What makes a person more repulsive to a mosquito? It could be the smell of coconut.
That was one of the more curious findings of a small study published Wednesday in the journal iSciencewho tested whether different scented soaps made people more or less attractive to mosquitoes.
They discovered that the answer was not as simple as using this soap, not that one. Instead, the interaction of odors between human bodies and the products they used turned out to be much more complicated.
«It’s a simple question with a very complex answer,» said the study’s lead author, Clement Vinauger, an assistant professor of biochemistry at Virginia Tech who studies the molecular genetics of how mosquitoes choose their prey. «What really matters is how the chemicals in the soap combine with the chemicals in the individual person.»
That might explain why coconut seemed to repel mosquitoes, while citrus scents known to repel pesky bugs seemed to attract them.
A winning combination
Only female mosquitoes forage for blood, and only after mating, when they need its nutrients for their eggs to develop.
The rest of the time they feed on sweet-scented flowers.
When we use scented products on our skin, «we blur the lines between humans and plants,» Vinauger said. “Mosquitoes have a single resource that smells like both.”
A person’s odor comes from a unique combination of more than 350 chemicals, some of which are produced by the body and others by bacteria that live on and in us.
Everyone has the same chemicals, just in different ratios, some more attractive to mosquitoes than others, said Ali Afify, an assistant professor of biology at Drexel University in Philadelphia, who was not involved in the new study.
There are some known variables. Pregnancy or illness can change the chemical ratios and alter their attractiveness to mosquitoes. The same goes for drinking beer or being physically active. Perfumes, soaps, and lotions also play a role.
“Everything you use on your skin can make you more or less attractive to mosquitoes,” Afify said.
To see which chemicals might tip the scales in either direction, four volunteers were asked to wash with four different brands of soap: Dial, Dove, Native, and Simple Truth. In each case, the volunteer washed one forearm while leaving the other intact, then wore nylon sleeves on both arms for one hour. They repeated the process with the other three soaps. After the hour was up, the investigators removed the odor-soaked sleeves. They placed each sleeve in a cup and put the cups in a mesh cage filled with Aedes aegypti mosquitoes The species is found in the southern half of the US and as far as the east coast of Connecticut.
The scent in the cup that attracted the most mosquitoes was considered the most attractive, and the results surprised the scientists.
All four soaps contained limonene, a compound found in citrus fruits that is a known mosquito repellent. But that didn’t seem to matter in three of the four soaps. They made people more attractive to mosquitoes.
Scent samples taken from the washed arms also had higher amounts of a chemical called terpene, a compound commonly found in essential oils that gives cannabis its aroma. The fact that the terpenes in the soap seemed to make people more attractive to mosquitoes didn’t seem to add up.
«Terpenes tend to be repellent,» said Christopher Potter, an associate professor of neuroscience at the Johns Hopkins University School of Medicine in Maryland, who studies mosquito smell.
The fact that the soaps were not repellent «suggests that what mosquitoes pick up is much more complicated,» said Potter, who was not involved in the new research.
It could be that certain odors amplify repellent or attractive compounds that naturally exist in humans, rather than triggering those responses themselves, he said.
In the end, the team identified four chemicals that were associated with being slightly more attractive to mosquitoes and three that seemed to repel them, but the results were generally weak and variable for all but one of the chemical odorants tested: coconut.
«It confirms what previous studies have found, that mosquitoes don’t like coconut-scented products, so our safest bet right now is to use them,» Vinauger said.
The new research was a proof-of-concept study, meaning the team set out to determine whether or not there was a phenomenon that warranted further investigation. It’s not yet clear if the coconut scent itself repels mosquitoes or if it enhances one of the natural chemicals in human skin that is a repellent. It’s also not clear if that’s true for all of the 200 or so species of mosquitoes that feed on humans or just the ones. Aedes aegypti mosquitoes used in the study.
Because odors are complex (a specific combination of about 12 chemicals is responsible for the smell of strawberry), it’s not likely that a single odor or compound is solely responsible for how attractive a person is to mosquitoes.
«Some may play a larger role, but there won’t be one smoking gun that explains the attraction,» Potter said.
CORRECTION (May 10, 2023, 12:38 pm ET): An earlier version of this article misrepresented the name of the university where Clement Vinauger is an assistant professor. It’s Virginia Tech, not Virginia Tech University.