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First published online July 25, 2005
Journal of Experimental Biology 208, 2963-2972 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01736

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Carbon dioxide instantly sensitizes female yellow fever mosquitoes to human skin odours

Teun Dekker^1,*, Martin Geier² and Ring T. Cardé¹

¹ Department of Entomology, University of California, Riverside, CA 92521, USA
² Institut fur Zoologie, Universität Regensburg, Universitätsstrasse 31, Regensburg 93040, Germany

^* Author for correspondence at present address: Division of Chemical Ecology, Department of Crop Science, SLU, Alnarp, SE-23053, Sweden (e-mail: teun.dekker{at}vv.slu.se)

Accepted 6 June 2005

Female mosquitoes are noted for their ability to use odours to locate a host for a blood meal. Two sensory organs contribute to their sense of smell: the maxillary palps, which measure the level of CO₂, and the antennae, which detect other host-released odours. To establish the relative importance and interactions of CO₂ and other body emissions in freely flying mosquitoes, we presented female yellow fever mosquitoes Aedes aegypti L. with broad plumes of human skin odour and CO₂ at natural concentrations and dilutions thereof in a wind tunnel. 3-D video-recorded flight tracks were reconstructed. Activation, flight velocity, upwind turning and source finding waned quickly as skin odours were diluted, whereas in the presence of CO₂ these parameters remained unchanged over more than a 100-fold dilution from exhaled concentrations. Although mosquitoes were behaviourally less sensitive to skin odours than to CO₂, their sensitivity to skin odours increased transiently by at least fivefold immediately following a brief encounter with a filament of CO₂. This sensitization was reflected in flight velocity, track angle, turning rate upon entering and exiting the broad odour plume and, ultimately, in the source-finding rate. In Ae. aegypti, CO₂ thus functions as a `releaser' for a higher sensitivity and responsiveness to skin odours. The initially low responsiveness of mosquitoes to skin odours, their high sensitivity to CO₂, and the sensitization of the olfactory circuitry by CO₂ are ecologically relevant, because rapidly fluctuating CO₂ levels reliably signal a potential host. Possible mechanisms of the instantaneous sensitization are considered.

Key words: Aedes aegypti, mosquito, orientation, sensitisation, host odour, carbon dioxide, human skin odour

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