Influence of plume structure, composition and concentration on orientation behavior of mosquitoes to host odors

Mosquitoes rely partially on odors to find a blood host. We addressed the question of how the structure, composition and concentration of host odor plumes may affect mosquito orientation to odor sources.;In a Y-tube study we found that L-Lactic acid, a human signifying skin odor compound that is hardly attractive alone, is a key component in combination with other odors and mediates intra- and interspecific host selection by Anopheles gambiae mosquitoes.;The effect of plume structure on upwind flight and trap capture was tested in a dual-choice olfactometer. We demonstrated that homogeneous CO2 plumes inhibit upwind flight and trap capture of Ae. aegypti and An. gambiae, whereas homogeneous skin-odor plumes do not. Turbulent CO2 plumes, however, induce upwind flight and trap capture.;Further tests with Ae. aegypti were done in a wind tunnel. Brief CO2 filament encounters induced brief upwind surges followed by casting, but brief encounters of skin odor did not influence the flight behavior noticeably. In broad turbulent skin-odor plumes upwind flight was reduced compared to homogeneous plumes. We postulated that recognition of across-fiber encoded odors (such as skin odor) is typically slower than labeled-line encoded odors (such as CO2). Honey bees conditioning to a 300 ms odor pulse discriminated less between odors in subsequent tests compared to bees trained to a 2 s odor pulse. This supports our hypothesis of slow recognition of odor in across-fiber encoded odors.;We also demonstrate that Ae. aegypti was less sensitive to dilutions from natural concentrations of skin odor than to dilutions of CO2. We found, however, that previous exposure to brief CO 2 filaments increased their sensitivity. Apparently CO2 sensitizes the central olfactory system for skin odor.;Finally, detailed analysis of the flight tracks demonstrated that mosquitoes by actively steering upwind after contacting a CO2 filament distort a steady relationship between longitudinal and transverse visual flow across the ventral ommatidia of the mosquito's eye, which transiently negates optomotor anemotaxis. Because our analysis showed that during these maneuvers mosquitoes keep their coriolis forces input constant, we postulate that mosquitoes exhibit for 500--600 ms a haltere-mediated fixed-action pattern type of flight maneuver, which is imprecisely aimed upwind.