New evidence published online in Pest Management Science reports the first signs of resistance to pyrethroid insecticides in a population of mosquitoes from Marin County, California. The species in question is not only a major pest, but also acts as a vector of West Nile virus, a virus that spread rapidly westward across the United States after it first invaded the new world in New York in 1999. The study, carried out by researchers in California, determined that the Culex pipiens complex mosquitoes displayed tolerance to pyrethroids, which are commonly used agricultural pesticides. Mosquitoes of the same species in Africa and Asia have displayed pyrethroid tolerance, but this report is the first to indicate tolerance in North American mosquitoes. (...)