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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

View larger version (30K): [in a new window]   Fig. 1. Wind tunnel setup and plume generators. Superimposed ribbon plume generator (A), and the continuous (see also Fig. 2) plume generators (B,C). The superimposed plume was generated by a pipette positioned 100 cm downwind and 30 cm upwind of the release cage. This configuration created a ribbon CO2 plume that passed through the centre of the release cage. The continuous plume generator (B) was placed behind the stainless steel laminising screens. It had two inlets, one for the odour, and the other for a 4 l min–1 clean air `jet' to mix the mixture. We also tested a continuous skin odour plume by inserting a hand from outside the wind tunnel directly in the continuous plume generator through a tube (C) upwind from the laminising screens.

View larger version (18K): [in a new window]   Fig. 2. Skin-odour tube. The experimenter's arm was inserted in one end. An airtight connection prevented leakage of air. A fan at the end of the tube created a flow over the arm and ensured high skin-odour uptake. The air stream could be split to create skin-odour plumes of lower concentration. Flow rates were verified `off-line' using a bubble flow meter.

View larger version (23K): [in a new window]   Fig. 3. Sample propylene density plots measured using the photoionization detector (PID) from the centre to outside of the continuous broad plume at 50 cm from the source. The sampling rate was 100 Hz, and the distance of the measuring point from the centre of the plume was 0–10 cm. Only small concentration fluctuations were observed within the plume. The values given by black trace `10' reflect background (no detectable propylene) values.

View larger version (26K): [in a new window]   Fig. 4. Activation by skin odour and CO2 (A) skin odour series: 100% skin odour (N=64), 20% skin odour (N=67), 4% skin odour (N=70), `hand' (N=64), clean air (N=59); (B) CO2 series: 1% CO2 (N=40), 0.3% CO2 (N=40), 0.1% CO2 (N=55), 0.05% CO2 (N=55), `hand' (N=46). The shape parameter was in all cases <1 (between 0.5 and 0.6), which implies a higher activation rate at the beginning of the experiment.

View larger version (28K): [in a new window]   Fig. 5. Average change in track angle over time when flying in (right; mosquitoes entering the plume) or out (left; mosquitoes leaving the plume) of a plume consisting of skin odour or CO2. Straight upwind flight would be 0°. The grey vertical line represents the plume boundary. Values are means ± S.E.M., averaged over all mosquito tracks (see N values) (A) Skin odour series: 100% skin odour (N=36), 20% skin odour (N=28), 4% skin odour (N=22), `hand' (N=49), clean air (N=17); (B) CO2 series: 1% CO2 (N=31), 0.3% CO2 (N=26), 0.1% CO2 (N=30), 0.05% CO2 (N=25), `hand' (N=23); (C) sensitization series: 100% skin odour (N=24), 20% skin odour (N=38), 20% skin odour + CO2 (N=40), CO2 (N=19).

View larger version (83K): [in a new window]   Fig. 6. Sample tracks of mosquitoes in response to (A) a ribbon CO2 plume, (B) 20% skin odour, (C) 20% skin odour after contact with a ribbon plume of CO2, and (D) undiluted skin odour. Dots represent the mosquito's position at intervals of 100 ms. Contact with the odour plume is indicated by a light blue colour. B (left) shows two typical tracks of mosquitoes flying through a diluted skin odour plume. Only rarely did 20% skin odour induce sustained upwind flight to the odour source (right).