Effects of agricultural pesticides on the susceptibility and fitness of malaria vectors in rural south-eastern Tanzania

Abstract

Background Agricultural pesticides may exert strong selection pressures on malaria vectors during the aquatic life stages and may contribute to resistance in adult mosquitoes. This could reduce the performance of key vector control interventions such as indoor-residual spraying and insecticide-treated nets. The aim of this study was to investigate effects of agrochemicals on susceptibility and fitness of the malaria vectors, Anopheles arabiensis across farming areas in Tanzania. Methods An exploratory mixed-methods study was conducted to assess pesticide use in four villages (V1-V4) in south-eastern Tanzania. An. arabiensis larvae were collected from agricultural fields in the same villages and their emergent adults examined for insecticide susceptibility, egg-laying, and wing lengths (as proxy for body size). These tests were repeated using two groups of laboratory-reared An. arabiensis, one of which was pre-exposed for 48hrs to sub-lethal aquatic doses of agricultural pesticides found in the villages. Results Farmers lacked awareness on the linkages between public health and agriculture sectors but were interested in being more informed. Agrochemicals usage was reported as extensive in V1, V2 & V3 but minimal in V4. Similarly, mosquitoes from V1-V3 but not V4 were resistant to pyrethroids, and either pirimiphos-methyl, bendiocarb or both. Adding the synergist, piperonyl butoxide, restored potency of the pyrethroids. Pre-exposure of laboratory-reared mosquitoes to pesticides during aquatic stages did not affect insecticide susceptibility in emergent adults of the same filial generation. There was also no effect on fecundity, except after pre-exposure to organophosphates, which were associated with fewer eggs and smaller mosquitoes. Wild mosquitoes were smaller than laboratory-reared ones but fecundity was similar. Conclusions Safeguarding the potential of insecticide-based interventions requires improved understanding of how agricultural pesticides influence important life-cycle processes and transmission potential of mosquito vectors. In this study, susceptibility of mosquitoes to public health insecticides was lower in villages reporting frequent use of pesticides compared to villages with little or no pesticide use. Variations in the fitness parameters, fecundity and wing length, marginally reflected the differences in exposure to agrochemicals, and should be investigated further. Pesticide use may exert additional life-cycle constraints on mosquito vectors, but this likely occurs after multi-generational exposures.

The ability of Anopheles funestus and A. arabiensis to penetrate LLINs and its effect on their mortality

“…Although wing length for both mosquito species were not measured in the current study, estimates of mosquito body sizes of F1-laboratory reared mosquitoes were extracted from recent studies that used the same colonies and/or collected adults for F1 from the same village 34,35 . Compared to A. arabiensis with relatively large body size (2.6 -3.6 mm), the marked ability of A. funestus to penetrate all net types might be attributed to its small body size (2.6 -2.7 mm) relative to the mesh holes size of the net 7,35 .…”