When it comes to mosquitoes, the annoying buzzing and incessant biting can leave many people seeing red.
But as it turns out, that’s exactly what the insects themselves are picturing, and researchers say that’s why they’re drawn to human skin.
A new study led by scientists at the University of Washington suggests that mosquitoes — after detecting a telltale gas that we exhale — fly toward specific colours, including red, orange, black and cyan.
Conversely, they ignore green, purple, blue and white.
The researchers believe this helps explain how mosquitoes find hosts, since human skin, regardless of overall pigmentation, emits a strong red-orange ‘signal’ to their eyes.
Mosquitoes are drawn to specific colours including red, orange and black, a study has found
‘Mosquitoes appear to use odours to help them distinguish what is nearby, like a host to bite,’ said senior author Jeffrey Riffell, a University of Washington professor of biology.
‘When they smell specific compounds, like CO2 from our breath, that scent stimulates the eyes to scan for specific colors and other visual patterns, which are associated with a potential host, and head to them.’
The findings reveal how a mosquito’s sense of smell — known as olfaction — influences the way it responds to visual cues.
Knowing which colours attract hungry mosquitoes, and which ones do not, can help design better repellants, traps and other methods to keep mosquitoes at bay, the researchers said.
‘One of the most common questions I’m asked is, “What can I do to stop mosquitoes from biting me?”‘ said Riffell.
‘I used to say there are three major cues that attract mosquitoes: your breath, your sweat and the temperature of your skin.
‘In this study, we found a fourth cue: the colour red, which can not only be found on your clothes, but is also found in everyone’s skin.
‘The shade of your skin doesn’t matter, we are all giving off a strong red signature. Filtering out those attractive colours in our skin, or wearing clothes that avoid those colours, could be another way to prevent a mosquito biting.’
In their experiments, the team tracked the behaviour of female yellow fever mosquitoes, Aedes aegypti, when presented with different types of visual and scent cues.
Like all mosquito species, only females drink blood, and bites from A. aegypti can transmit dengue, yellow fever, chikungunya and Zika.
The researchers tracked individual mosquitoes in miniature test chambers, into which they sprayed specific odours and presented different types of visual patterns — such as a coloured dot or a tasty human hand.
Without any odour stimulus, mosquitoes largely ignored a dot at the bottom of the chamber, regardless of its colour.
After a spritz of CO2 into the chamber, however, the mosquitoes continued to ignore the dot if it was green, blue or purple. But if the dot was red, orange, black or cyan, mosquitoes would fly toward it.
In their experiments, the team tracked the behaviour of female yellow fever mosquitoes, Aedes aegypti (pictured), when presented with different types of visual and scent cues
Humans can’t smell CO2, which is the gas we and other animals exhale with each breath, whereas mosquitoes can.
Previous research, including by Riffell’s team, has shown that smelling CO2 boosts female mosquitoes’ activity level — searching the space around them, presumably for a host.
The coloured-dot experiments revealed that after smelling CO2, these mosquitoes’ eyes prefer certain wavelengths in the visual spectrum.
It’s similar to what might happen when humans smell something good.
‘Imagine you’re on a sidewalk and you smell pie crust and cinnamon,’ said Riffell.
‘That’s probably a sign that there’s a bakery nearby, and you might start looking around for it. Here, we started to learn what visual elements that mosquitoes are looking for after smelling their own version of a bakery.’
Most humans have ‘true colour’ vision. We see different wavelengths of light as distinct colours: 650 nanometers shows up as red, while 450 nanometer wavelengths look blue, for example.
The researchers do not know whether mosquitoes perceive colours the same way that our eyes do. But most of the colours the mosquitoes prefer after smelling CO2 — orange, red and black — correspond to longer wavelengths of light.
Human skin, regardless of pigmentation, also gives off a long-wavelength signal in the red-orange range.
When Riffell’s team repeated the chamber experiments with human skintone pigmentation cards — or a researcher’s bare hand — mosquitoes again flew toward the visual stimulus only after CO2 was sprayed into the chamber.
If the scientists used filters to remove long-wavelength signals, or had the researcher wear a green-colored glove, then CO2-primed mosquitoes no longer flew toward the stimulus.
‘These experiments lay out the first steps mosquitoes use to find hosts,’ said Riffell.
The experts said more research was now needed to determine how other visual and odour cues — such as skin secretions — help mosquitoes target potential hosts at close range.
The study has been published in the journal Nature Communications.
