Lactose shortens the lifespan of <i>Anopheles stephensi</i> (Diptera: Culicidae) due to osmotic diarrhea-like symptoms

Anopheles mosquitoes spread malaria, which is caused by Plasmodium parasite infection. In nature, both male and female mosquitoes typically ingest sugars from floral nectar; however, few studies have investigated the effects of sugars as an insecticide for mosquitoes. During our previous work, the lifespan of An. stephensi was shortened by the feeding of high concentrations of lactose. The mechanism by which lactose can shorten the lifespan of mosquitoes is still unidentified. The present study aims to evaluate the potential of lactose as an insecticide against mosquitoes. A sublethal concentration of lactose decreased the lifespan of An. stephensi but did not affect their blood-feeding ratio or the number of eggs they laid. Female Anopheles mosquitoes refused to ingest lactose-containing sugars when a lactose-free sugar is available; however, male mosquitoes ingest them. Both sexes excreted lactose from their bodies after feeding, while other sugars were completely absorbed and digested. An ingestion volume assay using food dye revealed that lactose reduces the volume of the solution in the mosquito body. Female mosquitoes forced to ingest lactose for 3 wk showed a slight change in the composition of their midgut microbiota but not in their relative quantity. These findings suggest that lactose feeding causes osmotic diarrhea-like symptoms in Anopheles mosquitoes. The use of attractive toxic sugar baits with lactose as an insecticide is expected to control male Anopheles mosquitoes.