The emergence and widespread distribution of insecticide resistance in Anopheles gambiae s.l. raise serious concerns regarding the long-term effectiveness of vector control tools. Assessing resistance intensity provides more operationally relevant information than diagnostic-dose bioassays alone. The phenotypic intensity of resistance to pyrethroids (permethrin and deltamethrin), bendiocarb, chlorfenapyr and clothianidin in Anopheles gambiae s.l. was evaluated 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% to 10–45%, respectively, depending on the study sites. Even at 5 × and 10 × concentrations, pyrethroid-induced mortality often remained below 90%, indicating high-intensity resistance. Resistance levels were consistently higher for permethrin than for deltamethrin, in both WHO tube tests and CDC bottle bioassays. In contrast, bendiocarb induced high mortality rates, generally exceeding 90–98% in most localities, with low to moderate resistance detected in only a few sites. Complete susceptibility of Anopheles gambiae s.l . was observed to chlorfenapyr and clothianidin, with mortality rates ≥ 98% across all communes. The intense and widespread pyrethroid resistance observed in Benin represents a major threat to interventions relying on this insecticide class. WHO tube tests and CDC bottle bioassays showed similar qualitative trends, although differences in mortality levels were observed between the two methods. The relatively preserved susceptibility to bendiocarb, as well as to chlorfenapyr and clothianidin, supports their potential use in insecticide rotation strategies, including bi-treated insecticidal nets (e.g., Interceptor ® G2, PermaNet ® Dual, Yorkool ® G5, and Mont Inari) as well as indoor residual spraying campaigns using bendiocarb or clothianidin.
Kpanou et al. (Sun,) studied this question.