A group of scientists from the Unversity of Washington has found out that mosquitoes are usually flying towards specific colors such as red, orange, black, and cyan, after detecting a telltale gas from our breaths. And the little creatures often ignore colors like green, purple, blue, and white.
These findings may lead us to learn how they find their hosts, as the human skin emits a strong red-orange signal to their eyes.
Jeffrey Riffell, a UW professor of biology and the senior author of the study said, “Mosquitoes appear to use odors to help them distinguish what is nearby, like a host to bite”. When the mosquitoes 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” Riffell added.
The results of the study, published on February 4 in Nature Communications, reveal how the mosquito’s sense of smell — known as olfaction — influences how the mosquito responds to visual cues. Knowing which colors attract hungry mosquitoes, and which ones do not, can help design better repellants, traps, and other methods to keep mosquitoes at bay.
Riffell says that one of the most common questions he keeps being asked is, “what to do in order to stop mosquitoes from biting” and continued, “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 color 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 colors in our skin, or wearing clothes that avoid those colors, could be another way to prevent a mosquito biting.”
The team tracked the behavior of, Aedes aegypti, female yellow fever mosquitoes, when presented with different types of visual and scent cues. As is the case in all mosquito species, only females drink blood, but A. aegypti bites can also transmit dengue, yellow fever, chikungunya, and Zika. The researchers tracked individual mosquitoes in miniature test chambers that were sprayed with specific odors and featured different types of visual patterns — such as a colored dot or a tasty human hand.
In the absence of an odor stimulus, mosquitoes largely ignored a dot at the bottom of the chamber, regardless of color. After a spritz of CO2 into the chamber, mosquitos continued to ignore the dot if it was green, blue, or purple in color. But if the dot was red, orange, black, or cyan, mosquitoes would fly toward it.
Prior researches from various researchers indicate that female mosquitoes’ hunting activities increase and start searching a perimeter upon smelling CO2, a gas that we humans are unable to smell. The colored-dot experiments revealed that after smelling CO2, these mosquitoes’ eyes prefer certain wavelengths in the visual spectrum.
Imagine you’re on a sidewalk and smell some baked goods, you would start looking for a bakery sign right? The mosquitoes do too.
We, humans, see color in different wavelengths of light, and the team currently does not know whether the mosquitoes perceive color as we do. But according to the experiments, mosquitoes prefer longer wavelengths of light upon smelling CO2.
Unfortunately for us though, human skin emits a long-wavelength signal in the range of red-orange, regardless of its pigmentation. To mosquitoes, we're all just tasty snacks walking about.
When Riffell’s team repeated the chamber experiments with human skin-tone 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 researchers 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.
Genes determine the preference of these females for red-orange colors. Mosquitoes with a mutant copy of a gene needed to smell CO2 no longer showed a color preference in the test chamber. “These experiments lay out the first steps mosquitoes use to find hosts,” said Riffell.
The research was funded by the National Institutes of Health, the Air Force Office of Scientific Research, the UW, and the U.S. Army Research Office.