Why do mosquitoes bite one individual but not another? I am travelling around Asia with my girlfriend. I don鈥檛 appear to have been bitten at all, while she has been bitten endlessly. We sleep in the same bed and share a similar diet and neither of us wears scented products.
鈥� Are you sure that you don鈥檛 just fail to react noticeably to mosquito bites? Some individuals hardly react at all to bites, which interferes with reliable bite-scoring.
Attractiveness to mozzies correlates with obesity, maleness, temperature and sweat, but personal differences in sweat composition and skin bacteria have unpredictable effects. Mosquitoes are said to favour certain blood groups, but there is more argument than hard evidence on which groups they prefer, if any, or on whether diet makes any difference.
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The most definite attractants are lactic acid, carbon dioxide, warmth, certain fatty acids and other compounds from sweat or its bacterial decomposition. Notoriously, Limburger cheese and unwashed socks are rich sources of such chemicals. Some cosmetics are effective repellents or, more accurately, they counteract attractants by confusing the mosquitoes鈥� senses.
Jon Richfield, Somerset West, South Africa
鈥淢osquitoes are most attracted by lactic acid, carbon dioxide, warmth, certain fatty acids and other compounds from sweat鈥�
鈥� While it is true we all produce a bouquet of natural chemicals, some of which attract biting insects, a fortunate few give off an aroma that apparently masks these attractive chemicals, and so prevents these people from being bitten because the mozzies can鈥檛 track them down. James Logan and John Pickett at Rothamsted Research in Hertfordshire, UK, have pinpointed the human chemicals that keep blood-sucking insects at bay and are developing a mosquito repellent based on these chemicals.
The team at Rothamsted first noticed that some individuals are less appealing to blood-sucking insects when they saw some herds of cattle fending off flies while others grazed undisturbed. Pickett鈥檚 group reasoned that animals in the undisturbed herd must simply smell less alluring to the flies, and examined the cows鈥� chemical profiles. Sure enough, they discovered that very distinctive chemical signals emanated from certain individuals, and these lucky cows were unattractive to flies.
Logan discovered that humans have distinct chemical signatures too. He let yellow fever mosquitoes (Aedes aegypti) fly along a Y-shaped maze and wafted the scent from volunteers鈥� hands down the prongs of the maze. While some chemicals attracted mosquitoes, others repelled them, and so the insects flew down the more attractive prong. Logan has since been able to identify which chemicals the insects respond to by strapping miniature electrodes to the antennae of female mosquitoes and checking their responses. It turns out that only some humans produce the masking chemicals that mosquitoes find unappealing.
Yfke van Bergen, Manchester, UK