Listening to a mosquito's whining buzz can be an unnerving experience. At best you'll end up with an itchy bite, but in some parts of the world it could prove fatal. Mosquito traps offer some protection, luring the insects with plumes of attractive odours. However, when Teun Dekker and his colleagues tested how yellow fever mosquitoes responded to odour plumes with different widths and structures in 2001, they were astonished to see that although the insects were extremely attracted to continuous plumes of human odour, they completely ignored continuous plumes of CO2. This was surprising because a sniff of CO2 is usually one of the most attractive odours a mosquito can encounter. ‘There was something wrong with the manner in which we presented these odours,’ recalls Dekker. Wondering how the structure of the odour plume might affect the insect's behaviour, Dekker and Ring Cardé from the University of California, Riverside, USA, decided to find out which aspects of odour plumes yellow fever mosquitoes ignore and which features are irresistible (p. 3480).
‘We built a wind tunnel with good laminar flow so we could predict where the plume was and where the mosquito was with respect to the plume and then analyse the behavioural patterns,’ explains Dekker.
Releasing four ravenous females into the wind tunnel with a ribbon-thin plume of CO2, Dekker filmed the insects' flight paths and was pleased to see that as soon as an insect encountered the odour, it switched direction and began heading upwind. Next, Dekker reduced the concentration of CO2 in the ribbon plume and was surprised to see that the mosquitoes reacted even when the CO2 concentration was barely above atmospheric levels.
However, when Dekker tested the mosquitoes' responses to a thin filament of the odour from his own forearm, he was surprised to see that the insects were oblivious to it. They just flew straight through without flinching.
Next he designed broader uninterrupted plumes and found that the mosquitoes were attracted to both odours, flying upwind in the human odour plume while flying in and out along the edge of the CO2 plume. Dekker also disturbed the two plumes to produce intermittent turbulent puffs of each odour and found that the mosquitoes were most responsive to these intermittent plumes, turning into the flow and heading upwind.
Having discovered that yellow fever mosquitoes are oblivious to thin plumes of skin odour, Dekker suspects that the 50 ms encounter with the smell was insufficient to allow them to process the information and steer upwind. However, he explains that the insect's CO2 receptors directly control their behaviour, allowing to respond almost instantly to even the faintest of whiffs.
He also adds that it makes sense for mosquitoes to respond strongly to fluctuating CO2 because it indicates the presence of a live host, unlike skin odour that can linger after the victim has departed. However, mosquitoes do not follow CO2 trails slavishly to the source – otherwise they'd fly into your mouth. He suggests that they lock on initially to a fluctuating CO2 trail before switching to follow a broad skin odour plume to home in on lunch.