Susceptibility of the invasive malaria vector Anopheles stephensi in Ethiopia to novel chemical insecticides and insect growth regulator

The presence of invasive Anopheles stephensi has been confirmed from field studies in several urban, peri-urban and rural areas of Ethiopia. Recent studies have shown that these mosquitoes are resistant to common public health insecticides, namely pyrethroids (deltamethrin, permethrin, and alpha-cypermethrin), Carbamates (bendiocarb and propoxur), and organophosphates (pirimiphos-methyl). This study aimed to assess the susceptibility of populations of An. stephensi from Ethiopia to novel chemical insecticides recently recommended for vector control, namely, broflanilide (a meta-diamide targeting GABA receptors), clothianidin (a neonicotinoid disrupting nerve signals), and chlorfenapyr (a pyrrole disrupting respiratory pathways ), as well as pyriproxyfen (an insect growth regulator mimicking an insect juvenile hormone which results in adult mosquito emergence inhibition), which is recommended for mosquito larval control. The WHO bottle bioassay test was conducted to assess the susceptibility level of populations of An. stephensi reared from larval collections exposed to recommended diagnostic doses of broflanilide, clothianidin, chlorfenapyr, and pyriproxyfen at 18 µg/bottle, 10 µg/bottle 100 µg/bottle, and 100 µg/bottle, respectively. The solvent used was either a mixture of Mero and acetone or acetone alone. Control bottles were treated with solvent only. Mortality was recorded 24 hours post- exposure for all the insecticides and continued to 72 hours post- exposure for chlorfenapyr and pyriproxyfen. The effect of pyriproxyfen on mosquito fecundity (oviposition) was also assessed over four days after 72 hours holding period.Mortality rates of An. stephensi 24 hrs post-exposure to broflanilide and clothianidin was 100% (100/100), while mortality rates at 24 and 48 hrs post-exposure to chlorfenapyr was 66% (66/100), and 100% (100/100), respectively indicating high susceptibility of populations of An stephensi to the three candidate chemical insecticides. Mortality rates of An. stephensi 24hr , 48hr and 72 hr post-exposure to pyriproxyfen was 0%, 11% (11/100), and 15% (15/100), respectively. There was no mosquito mortality in the controls. Moreover, oviposition inhibition rate in adult female of An. stephensi exposed to pyriproxyfen was 100% while oviposition rate in adult female of An.stephensi in the control was 100%.The study revealed that populations of An. stephensi from Ethiopia were fully susceptible to the three novel chemical insecticides (broflanilide, chlorfenapyr and clothianidin) and an insect growth regulator ( Pyriproxyfen) resulted in complete oviposition inhibition. These findings suggest that the tested candidate chemical insecticides and insect growth regulator (IGR) can be used for indoor residual spraying (IRS) and larviciding, respectively for the control of an invasive malaria vector An. stephensi in Ethiopia.