Bionomics and control of Glossina austeni and G.brevipalpis (Diptera : Glossinidae) in South Africa

Abstract

Glossina austeni Newstead and G. brevipalpis Newstead are responsible for transmitting nagana in the Zululand region of the KwaZulu-Natal Province in South Africa. Following the last major nagana outbreak in 1990-1992, little information has become available on the bionomics and control of these two tsetse fly species. To address this, an initial survey was undertaken of clinically sick animals in a communal farming area adjacent to a game reserve with high tsetse and game abundance. Results of this survey clearly showed that bovine trypanosomosis is prevalent in the vicinity of the Hluhluwe-iMfolozi Park, with buffy coat examinations indicating a 34.2% trypanosomal infection rate, while the PCR-RFLP indicated an infection rate of up to 60.5 %. In order to assist and focus control operations of G. austeni and G. brevipalpis, their preferred habitat types were then studied on the Ndlozi peninsula in Lake St Lucia. The index of apparent abundance for G. brevipalpis was high in the dense indigenous forest, as well as in open grassland and exotic plantations. This has important repercussions for the epidemiology of trypanosomosis in the study area, as G. brevipalpis is usually not considered to be an important vector because of its confinement to areas that are difficult to access by livestock and people. Data from this study suggest that they do occur in open areas where susceptible hosts can be present and although their potential role in the epidemiology of trypanosomosis depends largely on their infection rate, the role of habitat should not be underestimated. For G. austeni, the large majority of flies was captured in the indigenous forest, and this fly is associated with dense habitat and usually ranges close to its primary habitat in search of food. Compared to G. brevipalpis, the distribution of G. austeni is far more restricted. To evaluate the use of insecticide-treated targets for the control of both these tsetse species, a trial was carried out on 120km² of the Ndlozi peninsula. Tsetse targets treated with 0.8% deltamethrin (Glossinex®) were deployed at 4 per km² in year 1 of the study, increased to 8 targets per km² in year 2, and finally increased to 12 per km² in year 3. Target deployment was focused on the indigenous vegetation and forest clumps. Results showed a 99% reduction in G. austeni catches after 17 months from initial target deployment, and maintained for up to 14 months after all targets were removed. Targets deployed at a density of 8 per km² in wooded areas were sufficient to control G. austeni under prevailing conditions. In contrast, for G. brevipalpis, a moderate 60-89% reduction was achieved and could not be maintained. Results indicated differences in the effectiveness of odour-baited insecticide-treated targets in controlling each of the two Zululand tsetse species, attributed to substantial behavioural differences between the two species with regard to their habitat preferences. Following the reduction trial, a two-year study was done to monitor the tsetse population recovery and reinvasion of the previously treated areas. Different rates of tsetse re-invasion and population recovery in the previously treated area were evident for the two tsetse species. For G. austeni, a target barrier, approximately 7 km wide, with 8 targets per km² appears very effective in preventing population recovery and invasion. In contrast, the highly mobile G. brevipalpis dispersed effectively through the barrier area. To evaluate the potential environmental impact of tsetse control on non-targeted horse flies, collections were made of horse flies in the areas treated and untreated with targets during the previously mentioned reduction trial. Results indicated that the H-trap is an efficient tool for collection of horse flies, with a total of 23 species collected. Over a four-year study period, no negative effect was evident on the monthly catches of horse flies. This indicated the negligible impact of insecticide treated targets on horse flies, and their value as an environmentally acceptable method of tsetse control, especially in conservation sensitive areas. Finally, to support local farmers in tsetse-infested areas, two low-cost, low-technology techniques of tsetse control were evaluated. In preparation for the use of restricted application of insecticide to cattle, observations were done on the preferred landing sites of G. austeni and G. brevipalpis on cattle. Results indicated a preference for the lower limbs and belly of cattle as landing sites for both species. The localized control of tsetse flies by treating only the preferred landing sites, i.e. legs and belly of cattle, appears promising as an additional tool in the arsenal against trypanosomosis in KwaZulu-Natal. Experiments were also conducted to determine the effectiveness of protective netting in reducing the challenge by G. austeni and G. brevipalpis. The results showed that physical obstruction of tsetse flies by netting of appropriate height is an effective means of preventing tsetse flies from entering an area, depending on its height and varies between Glossina species.