Dynamics of insecticide resistance in the malaria vector Anopheles gambiae s.l. from an area of extensive cotton cultivation in Northern Cameroon.

To explore temporal variation in insecticide susceptibility of Anopheles gambiae s.l. populations to the four chemical groups of insecticides used in public health and agriculture, in close match with the large-scale cotton spraying programme implemented in the cotton-growing area of North Cameroon. Mosquito larvae were collected in 2005 before (mid June), during (mid August) and at the end (early October) of the cotton spraying programme. Larvae were sampled in breeding sites located within the cotton fields in Gaschiga and Pitoa, and in Garoua, an urban cotton-free area that served as a control. Insecticide susceptibility tests were carried out with 4% DDT (organochlorine), 0.4% chlorpyrifos methyl (organophosphate), 0.1% propoxur (carbamate), 0.05% deltamethrin and 0.75% permethrin (pyrethroids). Throughout the survey, An. gambiae s.l. populations were completely susceptible to carbamate and organophosphate, whereas a significant decrease of susceptibility to organochlorine and pyrethroids was observed during spraying in cotton-growing areas. Tolerance to these insecticides was associated with a slight increase of knockdown times compared to the reference strain. Among survivor mosquitoes, the East and West African Kdr mutations were detected only in two specimens of An. gambiae s.s. (n = 45) and not in Anopheles arabiensis (n = 150), suggesting metabolic-based resistance mechanisms. Environmental disturbance due to the use of insecticides in agriculture may provide local mosquito populations with the enzymatic arsenal selecting tolerance to insecticides.