The Another

    Certain soaps make humans more attractive to mosquitoes



    Mosquito (Aedes aegypti) biting human skin, March 2015.

    ‘Tis the season, and mosquitoes are out and about. Researchers at the Virginia Polytechnic Institute and State University – known as Virginia Tech – have come up with some bad news: Most of the scented soaps used in the US increase the appetite of egg-laying female mosquitoes – the only ones who bite.

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    Clément Vinauger – an assistant professor at the American university and joint coordinator of this study – explained the mechanism, described in the newspaper iScience : “Adult hematophagous female mosquitoes require nutrients and proteins from vertebrate blood to produce progeny.” “To detect and locate blood sources, mosquitoes rely to a large extent on olfactory cues released by their hosts.”

    But what makes us different from other animals is that every day we use cosmetic or hygiene products, such as soaps, and apply them to our skin.” These products are designed to please our sense of smell, often emitting sweet plant scents. “From the mosquito’s point of view, we’re then a resource that smells like both an animal and a plant,” continued the researcher. “However, the effect on mosquito response of adding these plant-emitted compounds to our body odor had never been tested .” He adds: “The increasing use of synthetic fragrances, inspired by floral extracts to alter the scent of people (eg, perfumes and soaps) and items (eg, laundry detergent), resulted in the introduction of compounds used by insects outside of their natural context. For example, personal care products, such as shampoos and soaps, are widely used on a daily basis and in variable quantities and contain numerous volatile organic compounds that naturally mediate plant- and host-mosquito interactions.”

    To undertake this study, the team led by Vinauger together with Frenchwoman Chloé Lahondère used methods that the researcher describes as “fairly conventional.” Four volunteers were recruited to successively test the four main soaps sold in the United States: Dial, Dove, Simple Truth, and Native. Each time, they were asked to wash one of their arms, to rinse the other, and then to wear nylon sleeves for an hour so that the fabric could be impregnated with the odor emitted: That of the body on one side, and that of the body and the soap on the other. All that remained was to study the two pieces of fabric for each volunteer, an operation repeated over several months.

    Landing count

    Using gas chromatography-mass spectrometry, the researchers quantified how human odor is modified by soap application. And the results were remarkable: While human body odor is composed of 80% aldehydes, ketones, carboxylic acids, and alcohol, “what we smell after washing comes more than 50% from the soap and is largely dominated by a chemical class called terpenes, which are typically produced by plants,” said Vinauger.

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