Effects of a caffeine-based diet on insecticide resistance and longevity in infected Anopheles coluzzii

Alkaloids such as caffeine can be toxic for insects. However, although mosquitoes feed on many plants with nectar containing alkaloids, their impact on the vectorial capacity of mosquitoes is not known, in particular in the face of increasing resistance to insecticides. We assessed with the mosquito Anopheles coluzzii how caffeine affects several measures of resistance to deltamethrin – the rate at which mosquitoes are knocked-down during exposure, the mortality within 24 hours of exposure, and the longevity following exposure – and we compared these traits for mosquitoes that were uninfected or infected by the malaria parasite Plasmodium falciparum . The mosquitoes were fed throughout their lives on a 10% sugar solution supplemented with 0, 50, or 200 ppm caffeine. Three or four days after emergence, they were given an infected or an uninfected blood meal. Another three days later, they were exposed to deltamethrin or to a sham and checked for knock-down during the exposure and death within the next 48 hours. We monitored the surviving mosquitoes for longevity and assessed their infection status when they died. The rate of mosquitoes knocked down by the insecticide increased with higher caffeine concentrations, but neither the infection status nor its interaction with caffeine concentration influenced the knockdown rate. Similarly, caffeine increased the mortality of insecticide-exposed mosquitoes within 48 hours after exposure. The mortality was highest if mosquitoes had fed on infected blood but harbored no parasites, and lowest if they had not fed on infected blood. The longevity, once the mosquitoes had survived the first 48 hours, was not affected by the concentration of caffeine or by any of the combination of caffeine with infection status or insecticide, but, the mosquitoes that had been exposed to the insecticide lived longer than unexposed ones, in particular if they had fed on infected blood but were not infected.Overall, our experiment highlights that the level of resistance to an insecticide is affected by complex interactions between the mosquito’s diet and infection by malaria.