Assessment of temephos resistance in Aedes aegypti and Anopheles stephensi populations in Tamil Nadu, India

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BACKGROUND OBJECTIVES: Monitoring insecticide resistance among mosquito populations is critical for guiding the selection of effective insecticides and implementing sustainable management strategies. This study evaluated the susceptibility of Aedes aegypti and Anopheles stephensi larvae to temephos in three major cities of Tamil Nadu, India. METHODS: Larval and ovitrap collections of Ae. aegypti were carried out in the urban and peri-urban areas, whereas immature stages of An. stephensi were collected exclusively from urban settings. Diagnostic concentration bioassay and dose-response bioassays were performed to determine larval mortality and to estimate resistance ratios relative to a susceptible laboratory strain. RESULTS: An. stephensi populations remained fully susceptible, exhibiting 96.98% - 98.18% mortality at the diagnostic concentrations. In contrast, Ae. aegypti populations showed widespread resistance, with corrected mortality ranging from 13.32% to 72.7% at the diagnostic concentration. Subsequent dose response bioassays indicated moderate to high resistance levels in urban populations. Peri-urban populations exhibited comparatively lower resistance than their urban counterparts. INTERPRETATION CONCLUSION: The findings demonstrate significant resistance to temephos in Ae. aegypti populations across urban areas of Tamil Nadu, underscoring the importance of regular resistance monitoring system and the implementation of integrated insecticide resistance management strategies. Although An. stephensi remains susceptible, continuous surveillance is important to maintain larvicidal efficacy. The outcomes of the study provide essential baseline data to inform and strengthen vector control programmes.

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