L-Lactic Acid as a Factor in the Attraction of Aedes aegypti (Diptera: Culicidae) to Human Hosts2

L(+)-lactic acid in the concentrations given off by human hands was shown in olfactometer tests to be an attractant for avid female Aedes aegypti (L.). The amount recovered by rinsing the hands with acetone ranged from 258 to 1225 micrograms; the amount evaporating from the hands ranged from 23 to 133 micrograms per hour. The regression lines (micrograms lactic acid per percent mosquitoes trapped) obtained in 4 tests with each of 11 subjects were significant, but other factors also influenced individual differences in attractiveness. Vapors from authentic L(+)-lactic acid released from glass sample tubes at rates comparable to those released from hands were attractive either in combination with carbon dioxide (CO2) in filtered air, or alone in room air. However, the synergistic effect of CO2 on the response to lactic acid did not persist more than a few seconds after the mosquitoes left an atmosphere rich in CO3. Breath contained more L(+)-lactic acid than vapor from the hands but was less attractive, probably because it also contained an excess of moisture. D(—)-lactic acid was less attractive than L(+)- lactic acid at low concentrations and in direct competition at higher concentrations. A stocking treated with L(+)-lactic acid at 3.56 milligrams per square centimeter (about 1000–8000 times the amount on hands) was repellent, i.e., the treatment reduced the number of mosquitoes landing and biting. However, in the olfactometer, vapors from this stocking were attractive, at least at a distance of 45 centimeters. Application of a repellent (deet) to the hands did not reduce the amount of lactic acid vapor released.