The Mosquitoes of Bwamba County, Uganda. VI.—Mosquito breeding in Plant Axils

Abstract

1. A study of mosquito breeding in plant axils was carried out in Bwamba County, an area with much heavy forest in the extreme west of Uganda. Attention was directed mainly to Aëdes (Stegomyia) simpsoni, a known vector of human yellow fever in Bwamba.2. The various plants studied are described.3. The results of preliminary surveys are discussed. These confirmed that, as far as mosquito breeding is concerned, the most important plants in Bwamba are the lowland colocasia (Xanthosoma sagittifolium), the “gonja” variety of cultivated banana and the pineapple (Ananas comosus). It was shown that A. simpsoni is the commonest axil-dwelling larva in the inhabited lowland areas, and that it occurs in widely different types of lowland country. Records are given of the occurrence of larvae of Aëdes (Stegomyia) aegypti in plant axils.4. A survey of 12 different kinds of plants is described, and it is shown that the larval populations depend to a considerable extent on the amounts of water retained by the axils of the various plants concerned. In Bwamba, medium-sized axils with a capacity of 4 to 8 cc. appear to be the most favourable. Certain plants which yielded large numbers of larvae—such as the wild banana (Musa sp.)—were not found to harbour A. simpsoni. Apparently this is due to the fact that they are confined to forest and mountain areas, where A. simpsoni is scarce or absent.5. A monthly survey of Xanthosoma axils, carried out over a period of 12 months, showed that A. simpsoni larvae were continually present and that the fluctuations in larval population were dependent on growth and harvesting of the plants rather than on rainfall.6. Scrapings from dry Xanthosoma axils, taken during a short period of drought, were placed in bowls of water in mosquito-proof cages. Larvae of A. simpsoni and Harpagomyia taeniarostris hatched from this material in 2 to 3 days.7. Temperature measurements in a Xanthosoma axil showed that the water therein was cooler than the surrounding air and also that the axil temperature was remarkably equable.8. Systematic collections of pupae from Xanthosoma axils showed that the mosquito output (which cannot be gauged directly from the population of larvae) is high and constant. It is estimated that a plot of 500 plants of this species may, under favourable conditions, produce something like 5,000 A. simpsoni (2,000 females) per month.The influence of predatory larvae (Eretmapodites spp.) on the populations of other species is thought to be considerable.9. The fauna of plant axils, apart from mosquito larvae, is described briefly.