Making scents of who mosquitoes bite

By Jasminee Sahoye

HealthWhile many of us will be enjoying the outdoor weather, there’s the possibility you may encounter mosquitoes, those bloodsucking insects that can leaves marks on your skin from their bites.

The way we smell is a big factor in getting bitten, according to Zainulabeuddin Syed, a mosquito biologist with the University of Notre Dame’s Eck Institute for Global Health.

The aggravating bites of mosquitoes can be passage for hitchhiking pathogens to worm their way into our bodies. Mosquitoes spread malaria, dengue, yellow fever and West Nile virus, among others.

Syed studies olfaction in mosquitoes and other insects and points out that mosquitoes have an extraordinary sense of smell. A big part of their brains are devoted to this sense.

Only female mosquitoes feed on blood meals and they use the blood to produce eggs. And female mosquitoes find their blood meals through the use of smell.

An understanding of the olfactory behaviour of mosquitoes that leads them to feed on humans can play an important role in developing more effective methods of mosquito and disease control.

Syed is also researching the role that plants play in mosquito behaviour. He points out that despite our occasional feeling that we’re surrounded by hordes of hungry mosquitoes, they spend a relatively short amount of time feeding.

Rather, they spend considerable time on plants taking the sugars that provide energy for those occasions when they do not feed.

In another study, a team of biologists from the University of Washington (UW) and the California Institute of Technology has cracked the cues mosquitoes use to bite a tasty, unsuspecting, person.

They reported that the tiny insects employ a razor-sharp sense of smell to tip them off that a warm-blooded meal is nearby and then use vision and other senses to home in on the feast.

“Very little was known about what a host looks like to the mosquito and how a mosquito decides where to land and begin to feed,” said UW biologist Jeff Riffell, co-author on the report and one of three professors collaborating on these efforts.

Other scientists in their experiments have suggested that the mosquito’s sense of smell might activate other senses in the quest for a host.

However, Riffell and his two colleagues wanted to understand what those triggers are and which sensory pathways are most critical for finding a meal. They used wind tunnels to observe mosquitoes, placing them in an enclosed environment where they could record and track their behaviour.

“What’s great about this wind tunnel is that it provided a nice control of wind conditions and the environment these mosquitoes are flying around in,” said Riffell. “We can really test different cues and the mosquito’s response to them.”

The wind tunnels were mostly featureless, with the exception of a small dark dot on the floor. To test the role scent played in mosquito behaviour, the researchers released a plume of carbon dioxide – the gas we exhale – into the wind tunnel and observed how mosquito behaviour changed. It turned out that carbon dioxide triggered a strong response in the mosquitoes.

“When we gave them the odour stimulus, all of a sudden they were attracted to this black dot. It’s almost like the carbon dioxide gas turned on the visual stimulus for the mosquitoes to go to this black dot,” said Riffell.

He believes the mosquitoes went to the black dot – a high-contrast spot in an otherwise featureless environment – thinking that a warm-blooded host was nearby.

These results might mean that mosquitoes control or “gate” their sensory systems. They may not seek a host until they smell one – in this case, due to the scent of our exhaled breath.

If this theory is correct, the scents picked up by the mosquito may determine whether or not it engages other sensory systems in the search, especially vision.

Adding heat or water vapour to the black dot increased the mosquitoes’ affinity for the dot after carbon dioxide was released. Riffell and his colleagues plan to study how other scents might affect mosquito behaviour.