Distribution of insecticide resistance and molecular mechanisms involved in the West Nile vector Culex pipiens in Morocco

BACKGROUND Mosquitoes of the Culex pipiens complex are the vectors of several arboviruses and are thus subjected to insecticide control worldwide. However, overuse of insecticides selects for resistance. While assessing the resistance status of the vectors is required for effective and sustainable disease control, resistance has so far only been sparsely studied in Morocco. In this study, we establish a first countrywide assessment of the levels of resistance to various insecticides and the potential responsible mechanisms involved. Cx. pipiens larvae were collected from natural populations of five regions of Morocco, and their taxonomic status was determined (molecular forms). The level of their susceptibility to insecticides was assessed by single-diagnostic-dose bioassays. Molecular identification of known resistance alleles was investigated to determine the frequency of target-site mutations. RESULTS This study confirms that Moroccan populations are an interbreeding mix of pipiens and molestus forms, with large gene flow for the resistance alleles. We also found that Cx. pipiens mosquitoes are resistant to all insecticide families, all over Morocco: resistance is high for insecticides used in mosquito control, but also present for other pesticides. Resistance alleles are similarly more frequent for mosquito control insecticides. However, their distribution is heterogeneous in the five regions, with significant genetic differentiation between populations, revealing the crucial role of local insecticide treatment practices. CONCLUSION This study provides reference countrywide data that highlight the need for further research to refine the distribution of resistance in Morocco and to understand the role of agriculture/urban residuals in its spread.

Automated summary

This study showed that populations of Culex pipiens in Morocco are a mix of pipiens and molestus forms, with significant gene flow for resistance alleles. The mosquitoes are resistant to all insecticide families across the country, with higher resistance to those used for mosquito control. Resistance alleles are more prevalent for mosquito control insecticides, but their distribution varies between regions, highlighting the importance of local insecticide treatment practices. Further research is needed to understand the spread of resistance in Morocco and the role of agriculture/urban residuals.