Status of insecticide susceptibility in Anopheles arabiensis from Mwea rice irrigation scheme, Central Kenya

Susceptibility to DDT (an organochlorine), fenitrothion (an organophosphate), bendiocarb (a carbamate), lambdacyhalothrin and permethrin (both pyrethroids) was tested using standard WHO diagnostic bioassay kits. Bioassays were performed on non-blood fed mosquitoes one- to three-day old. Knockdown was recorded every 10 min and mortality 24 h post-exposure was noted.Mortality 24 h post-exposure was 100% for all insecticides except for lambdacyhalothrin, which averaged 99.46%. Knockdown rates at 10 min intervals were not significantly different between the Mwea population and the susceptible KISUMU strain of Anopheles gambiae sensu stricto control. The KDT50 and KDT95 values for the Mwea population were either lower than those for the control or higher by factors of no more than 2 for most comparisons and compared well with those of An. gambiae sensu lato categorized as susceptible in other studies.These results suggest that the Mwea population of An. arabiensis is susceptible to all the insecticides tested. This implies that vector control measures employing any of these insecticides would not be hampered by resistance.Anopheles gambiae sensu stricto, Anopheles arabiensis and Anopheles funestus are the most important vectors of malaria in sub-Saharan African and occur in sympatry across most of their range [1]. Studies show that the use of insecticides both for Indoor Residual Spraying (IRS) programmes and in the treatment of bed nets has resulted not only in a reduction in vector population densities but also in morbidity and mortality due to malaria [2-4]. There is, however, evidence that malaria vectors are developing resistance to commonly used insecticides [5]. In Western Kenya, resistance was first reported in the context of Insecticide-Treated Net (ITN) use [6]. Although more recent studies indicate that resistance levels have increased only marginally [7], there is concern that continued and/or increased use of insecticides may result in increased resistance t