Mosquitoes target humans using a combo of sensory cues

New York: Ever wondered why mosquitoes attack certain people more than others? Mosquitoes are actually selecting their targets based on a combination of sensory cues that attract the creatures to humans, a new study has found.

The Rockefeller University researchers looked at the interaction of different sensory cues — carbon dioxide, heat and odour — that attract mosquitoes to humans, and found that it takes a combination of at least two of these to send the bugs biting.

The mutant was missing a specific gene, known as Gr3, which codes for a carbon dioxide receptor.

Without Gr3, the mosquitoes were unable to detect the gas. Researchers first looked at the mutant mosquitoes' behaviour inside a chamber that held a plate heated to the temperature of human skin.

Normal mosquitoes weren't attracted to the warmth unless carbon dioxide was also emanating from it, and mutants weren't drawn to the plate at all. The team found a similar interaction between lactic acid — a compound in human breath and skin odour — and CO2: attraction to the odour was dependent on the presence of carbon dioxide. "Relying on multiple sensory cues helps organisms make informed decisions about context-dependent behaviours.

In the case of a female mosquito, this would allow her to accurately hone in on a human host to blood feed," said lead researcher Conor McMeniman. When the scientists tested the ability of these CO2-blind mutants to find humans in a real-world set-up, the difference between mutants and normal mosquitoes was less significant.

The insects were examined in a humid, greenhouse-type enclosure in Australia, where human volunteers sat and captured the insects as they landed on them. McMeniman devised a way to make the mutant mosquitoes glow in the dark, and counted them afterward.

Those without CO2-sensing abilities were only impaired by 15 per cent.

The mutants were clued in by other factors — the combination of heat and odour coming off the human subjects.

However, when a similar concept was tested on a larger scale, with a mouse as the subject, the mutants were much less likely to bite, meaning that without carbon dioxide as a cue, the effect of odour and heat are diminished as the insect moves farther away from the host. "There are a lot of things that give off heat, and it would be a waste for a mosquito to try to bite all of them.

But with several factors present, the insect can increase her chances of a fruitful bite in a cluttered sensory environment," said McMeniman.