The first mosquito bites have already been scored. So turn off the lights when the window is open? well, says a webpage from the Red Cross with facts and fables about mosquitoes: “Mosquitoes can hardly see anything.”
magazine On the road says the same, quest even reports that mosquitoes would be “shy” of light.
But Jeroen Spitzen, mosquito researcher at Wageningen UR, has new information. “Just in the last two years, there have been articles that have forced us to revise our mantra,” he says, “or at least nuance it.”
Closer to her goal
Light is not the main stimulus for mosquitoes, he agrees. That’s the exhaled carbon dioxide, combined with body odor and heat as the mosquito gets closer to its target. “But we now know that certain visual contrasts influence flight behaviour.”
In houses where light can be seen from outside, you are 84 percent more likely to find mosquitoes than in unlit houses, Spitzen says. That stood last February in Malaria Journal† But another factor is more important: ventilation. In well-ventilated homes, the chance of a mosquito flying in is 99 percent lower than in poorly ventilated homes, regardless of lighting conditions. Ventilation lowers the temperature and the carbon dioxide content.
“But it is nuanced,” says Spitzen. “We don’t know what light alone does. It would have been nice if the researchers also compared houses where no people were sleeping. So only houses with and without light. I suspect that they would have found less difference.”
The different stimuli – light, smell, carbon dioxide – appear to modulate each other: one determines how you respond to the other. American researchers reported that earlier this year in Nature Communications† They analyzed some 1.3 million filmed mosquito flight paths under a variety of conditions. “We see that carbon dioxide creates a strong attraction for specific colors,” they write, “including the colors humans perceive as cyan, orange, and red.” Precisely the colors that are dominant in human skin, they note.
Light-sensitive proteins
The researchers did experiments with mosquitoes in which genes were switched off for CO2-detection, or genes that code for certain opsins: light-sensitive proteins in the eyes, in this case those for the red tones. Both types of mosquitoes lost interest in humans. “Very cool,” says Spitzen. “Really very new.”
Based on that, you could experiment with strips of different colors to deflect mosquitoes away from doors, windows or vents, he notes. He himself is experimenting with traps that catch mosquitoes at entrance openings. “We work with synthetic human scent, but also with contrasts.” Spitzen and his colleagues showed that heat and humidity cause these to fall attract more mosquitoes †Malaria Journal, 2020). These are also modulating effects.
Mosquitoes therefore do not need light to reach their target, Spitzen summarizes. “They know where to find you in the dark. But visual stimuli do help mosquitoes to fly towards their target more efficiently.” You could make the comparison with humans: “We also use our senses in different proportions, depending on the circumstances.”