Combining unrelated insecticides for improved control and management of insecticide resistant African malaria vectors

Background Alternative long-lasting, practical and effective tools for applying insecticides on home walls against malaria vectors need to be developed. The use of wall hangings made from netting on interior walls for aesthetic purposes is a common practice in rural communities. Insecticide-treated net wall hangings can be produced in a longlasting format and used in an approach that simulates indoor residual spraying (IRS). Methods The efficacy of net wall hangings (NWH) treated with the residual organophosphate insecticide, pirimiphos methyl (1g/sq m), was evaluated in experimental huts against malaria vectors in Muheza, Tanzania. To determine the optimum level of wall coverage required, NWH were tested on ceiling only, two walls, four walls, or four walls plus ceiling. Comparison was made with deltamethrin-treated NWH on two walls. Results Pirimiphos methyl (p-methyl)-treated NWH (on two walls) killed significantly higher proportions of anophelines (92% of Anopheles gambiae and 79% of Anopheles funestus) than the deltamethrin-treated NWH (15% of An. gambiae and 17% of An. funestus) (P<0.001). WHO susceptibility tests showed that the local vector population was susceptible to the organophosphates but resistant to pyrethroids. Mortality rates were significantly higher in huts with p-methyl NWH on two walls (92% for An. gambiae and 79% for An. funestus) than on ceiling only (61% for An. gambiae and 62% for An. funestus, P<0.05). There was no improvement in mortality when wall coverage with p-methyl NWH increased beyond two walls. Blood-feeding rates with p-methyl NWH were generally high across all the treatments (52-77%) and did not differ significantly from the control (64-67%). There was no evidence of reduced blood-feeding or increased exiting with increase in wall coverage with pmethyl NWH.