R. Brightwell

Control of tsetse fly (Diptera: Glossinidae) populations using traps at Nguruman, south-west Kenya.

A field trial was carried out in a Maasai group ranch to assess the use of odour-baited traps for suppression of a population of the tsetse fly Glossina pallidipes Austen. In January, 1987, local people made 100 NG2B traps in their homesteads. These were then deployed within the suppression zone of about 100 km 2 , primarily in the areas of woodland where flies aggregate in the dry season. Traps were baited with acetone (ca. 150 mg/h) and cow urine (ca. 1000 mg/h) and checked at monthly intervals in order to replenish odours and repair damage. A further 90 traps were added between October and December to enlarge the suppression area slightly and to strengthen the trap barriers. The population was monitored using biconical and NG2B traps as well as by mark-release-recapture estimates of population size. By October the number of G. pallidipes in the suppression zone was reduced by 98–99% relative to the number 3 km outside the suppression zone. Some reinvasion, mainly of parous females, occurred in November during the short rains but these flies were rapidly trapped out again. Average mortality rates due to trapping were estimated at 4–5% per day, which, combined with the natural mortality, reduced the adult population at a rate of about 2.6% per day during the dry season. The traps had less effect on the smaller population of G. longipennis Corti but still gave a reduction of up to 90% in the dry season. The use of this low technology approach offers good prospects for future community-based tsetse control operations.

Development of a low-cost tsetse trap and odour baits for Glossina pallidipes and G. longipennis in Kenya.

Abstract. Experiments were carried out to improve the NG2B tsetse trap (Brightwell et al., 1987), baited with acetone and cow urine, for use by rural communities to control G.pallidipes Austen and G. longi‐pennis Corti. Modifications included a lower dose rate of acetone, a new cage design and raising the trap about 15‐20cm. Research on different trap cone materials showed that the degree of light transmission of the netting, rather than its colour, was the crucial factor affecting the catch of G.pallidipes. Adding an additional metre of blue cloth to one side of the trap increased catches of females of both species by about 60%. Traps baited with synthetic phenols yielded similar numbers of G.pallidipes and significantly more G.longipennis than those baited with natural cow urine. The latter difference was not apparent when octenol was also used, so cow urine was retained as one of the odour baits in preference to the imported phenols. Although octenol increased catches of G.pallidipes by only about 30%, catches of G.longipennis were increased 2–4‐fold, making it a very useful attractant for the latter species. The cost of the trap/odour‐bait system was estimated to be US

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

8.5 per unit per annum. The economics of this method of tsetse control are discussed.

A new trap for Glossina pallidipes

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.

Size and mortality rates of Glossina pallidipes in the semi-arid zone of southwestern Kenya

Abstract New tsetse trap designs, based on the Zimbabwe F3 trap, were compared to the biconical trap. When baited with acetone and cow urine, the most successful of these, the NG2B, was on average three times as effective for female Glossina pallidipes (Austen) as a similarly baited biconical trap. The index of increase was shown to be dependent upon temperature, and the implications of this are discussed. Less material is required than for the biconical or F3, and construction is considerably easier. Constructional details of the traps are given using locally available materials. The next step is to test the trap as a component of an appropriate technology for community‐participation control of G. pallidipes.

Changes over twelve years in populations of Glossina pallidipes and Glossina longipennis (Diptera: Glossinidae) subject to varying trapping pressure at Nguruman, south-west Kenya

ABSTRACT. Seasonal changes in the mean size of tsetse, Glossina pallidipes Austen, as indicated by wing vein length, were monitored during 1983‐86 at Nguruman, southwestern Kenya. Changes in size of nulliparous females and wing fray category 1 males were shown to be correlated with the relative humidity 2 months before they were captured. Soil temperature when flies were in the pupal stage had much less effect. Size dependent mortality was demonstrated, with the mean size of flies emerging from pupae significantly less than that of field‐caught flies. This mortality must occur at emergence, since there was no evidence of size‐dependent mortality once the flies became available to the trap. Size was correlated with density‐independent mortality acting on the parent population 2 months previously. It might therefore be possible to use size as an index of the intensity of such mortality. This could be useful when assessing the level of additional mortality required to suppress tsetse populations.

Factors affecting seasonal dispersal of the tsetse flies Glossina pallidipes and G. longipennis (Diptera: Glossinidae) at Nguruman, south-west Kenya

Long term changes in the size of populations of the tsetse Glossina pallidipes Austen and G. longipennis Corti were monitored over a 12 year period at Nguruman in south-western Kenya. Tsetse populations were subject to droughts of varying intensity and, from 1987, to trapping, initially by a research organization, and later by a community-based development project. Populations were mainly sampled using odour-baited biconical traps, with data from other monitoring traps corrected accordingly. Mark–release–recapture studies were carried out to relate trap catches to absolute population size, and to quantify movement between subpopulations. Trypanosomiasis incidence rates in a herd of local cattle were also monitored for much of this period. Trap catches were shown to be well correlated with estimates of absolute population size, with no evidence of any seasonal change in trap efficiency. The intensity of trapping and level of seasonal immigration appeared to be the main determinants of population trends, with effective control being achieved when traps were well maintained. Movement between the two lowland subpopulations was shown to be greater for females, and to be inversely related to temperature. An analytical model was used to investigate the responses of a partially isolated population to trapping pressure. Predictions of a deterministic simulation model demonstrated that the observed changes are consistent with an adult trapping mortality of 4–8% per day, and immigration of 100,000 G. pallidipes females per month in the long rains (April and May), 5000 per month in the short rains (November), and about 500 per month during the dry seasons. Trypanosomiasis incidence in local cattle was greatly reduced during the period of community-based tsetse control. When tsetse were sampled exactly where the cattle were grazing, disease incidence was shown to be linearly related to G. pallidipes catches. Arguments for trap resistance and residual populations were examined, and found to be inconsistent with the data. The future for tsetse control by the Nguruman community is considered.

Control of tsetse flies and trypanosomiasis: Myth or reality?

Seasonal changes in the distribution of the tsetse flies, Glossina pallidipes Austen and G. longipennis Corti, along a transect from riverine thickets out into open plains were monitored along with tsetse density, climatic factors, vegetation and host abundance. Dispersal of tsetse into open country was quantified using the mean spread. During and after the rains both species extended their distribution out into open country up to at least 3.5 km from riverine thicket areas. The mean spread of G. longipennis was greater than that of G. pallidipes . The spread of males and females was very similar, as was the spread of different age categories of parous females, but nulliparous flies and females carrying second instar larvae were under-represented in samples from open areas. This seasonal dispersal can be accounted for by random diffusion with an average root mean square displacement of about 175 m per day. Observed degrees of spread were best correlated with humidity conditions prior to sampling, but multiple regression models suggested that host abundance, vegetation as measured by Normalised Difference Vegetation Indices (NDVI), and, in the case of G. longipennis , tsetse density, were also factors in determining the degree of spread. The significance of these findings in relation to tsetse control is discussed.

Tsetse fly (Diptera: Glossinidae) population dynamics and the estimation of mortality rates from life-table data

The African trypanosomiasis are among Africas most devastating diseases. The human disease, sleeping sickness, and the animal disease, nagana, are caused by trypanosomes, protozoan parasites transmitted by tsetse flies, Glossina spp. Attempts have been made to control tsetse and trypanosomiasis for over 70 years, supported by ever-increasing amounts of foreign aid. Although progress has been made in the control of sleeping sickness, this disease still persists in many countries. Nogono excludes cattle from many of the potentially most productive areas of Africa and is a major constraint on economic development. In this paper, Robert Dransfield, Brian Williams and Robert Brightwell review the control of tsetse and trypanosomiasis in the light of recent progress in our understanding of tsetse population dynamics, with special reference to the experience gained in tsetse control on a Maasai ranch at Ngurumon in the Rift Valley of Kenya, and make suggestions for the management and funding of future control programmes in relation to rural development.

Monitoring tsetse fly populations. I. The intrinsic variability of trap catches of Glossina pallidipes at Nguruman, Kenya

The estimation of tsetse fly mortality rates from life-table data is central to opulation dynamics studies and to the development of tsetse fly control programmes. For a population at equilibrium with a stable age distribution, the age-specific mortalities may be estimated directly from the number of individuals in each age class, but a correction must be applied when the population is growing or declining. Furthermore, if the mortality rates are changing with time, inaccuracies will be introduced into estimates of the mortality rates derived from the age structure of the population since the population will take time to reach a new stable age distribution. In this paper we use the Euler-Lotka equation, which relates the age-specific mortality and fecundity to the overall growth rate of the population, to study the loss rate of the tsetse fly Glossina pallidipes (Austen) as a function of pupal mortality, adult mortality and mortalities applied to each age class seperately. We then present a simulation model in order to quantity and to set limits on the precision of estimates of mortalities when the mortalities are themselves changing.