The emergence and spread of insecticide resistance among the main malaria vectors is threatening the effectiveness of vector control interventions in Senegal. The main drivers of this resistance in the Anopheles gambiae complex (e.g., An. gambiae and Anopheles coluzzii) remains poorly characterized in Senegal. Here we characterized the main target site and metabolic resistances mechanisms among the An. gambiae and An. coluzzii populations from their sympatric and allopatric or predominance area in Senegal. Larvae and pupae of An. gambiae s.l. were collected, reared to adulthood, and then used for insecticides susceptibility and synergist assays using the WHO (World Health Organisation) test kits for adult mosquitoes. The TaqMan method was used for the molecular characterization of the main target site insecticide resistance mechanisms (Vgsc-1014F, Vgsc-1014S, N1575Y and G119S). A RT-qPCR (Reverse Transcriptase-quantitative Polymerase Chaine Reaction) was performed to estimate the level of genes expression belonging to the CYP450 (Cytochrome P450) family. Plasmodium infection rate was investigated using TaqMan method. High levels of resistance to pyrethroids and DDT and full susceptibility to organophosphates and carbamates where observed in all three sites, excepted a probable resistance to bendiocarb in Kedougou. The L1014F, L1014S, and N1575Y mutations were found in both species. Pre-exposure to the PBO (Piperonyl butoxide) synergist induced a partial recovery of susceptibility to permethrin and full recovery to deltamethrin. Subsequent analysis of the level of genes expression, revealed that the CYP6Z1 and CYP6Z2 genes were over-expressed in wild-resistant mosquitoes compared to the reference susceptible strain (Kisumu), suggesting that both the metabolic resistance and target site mutation involving kdr mutations are likely implicated in this pyrethroid resistance. The presence of both target-site and metabolic resistance mechanisms in highly pyrethroid-resistant populations of An. gambiae s.l. from Senegal threatens the effectiveness and the sustainability of the pyrethroid-based tools and interventions currently deployed in the country. The Kdr-west mutation is widely widespread in An. coluzzii sympatric population. PBO or Duo nets and IRS (Indoor Residual Spraying) with organophosphates could be used as an alternative measure to sustain malaria control in the study area.

Background Anopheles gambiae s.s. and An. coluzzii, two major malaria vectors in Africa, exist in the nature as two incipient, sympatric or allopatric species. In most of their sympatric areas, the reproductive isolation between these two distinct species is thought to be the main barrier to hybridization. However, in Senegal, barriers to the gene flow seems to be leaky in some areas with relatively higher than expected hybridization rates. Here, we characterized the swarming behavior of these two species to investigate its role in the observed high hybridization.Methods The study was carried out in the south and center of Senegal during the 2018 rainy season. Swarms were surveyed at sunset towards the lightest part of the sky, about 0.5–4 m above the ground. Once located, swarm were collected using a net. Indoor resting populations were also collected during the same period from each sentinel village by pyrethrum spray catch earlier the morning. All specimens collected were identified morphologically followed by PCR to estimate the frequency of the two species and female hybrids.Results Results showed that An. gambiae swarmed mainly over bare ground whereas An. coluzzii swarms over various objects forming a dark-light contrast with the ground. The height of swarms varied between 0.5 to 2.5 meters and the swarming duration was about 10 minutes. Start of swarming was mainly correlated with sunset and no mixed swarms were found in areas of sympatry despite the high level of hybridization rate (2.4% − 4.4%).Conclusion As found elsewhere in West Africa, swarming site segregation is an important pre-mating reproductive barrier between An. coluzzii and An. gambiae in Senegal. No link was found between swarming behavior and hybridization, but the lack of mixed swarms may be the result of low number of samples obtained in the sympatric area.