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SNIFF AND SEARCH: SCIENTISTS DISCOVER HOW MOSQUITOES LOCATE A HOST

Using puffs of carbon dioxide, scientists have discovered how bloodthirsty mosquitoes are so adept at locating their prey over long distances.

A team of scientists, led by researchers at the University of Washington, USA, has discovered how the female mosquito brain uses its two key senses of vision and smell to identify, track and hone in on a potential host for her next blood meal.

The research, published in July 2019 in Current Biology, focused on the female mosquitoes’ ability to detect carbon dioxide, or carbon dioxide (as only female mosquitoes feed on blood, only females were analysed). Previous research conducted by one of the team showed that smelling carbon dioxide can ‘prime’ the mosquito’s visual system, getting it ready to hunt for a host. The aim of this latest study was to identify the precise changes in mosquito flight behaviour that are triggered by carbon dioxide and to understand how the mosquito brain responds to different smells and visual cues.

The researchers found that when the mosquito detects certain smells, it begins to use its visual system to scan its surroundings for specific types of shapes and fly towards them, presumably associating those shapes with potential hosts.

“Our breath is just loaded with carbon dioxide,” said corresponding author Jeffrey Riffell, Professor of Biology from the University of Washington. “It’s a long-range attractant, which mosquitoes use to locate a potential host that could be more than 100 feet away.”

Mosquitoes pick up carbon dioxide at long range before using visual cues closer to the host

The team also tested how Aedes aegypti mosquitoes responded to visual stimuli as well as puffs of carbon-dioxide-rich air. They found that being exposed to one-second puffs of air containing 5% carbon dioxide – just a fraction higher than the 4.5% carbon dioxide level found in human breath – prompted the mosquitoes to beat their wings more rapidly.

The mosquitoes showed little response to many visual stimuli, such as a fast-moving star field, but when showed a horizontally moving bar, mosquitoes beat their wings faster and attempted to steer in the same direction. This response was more pronounced if researchers introduced a puff of carbon dioxide before showing the bar.

“We found that carbon dioxide influences the mosquito’s ability to turn toward an object that isn’t directly in their flight path,” said Professor Riffell. “When they smell the carbon dioxide, they essentially turn toward the object in their visual eld faster and more readily than they would without carbon dioxide.”

The researchers repeated the experiments with a genetically modified Aedes aegypti strain, one whose neurons glow green when they are actively firing, and recorded the activity in the part of the brain responsible for visual processing. When shown the horizontal bar, two-thirds of its brain regions lit up, but when the researchers introduced a puff of carbon dioxide first and then showed the horizontal bar, 23% of the regions had even higher activity than before – indicating that smelling carbon dioxide prompted a greater response in these areas of the brain that control vision.

However, in the reverse experiment, showing the horizontal bar did not increase firing in the parts of the mosquito brain that control smell. “Smell triggers vision, but vision does not trigger the sense of smell,” said Professor Riffell.

Professor Riffell believes the findings align with the general picture of mosquito senses, as while mosquitoes can pick up a scent over long distances (more than 30 metres away) their eyesight is most effective for objects only 4-6 metres away.

The hope is that scientists can use the findings from this piece of research to help develop new methods for mosquito control and reduce the spread of mosquito-borne diseases.

Reworked extract from: University of Washington. ‘Scientists discover how mosquito brains integrate diverse sensory cues to find a host’. ScienceDaily. 18 July 2019.

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