Beating the bite of mosquitoes this spring and summer could hinge on your
attire and your skin. New research led by scientists at the University of
Washington indicates that a common mosquito species—after detecting a
telltale gas that we exhale—flies toward specific colors, including red,
orange, black and cyan. The mosquitoes ignore other colors, such as green,
purple, blue and white. The researchers believe these findings help explain
how mosquitoes find hosts, since human skin, regardless of overall
pigmentation, emits a strong red-orange "signal" to their eyes.
"Mosquitoes appear to use odors to help them distinguish what is nearby,
like a host to bite," said senior author Jeffrey Riffell, a UW 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 results, published Feb. 4 in Nature Communications, reveal how the
mosquito 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.
"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 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."
In their experiments, the team tracked behavior 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 odors and presented different types of visual
patterns—such as a colored dot or a tasty human hand.
Without any 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.
Humans can't smell CO2, which is the gas we and other animals exhale with
each breath. Mosquitoes can. Past research by Riffell's team and other
groups showed that smelling CO2 boosts female mosquitoes' activity
level—searching the space around them, presumably for a host. The
colored-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 color" vision: We see different wavelengths of light
as distinct colors: 650 nanometers shows up as red, while 450 nanometer
wavelengths look blue, for example. The researchers do not know whether
mosquitoes perceive colors the same way that our eyes do. But most of the
colors 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
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. Another strain of mutant mosquitoes,
with a change related to vision so they could no longer "see" long
wavelengths of light, were more color-blind in the presence of CO2.
"These experiments lay out the first steps mosquitoes use to find hosts,"
said Riffell.
More research is needed to determine how other visual and odor cues—such as
skin secretions—help mosquitoes target potential hosts at close range. Other
mosquito species may also have different color preferences, based on their
preferred host species. But these new findings add a new layer to mosquito
control: color.
Reference:
The olfactory gating of visual preferences to human skin and visible spectra
in mosquitoes, Nature Communications (2022).
DOI: 10.1038/s41467-022-28195-x