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Background  Insecticide resistance is threatening the effectiveness of efforts to control malaria vectors in Benin. This study explores the levels and mechanisms of insecticide resistance in  An. gambiae  s.l. to pyrethroids.    Methods  Larvae were collected from August 2017 to July 2018 in five communes in southern Benin (Adjohoun, Allada, Bohicon, Cotonou, and Porto-Novo) representing diverse ecological regions, and were reared in Benin’s insectary. Two- to five-day-old female mosquitoes from each district were exposed to multiple doses of deltamethrin and permethrin (1×, 2×, 5×, and 10×) using the WHO insecticide resistance intensity bioassay. The effect of pre-exposure to the synergist, piperonyl butoxide (PBO), was also tested at different pyrethroid doses. Molecular allele frequencies of  kdr  (1014F) and  ace-1 R (119S) insecticide resistance mutations and levels of detoxification enzymes were determined for mosquitoes sampled from each study area.    Results   An. gambiae  s.l. were resistant to pyrethroid-only exposure up to 10× the diagnostic doses in all the study sites for both deltamethrin and permethrin. Mortality was significantly higher in  An. gambiae  s.l. pre-exposed to PBO followed by exposure to deltamethrin or permethrin compared to mosquitoes exposed to deltamethrin or permethrin only ( p  < 0.001). The difference in mortality between deltamethrin only and PBO plus deltamethrin was the smallest in Cotonou (16–64%) and the greatest in Bohicon (12–93%). The mortality difference between permethrin only and PBO plus permethrin was the smallest in Cotonou (44–75%) and the greatest in Bohicon (22–72%). In all the study sites, the  kdr  resistance allele (1014F) frequency was high (75–100%), while the  ace-1  resistance allele (G119S) frequency was low (0–3%). Analysis of the metabolic enzymatic activity of  An. gambiae  s.l. showed overexpression of nonspecific esterases and glutathione S-transferases (GST) in all study sites. In contrast to the PBO results, oxidase expression was low and was similar to the susceptible  An. gambiae  s.s. Kisumu strain in all sites.    Conclusion  There is high-intensity resistance to pyrethroids in southern Benin. However, pre-exposure to PBO significantly increased susceptibility to the pyrethroids in the different  An. gambiae  s.l. populations sampled. The use of PBO insecticide-treated bed nets may help maintain the gains in  An. gambiae  (s.l.) control in southern Benin.    Graphical Abstract

Intensity and mechanisms of deltamethrin and permethrin resistance in Anopheles gambiae s.l. populations in southern Benin