Entomological studies of on-site sanitation systems in Botswana and Tanzania.
A survey of various types of on-site sanitation systems in Dar es Salaam and Gaborone showed that many were infested with larvae of flies (mainly Chrysomya putoria) and Culex mosquitoes (mainly Cx quinquefasciatus). The mosquitoes only occurred where the pit contents had a free water surface but the flies were found in both wet and scum covered pits. The infestation rate was much higher where the latrine vent pipe had no insect-proof screen. Exit traps were placed on the vent pipes and drop holes to determine by which route the emerging flies and mosquitoes left the pit. Provided that the latrine door was kept closed (thus producing a relatively greater light intensity in the vent pipe), over 90% of flies went up the vent pipe. In most cases about 80% of Cx quinquefasciatus did the same, but in two pits very dense mosquito infestations were found and over 1000 mosquitoes per night were caught leaving the pit through the drop hole. In such cases traps on the drop holes might be used as a semi-permanent control method. Entry traps were placed on the vent pipe and drop hole to catch females attempting to enter to lay eggs. All the flies and the majority of the mosquitoes caught were trying to enter the vent pipe which indicates that odour from this source is attractive to these insects. Various possible modifications to the methods of using latrines to minimize associated insect problems are discussed; the simplest and most important is to ensure that all vent pipes are effectively screened.