Effects of the diet of the mosquito Anopheles gambiae on its resistance to deltamethrin.

Background: While many aspects of the genetic basis of mosquito resistance to insecticides are understood, the degree to which this resistance is affected by environmental parameters such as diet remains unclear. Therefore, two experiments were performed to evaluate how the diet of Anopheles gambiae, that is its sugar and blood meals, influences its resistance to deltamethrin.

Methods: The first experiment 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 them received a blood meal. After 15-18 h (i.e., at a time when digestive genes are upregulated) or 60-63 h (i.e., after digestion). They were then exposed 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 h. The second experiment examined how the sugar-meal (consisting of the same three plants) interacted with the age at blood feeding. Mosquitoes were blood-fed or left unfed four or 11 days after emergence and exposed to the insecticide 1 day later.

Results: In the first experiment, the plant species 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, 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.

Conclusion: These results underline the complex role of the mosquitoes' diet on their response to insecticides.