L-lactic acid: a human-signifying host cue for the anthropophilic mosquito Anopheles gambiae

Using a dual‐choice olfactometer, the role of l‐lactic acid was investigated in relation to host‐seeking and selection by female Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) mosquitoes in a Y‐tube bioassay. l‐lactic acid alone was not attractive, but it significantly augmented the attractiveness of CO2, skin odour and skin‐rubbing extracts from humans and other vertebrates. Comparing the left and right index fingers of the same person, one could be made more attractive than the other by adding l‐lactic acid to the air stream over that finger. The difference in l‐lactic acid concentration between the two air streams offered to the mosquitoes fell within the natural range of variation emanating from a human hand, suggesting that l‐lactic acid modulates intraspecific host selection by An. gambiae. Analysis of skin rubbings from various vertebrates (carnivores, chickens, primates, rodents, ungulates) indicated that humans have uniquely high levels of l‐lactic acid on their skin. Tests with extracts of skin rubbings from cows and humans, with and without added l‐lactic acid, suggest that naturally lower levels of l‐lactic acid contribute to the lesser attractiveness of non‐humans to An. gambiae s.s.