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Do not feed these animals
(Image: Sinclair Stammers/Science Photo Library)
If you hate having to rub noxious chemicals onto your skin to deter
mosquitoes, take hope – the next generation of repellents could smell
like mint or caramel, and be more effective.
One of the ways mosquitoes are able to track us down is via
specialised neurons in the sensing pads that protrude from their head.
“For the longest time we thought the special neurons in these pads only
detected the carbon dioxide we exhale,” says Anandasankar Ray,
a neurobiologist at the University of California, Riverside. But a
graduate student wondered whether these “cpA” neurons would also respond
to her smelly feet after a hike.
To test the idea, the team attached a tiny electrode to the
mosquitoes’ cpA neurons. Sure enough, when the insects were exposed to
foot-odour the neurons fired, even in the absence of carbon dioxide.
What’s more, when cpA-disabled mosquitoes were released in a wind
tunnel, they were unable to track down foot-odour scented beads placed
at the other end. Mosquitoes with fully functioning cpA neurons had no
such problem. Taken together, this suggests that mosquitoes’ cpA
neurons are sensitive to smells that emanate from human skin.
“From a distance, we believe that these neurons are detecting carbon
dioxide from exhaled breath, but as the mosquito gets closer to a person
we think the neurons also pick up skin odours and guide the bug to the
arms, feet, and other exposed body parts,” says Ray.
Lured away
Common repellents like DEET target a different set of mosquito
olfactory neurons, which pick up odours but not carbon dioxide. They
also only work over a short range, and are too expensive, and
unpleasant, to be applied daily in poor sub-Saharan and Asian countries
where mosquito-borne diseases like malaria, yellow fever, and dengue are
rife. The other defence – devices that lure mosquitoes away from people
by burning propane or melting dry ice to release carbon dioxide – can
be difficult to maintain because of their costly, gas-delivering
mechanisms.
To investigate if the cpA neurons, with their newly discovered
dual-sensing capability, could be a target for more effective and
appealing mosquito control, Ray and his team ran close to half a million
chemical compounds through a computer programme to identify those with
structures that looked as if they would influence the action of the cpA
neurons.
Of the thousands of compounds the programme suggested that looked as
though they would repel mosquitoes or attract them for use in a lure
trap, the team picked 138 that were either cheap to produce, had a
pleasant smell or had already been approved as safe for use in foods and
fragrances, for example. Each of these was then tested to see whether
the mosquitoes’ cpA neurons would fire in response to the compound’s
presence, and the most promising were applied to human skin or tested in
a lure trap.
Mask and pull
One of the most effective repellent candidates was ethyl pyruvate, an
additive that gives a fruity rum or caramel flavour to breads, white
wine, and cocoa, and has been proven to be safe. Conversely, the most
promising attractant was cyclopentanone, a minty-smelling chemical in
some chewing gums and perfumes that powerfully attracted mosquitoes,
even in the absence of carbon dioxide. The next step is to test how
effective the compounds are in comparison to other methods already in
use.
“Our hope is that one day these compounds could be used in a
complimentary ‘mask and pull’ method,” Ray says. A repellent like fruity
ethyl pyruvate could mask humans from mosquitoes buzzing nearby, while a
minty cyclopentanone trap could lure the bugs to an unused corner of a
home.
Craig Montell,
a neurobiologist at the University of California, Santa Barbara, is
hopeful that Ray’s work could be the basis for new combinations of
mosquito control. But he points out that there are many other classes of
mosquito olfactory neurons that all need to be explored, as well as
neurons that detect the warmth of our skin.
