The mosquito, this small blood-drinking insect, is despite its size one of the greatest scourges of humanity. A vector of many diseases, many of which are fatal, everything in its behavior and in its genetic evolution makes it a predator in its own right. To better understand mosquitoes, and perhaps better protect against them, a recent study revealed that after being olfactorily stimulated, they are more attracted to long-wavelength colors such as red, orange, black and cyan. These colors, some of which are close to that of human skin, would therefore attract these insects more when we wear them.
Published in Nature Communications and conducted by the University of Washington, the study shows just how perfect mosquito-killing methods have become over thousands of years of evolution. They have real built-in radars that easily detect odors, heat sources, humidity, shapes... and make their technique almost unstoppable.
Research has already shown that the insect is easily spotted thanks to the CO2 exhaled by humans and warm-blooded animals. Thanks to this parameter, the mosquito can stalk its prey even beyond 10 meters away. It is also interested in objects that emit heat, and detects the hottest areas of the body. At less than 20 centimeters, it captures heat and humidity, and therefore the best sting areas.
Its hunting arsenal and the diseases it carries make the mosquito the perfect predator. It even sits at the top of the list of the most dangerous animals in the world. Contrary to what one might think, large carnivores like sharks, as well as the most venomous snakes and spiders, do not come close to it.
An infographic that appeared on Bill Gates' blog indicated that in one year, mosquitoes could kill 830,000 human victims, when the shark only kills six! Snakes make about 60,000 and scorpions 3,500.
Human skin: it emits a particular wavelength
As mentioned above, mosquitoes spot humans by the smell of CO2 on their breath. American researchers then conducted experiments by monitoring their reactions to olfactory and visual signals. After having detected the CO2, in fact, the mosquito's eyes take over in order to determine its origin.
The scientists selected female mosquitoes of the common species Aedes aegypti. They then observed how the insects reacted to dots of different colors at the bottom of a test chamber, with or without a CO2 spray.
The team then discovered that mosquitoes preferred to head for a dot if it was red, orange, black or cyan after smelling CO2. But without the gas, the insects ignored the dots, regardless of their color. However, they did not approach the green, blue, white, or purple dots — which are short-wavelength colors — even after being enticed to hunt (by imbuing those dots with CO2).
“When they smell specific compounds, like CO2 from our breath, this scent stimulates the eyes to look for specific colors and other visual patterns, which are associated with a potential host. This then stimulates them to move towards them, ”says Jeffrey A. Riffell, professor of biology at the University of Washington and lead author of the study, in a press release.
According to the researchers, insects prefer these colors because human skin, regardless of its hue, emits a strong red-orange signal. In follow-up tests, the researchers exposed bare human hands of different hues, and found that the insects only moved towards them if they smelled CO2 first. And on the contrary, if the CO2 was felt but the long wavelengths were filtered out, or if the hand wore a green glove, the insects did not approach it.
In addition, the research team conducted the same experiments on genetically modified mosquitoes. If the genes responsible for detecting the smell of CO2 or long-wavelength color vision were changed, the insects responded neither to visual nor to olfactory stimuli. This suggests that the two meanings are interdependent.
Ultimately, this research could eventually lead to new ways to control mosquitoes and prevent bites. Or would it perhaps be enough to wear white, green or purple in places particularly at risk? The manufacture of red traps emitting CO2 is also an avenue to explore.
In a next step, the study will look at other leads, such as skin secretions – which could attract mosquitoes to their hosts, say the researchers. They are also keen to point out that other species that feed on animals might possibly have different color preferences.