Phenotypic intensity of insecticide resistance in Anopheles gambiae sensu lato along the south–north transect in Benin

The emergence and widespread distribution of insecticide resistance in Anopheles gambiae s.l. raise serious concerns about the long-term effectiveness of vector control tools. Assessing resistance intensity provides more operationally relevant information than diagnostic-dose bioassays alone. Phenotypic resistance intensity to pyrethroids (permethrin and deltamethrin) and bendiocarb in Anopheles gambiae s.l. was assessed along a south–north transect in Benin using WHO tube tests and CDC bottle bioassays. Mortality was recorded at diagnostic doses and at higher concentrations in accordance with WHO guidelines. At diagnostic doses, mortality rates observed with permethrin and deltamethrin were low, ranging approximately from 5–30% and 10–45%, respectively, depending on the study site. Even at 5× and 10× concentrations, pyrethroid-induced mortality frequently remained below 90%, indicating high-intensity resistance. Resistance levels were consistently higher for permethrin than for deltamethrin in both WHO and CDC assays. In contrast, bendiocarb induced high mortality rates, generally exceeding 90–98% in most localities, with low to moderate resistance detected at only a few sites. Results obtained using both bioassay methods were concordant. The intense and widespread resistance to pyrethroids observed in Benin represents a major threat to interventions relying on this insecticide class. The relatively preserved susceptibility to bendiocarb supports its potential use within insecticide rotation strategies. Integrating resistance intensity data into national surveillance systems is essential to inform evidence-based vector control decision making.