Effects of the diet of the mosquito Anopheles gambiae s.l. on its resistance to an insecticide

Although genes responsible for resistance of mosquitoes against insecticides are common throughout malarious areas, it is not clear how much they affect the epidemiology of malaria, for resistance can be strongly affected by the environment. We therefore focused on the diet of Anopheles gambiae , an important aspect of their environment, with two experiments investigating how sugar and blood meals affect their resistance to deltamethrin. The first focused on sugar meals taken from different plants and on the time between the blood meal and the exposure to the insecticide. Mosquitoes had continuous access to Tevethia nerifolia , Ixora coccinea or Mandalium coromandelianum as sugar meals, and half of the mosquitoes received a blood meal. After 15–18 hours (i.e., at a time when digestive genes are upregulated) or 60–63 hours (i.e., after digestion) we exposed them to 0.5% deltamethrin for one hour and measured the proportion of mosquitoes that were knocked down during the exposure and that died within the next 24 hours. The plant had no effect on the rates of mortality or knock-down. If the mosquitoes were exposed earlier, blood-feds were 22.7% less likely to die and 10.0% less likely to be knocked down than unfeds,, but if they were exposed later, blood-feeding increased mortality by 8.7% and knock-down by 14.0%. In the second experiment, we explored how the sugar-meal (consisting of the same three plants) interacted with the age at blood feeding. The mosquitoes were blood-fed or left unfed four or 11 days after emergence and exposed to the insecticide one day later. Neither the plant nor its interactions with blood meal or age affected mortality, but younger mosquitoes had lower mortality (60.7%) than older ones (66.4%), independently of their blood-meal. Similarly, the plant had no effect on knock-down rate, but the blood meal increased it by 14.5% in young mosquitoes and reduced it by 21.5% in old ones. These results underline the complex role of the mosquitoes’ diet on their response to insecticides.