Emerging Resistance in Anopheles stephensi Larvae to the Larvicide Temephos in Chennai, India

This study aimed to evaluate the susceptibility of immature Anopheles stephensi to temephos formulation, a primary antilarval intervention currently in operational use. The assessment was conducted against both the WHO-recommended discriminatory dosage with temephos (0.25 parts per million ppm) and the operational field dosage with temephos 50% emulsifiable concentrate (EC; 1 ppm). F1 generation immature An. stephensi from two areas of Chennai, India was used for the study. The larvae (F1) from both areas were fully susceptible to the WHO discriminatory dosage. Larval susceptibility tests with temephos 50% EC observed 100% mortality in Thiruvanmiyur strains at 40 ppm (lethal concentration at which 99% of the exposed larvae were dead LC99: 37.47-45.06 ppm). However, Adyar strains exhibited 98.7% mortality at 50 ppm (LC99: 59.79 ppm). Anopheles stephensi larvae from both areas showed resistance to operational field dosage (1 ppm). Prolonged insecticide exposure precipitates resistance, highlighting the need for routine larvicide efficacy monitoring to support disease elimination efforts.