Insects have very keen powers of smell that direct them to their targets. But for researchers trying to figure out what attracts or repels the pests, sorting through the 300 to 400 distinct chemical odours that the human body produces has proved daunting.

Now scientists at Rothamsted Research in the UK have been making headway at understanding why some people can end up with dozens of bites, while others remain unscathed.

The researchers have identified a handful of the body's chemical odours, some of which may be related to stress, that are present in significantly larger concentrations in people that the bugs are happier to leave alone.

If efforts to synthesize these particular chemicals are successful, the result could be an all-natural mosquito repellent that is more effective and safer than products currently available.

"Mosquitoes fly through an aerial soup of chemicals, but can home in on those that draw them to humans," says James Logan, a researcher at Rothamsted, one of the world's oldest agricultural-research institutions.

But when the combination of human odours is wrong, he says, "The mosquito fails to recognise this signal as a potential blood meal."

Odours That Make a Meal?

The phenomenon that some people are more prone to mosquito bites than others is well documented. In the 1990s, chemist Ulrich Bernier, now at the US Department of Agriculture's Agricultural Research Service, began looking for what he calls the "magic compounds" that attract mosquitoes.

His research helped to show that mosquitoes are attracted to humans by blends of common chemicals such as carbon dioxide, released from the skin and by exhaling, and lactic acid, which is present on the skin, especially when we exercise.

But none of the known attractant chemicals explained why mosquitoes preferred some people to others.

Rothamsted's Dr Logan says the answer isn't to be found in attractant chemicals. He and colleagues observed that everyone produces chemicals that mosquitoes like, but those who are unattractive to mosquitoes produce more of certain chemicals that repel them.

"The repellents were what made the difference," says Dr Logan, who is interested in the study of how animals communicate using smell. These chemicals may cloud or mask the attractive chemicals, or may disable mosquitoes from being able to detect those attractive odours, he suggests.

So What Repels?

Currently, the most effective repellents on the market often contain a chemical known as Deet, which has been associated in some studies with potential safety concerns, such as cancer and Gulf War syndrome.

The Rothamsted team set out to get the mosquitoes' viewpoint. The researchers separated human volunteers into two groups - those who were attractive to mosquitoes and those who weren't. They then put each of the volunteers into body-size foil bags for two hours to collect their body odours.

The scientists were able to separate the chemicals. They then tested each of them to see how the mosquitoes responded. They found only a small number of body chemicals - seven or eight - that were present in significantly different quantities between those people who were attractive to mosquitoes and those who weren't.

They then put their findings to the test and found that the insects were either motivated by the person's smell or not. The chemicals were then tested to determine their impact on actual biting behaviour.

Significant Repellency

Two compounds with "significant repellency" have been determined. One of the compounds, 6-methyl-5-hepten-2-one, is a skin-derived compound that has the odour of toned-down nail-polish remover, according to George Preti, an organic chemist at the Monell Chemical Senses Centre in Philadelphia, who is involved in a separate line of research into insect-biting behaviour.

The other, identified in the paper as geranylacetone, has a pleasant odour, though there is some question about whether the chemical is formed by the human biochemical process or is picked up in the environment, Dr Preti says.

Dr Logan declined to comment about the specific chemicals because of proprietary concerns. He says the findings have been patented and the group is working with a commercial company to develop the compounds into a usable insect repellent.

One issue that still needs to be resolved: How to develop a formulation of the repellent chemicals that will stay on the skin, rather than quickly evaporating as they do naturally. The hope is to get a product to market within a year or two, he says.

Some of the chemicals researchers identified are believed to be related to stress, Dr Logan says.
Previous research has shown that these particular chemicals could be converted from certain other molecules and this could be as a result of oxidation in the body at times of stress, he says. However, it's not clear if the chemicals observed by the Rothamsted researchers were created in this way, and research is continuing to answer this and other questions.

Dr Logan suggests that mosquitoes may deem hosts that emit more of these chemicals to be diseased or injured and "not a good quality blood meal". Proteins in the blood are necessary for female mosquitoes to produce fertile eggs, and Dr Logan says it might be evolutionarily advantageous for mosquitoes to detect and avoid such people.