Dec 122013

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It’s a breakthrough in the fight against disease-carrying mosquitoes.

Researchers have found a chemical that disables the part of the insect’s brain that smells humans. Future bug repellents based on the compounds could give people an invisibility cloak against the winged pests. We talked to a scientist who worked on the discovery to find out more.

There are three main ways that mosquitoes zero in on their targets. The strongest mosquito attractant is carbon dioxide (CO2), which the insects can detect from a distance of up to 20 to 30 meters. “The reason mosquitoes and other blood-feeding insects have evolved to detect CO2 is because every living vertebrate is going to produce a lot of CO2 as turbulent plumes,” said Anandasankar Ray, an entomologist at the University of California, Riverside. “It doesn’t just dissipate; the prevailing wind will carry it almost like cigarette smoke gets carried away.”

At closer range, mosquitoes can sense different odors that are emitted from the skin (human skin odor is mostly the byproduct of skin microbes, which break down sweat to produce smelly volatiles). Finally, mosquitoes can detect body heat. Some research has also suggested that mosquitoes are attracted to certain blood types, which may be mediated by different odor molecules, but the evidence for this idea is not robust, Ray told io9.

The Carbon Dioxide Conundrum

Given mosquitoes’ remarkable ability to detect CO2 from a great distance, scientists have focused on determining just how, exactly, the insects can sense the gas, in hopes of someday blocking the ability. What it boils down to, they found, is a class of olfactory sensory neurons called cpA, which are housed in the mosquitoes’ maxillary palps, a type of sensory organ between the antennae near the mouth. In 2011, Ray and his colleagues discovered that they could use certain chemical odors to overstimulate the cpA neurons, and disrupt mosquitoes from detecting CO2.

“We then started wondering what would happen if the [CO2-blocked] mosquitoes would come closer to us and go towards our feet or arms,” Ray said. Even though the mosquitoes can’t detect people from the carbon dioxide on their breath, they could still sense skin odor, allowing them to find their prey. So the team, as well as other research groups, decided to pinpoint receptors that pick up on skin odors.

Experiments have demonstrated that skin odor alone can draw in mosquitoes, and scientists found that some odors did activate certain receptors in mosquito antennae. But, strangely, researchers couldn’t tie the activation of these receptors to the mosquitoes’ attraction behavior. In every case, the insects’ CO2 receptors also needed to be activated with carbon dioxide to elicit the attraction behavior. This left everyone scratching their head, trying to figure out why activating just the antennae receptors didn’t work. “At that point we hit a roadblock,” Ray said.

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