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, the impact of these alkaloids on the vectorial capacity of mosquitoes is not known, in particular regarding increased resistance to insecticides.

The effects of caffeine on Anopheles coluzzii female mosquitoes in terms of resistance to the insecticide deltamethrin, the rate at which mosquitoes are knocked-down during insecticide exposure, mortality within 48 h of insecticide exposure and longevity following exposure were studied. We also 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 allowed to feed on infected or uninfected blood, and 3 days later they were exposed to a deltamethrin-treated filter paper or to a sham (untreated paper). During the next 48 h the mosquitoes were checked for knockdown during the exposure and death. We measured the longevity of the surviving mosquitoes 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. Caffeine also increased the mortality of the insecticide-exposed mosquitoes within 48 h 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. In contrast, the longevity, once the mosquitoes had survived the first 48 h, was not affected by the concentration of caffeine or by its interactions with infection status or insecticide; however, 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, the results of this experiment highlight that the level of resistance to an insecticide is affected by complex interactions between the mosquito's diet and infection by malaria.