4), Tuyankro (4), Kouassi Perita (6), DegbSzere (3) and Koudougou (10). A 7-km-long track running between Degb6z6r6 and Koudougou was also sampled in two sections, the Degb6z6r6-Koudougou path (D-K path, 4 traps) and the KoudougouDegb6zer6 path (K-D path, 8 traps). Trap sites were chosen at the start of the investigation, November-December 1978, and were in general retained throughout the trapping period, ending in June 1981. They were