Perceive mosquitoes smell

One can imagine more enjoyable experiments than those of Peter Logan and John Pickett. The two zoologists from the UK's Rothamsted Research Center wanted to find out why some people seem to be magically attracted by mosquitoes while others are spared from bites.

To do this, they used a Y-shaped experimental set-up: a test person had to put their hand in each of the two long arms. Then the researchers let go of the mosquitoes at the short end and observed which of the two was bitten more often.

The scientists put test participants who were unattractive to the mosquitoes in a plastic bag up to their chins and collected their sweat. The mixture, which contains between 200 and 300 different substances, was separated into its individual substances using a gas chromatograph. Logan and Pickett sniffed mosquitos on each one whose antenna they had previously connected to electrodes. The more violently the insects reacted to a scent, the stronger the electrical signal.

Pickett recently tested the substance that mosquitoes were most disgusted with in the laboratory in the field: With bare arms he went to the Scottish Highlands, where the Scottish biting midge (Culicoides impunctatus) lives.

40,000 times an hour

"The animals can bite a person 40,000 times in an hour," says the mosquito researcher. Pickett rubbed alcohol on one of his arms and the mosquito repellent on the other. "After a short time, the unprotected arm was covered with stitches," he says. "Not a single mosquito bite into the protected arm". Pickett is currently testing in Africa whether his drug will also deter malaria and yellow fever mosquitoes.

The researchers want to keep the exact substance they have found secret until they have applied for a patent. Pickett, who presented his work at the Royal Society Summer Science Exhibition in London last week, reveals only this much: "The substance smells fruity in high concentrations and is produced by some plants as a stress reaction just before they die."

It has long been known that plants contain chemicals that deter mosquitoes. "Almost every people knows of some kind of plant that is supposed to drive away mosquitoes," says Andreas Rose, from Biogents, which tests potential insect repellants.

The people on the Mississippi, for example, rub the leaves of the love pearl bush (Callicarpa americana) and smear themselves with the juice. Its ingredients Callicarpenal, Intermedeol and Spathulenol are supposed to scare away the mosquitoes. "The problem is always that such natural chemicals are often harmful to human health," says Rose.

Martin Geier, head of the working group "Behavior of mosquitoes" at the University of Regensburg, believes it is theoretically possible that humans also produce substances that have a deterrent effect on mosquitoes. So far we only know which substances mosquitoes find particularly attractive in human vapors: a cocktail of lactic acid, ammonia and certain fatty acids.

"The individual components must be present in a very specific mixing ratio, otherwise it won't attract the mosquitoes," says Geier. In his opinion, one explanation for the fact that some people are hardly ever stung could therefore also be that they produce the individual substances in the "wrong" ratio.

The fairy tale of sweet blood

That the insects prefer "sweet blood", however, is only a legend. The claim of an American research group that people with high cholesterol are particularly likely to be victims of mosquitoes has not yet been published in any specialist journal. This suggests that it is not true.

Eating habits have been shown to have nothing to do with whether a person is often stung or not. Test subjects who ate a lot of garlic or swallowed vitamin B1 tablets were stabbed just as often in scientific tests as other people. "The only thing that scares the mosquitoes off is frequent washing," says Geier.

From the geometric structure and the size of the fragrance cloud around a living being, the insects can tell whether they are flying towards a person or a mouse, for example. Almost all mosquito species also identify their victims by the carbon dioxide flag in the exhaled air.

Species that are particularly strongly oriented towards CO2 tend to sting the head and upper body. The theory that mosquitoes, which prefer to bite the legs, based on the smell of cheese on the feet, was wrong, says Geier. An experiment clearly showed that. Blood-hungry mosquitoes with this preference were let loose on test persons who were lying on the ground with their feet in the air.

Lowest point on the body

In this position, they were no longer stabbed in the legs, but in the buttocks and back. "The insects seem to be looking for the lowest point on the body," says Geier. That also makes sense: "As a mosquito you are quickly killed and this risk is lower if you attack the deeper parts of the body." Because of the high risk, female mosquitoes only bite humans and animals when it cannot be avoided from their point of view - when they need the proteins in the blood to produce eggs.

Otherwise they are content with licking nectar and honeydew, or they prick plants and drink their juice. Males leave people alone.

A new mosquito repellent based on endogenous substances, such as Pickett wants to develop, would, in Martin Geier's opinion, be particularly useful if it would work longer than those that are already on the market.

The best keep mosquitoes at bay for four to five hours. Geier has often observed how the effect quickly wears off: on a person who has just rubbed insect repellent, the mosquitoes approach within a few centimeters, but do not land. Little by little, the distance becomes smaller and smaller, at the end the insects have already settled, but do not sting yet.

"As if in disgust, they bend their proboscis to the side and try to pull their legs up so that they don't have to touch the treated skin," says Geier. But if you forget the smallest spot while rubbing in, the mosquitoes will find it and sting. A means that keeps insects at a greater distance, because it surrounds people like a protective screen, would therefore also be a great step forward.

According to a study by Stiftung Warentest in 2004, the most effective mosquito repellants on the market are based on artificially produced chemicals, above all DEET (diethyltoluamide). But the substance is suspected of being nerve-damaging. DEET also attacks plastics and has probably ruined hundreds of watches as a result.

And if you have ever seen your watch being eaten away by a substance that you have just rubbed all over your body, you will find it difficult to believe the manufacturers' statements that DEET products are nonetheless skin-friendly. But natural mosquito repellants, which are mostly based on a mixture of essential oils, performed even worse. Many caused skin irritation. Neither of them really scared off the mosquitoes. And that, although most of them gave off a penetrating smell.