Troublesome North-American mosquitoes display resistance to insecticides

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.

The pyrethroid-resistance mosquitoes originated from a population that was breeding in pools of water under an apartment below ground. It is not known whether this below-ground population has genetic interaction with the above-ground species not displaying resistance to pyrethroids, but the possibility exists that the resistant genes will spread.

Before pyrethroids were introduced, DDT and other organochlorines were the major pesticides used; Cx pipiens developed tolerance to these chemicals as well as the succeeding organophosphates shortly after they were introduced. The pyrethroid-resistant mosquitoes in this study, however, did display a susceptibility to organophosphates that other groups of mosquitoes in California lacked. This indicates that perhaps the resistance to pyrethroids is developed at the expense of resistance to organophosphates.

Dr Anthony Cornel, a co-author of the paper, explains the significance of his findings: “Evidence of pyrethroid resistance in a medically important mosquito such as Cx. p.p.molestus in California indicates that an integrated approach between agricultural and public health use of chemicals needs to be taken to prevent extensive spread of pyrethroid resistant mosquitoes”.