A malaria-carrying mosquito inherited insecticide-resistance genes from a related species, around the time that bed nets treated with insecticide were increasingly used in West Africa.

Gregory Lanzaro at the University of California, Davis, and his colleagues analysed DNA from more than 1,000 specimens of Anopheles coluzzii and Anopheles gambiae in Mali from 2002 to 2012. They found that a group of genes, including one for insecticide resistance, from A. gambiae moved into A. coluzzii around 2006 when the two species mated.

Campaigns to encourage the use of insecticide-treated bed nets began in 2005 in this region, and the authors suggest that the nets favoured the selection of hybrid, insecticide-resistant mosquitoes.

Proc. Natl Acad. Sci. USA http://doi.org/x56 (2015)