With the arrival of heat, they hatched or came out of their winter diapause. There are therefore particularly many of them, at the moment, seeking our nourishing fresh blood. Scientists are looking for strategies to escape their unpleasant bites, which in some regions of the world are accompanied by the transmission of pathogens responsible for diseases such as malaria, yellow fever, dengue fever, chikungunya, and the Zika virus. or the Nile virus.
But to guide this research, it was first necessary to study their behavior and biology. During a conference she gave virtually this week at the Coeur des sciences de l’UQAM, entomologist Chloé Lahondère, professor in the Department of Biochemistry at Virginia Tech, in the United States, drew up a detailed portrait of the pests of our summer vacation in Quebec.
First, why do mosquitoes bite us?
Yep, it’s only the females that bite us. As soon as adult mosquitoes emerge from their pupae submerged in water, males and females mate. Then, the females go in search of a blood meal, because they find there the proteins necessary to produce their eggs. “Females can live long enough to complete several egg-laying cycles before dying. This is a problem, because if they take their first meal on a host that is infected, they will transmit the pathogens they picked up during their subsequent meals,” notes the researcher.
Males, on the other hand, do not digest blood. They feed mainly on the sugar they find in flower nectar (which also contains amino acids, vitamins and oils), but also in fruits, honeydew produced by aphids and the sap of certain trees.
Females also need sugar. “And the latter influences the number of eggs they will lay. If the female does not eat sugar, she produces fewer eggs, she flies shorter distances and her survival is limited,” explains the scientist.
To check if our flower gardens do not constitute in some way “self-service buffets for mosquitoes”, the team of Mme Lahondère placed mosquito traps in 10 gardens in Blacksburg, Virginia. From mid-May to the end of October, researchers collected mosquitoes that had been caught in these traps every week. They froze them, identified the species they belonged to, then mixed each crushed mosquito with a chemical compound that reacts with the fructose in the nectar, which allowed them to know if the insects had landed on a plant to eat sugar. “If the insect had fructose in its body, it must have been because it had visited a plant. We were then able to extract the DNA from the plant it had eaten on and sequence it to be able to identify the plants the insect had visited,” she explained.
The researchers found that mosquitoes obtained sugar, throughout the season, from 26 different species of plants with varied colors and shapes, including clover, plantain, periwinkle, maple, birch , plum tree, mint and sundew, a carnivorous plant. They also discovered that, towards the end of the summer season, they were also eating fruits and vegetables from our gardens. “Some plants are so abundant and present everywhere – clover, for example – that it would be too difficult to remove them in order to control mosquitoes,” said Ms.me Lahondère.
How do they bite us?
It is with its proboscis that the mosquito will draw blood from a small vessel present in our skin. Like a Swiss army knife, this horn is a real sheath containing six different styluses, each with its own specialty. Some will cut the skin, others will push the tissue and thus allow the tube to navigate inside the skin to locate a blood vessel. Still others will inject saliva, which makes you realize that you are being bitten. “Sometimes, this saliva has anesthetic properties which allow you not to feel the sting. » And there is also a stylus that allows blood to be sucked up and sent into the mosquito’s digestive tract.
How does the mosquito locate the humans it is trying to bite?
“He uses his different senses, just like humans,” replies the scientist. From a distance, the mosquito identifies its human and animal prey by the carbon dioxide they release when they breathe. Then, as it gets closer, it traces them to the bouquet of odors emanating from their skin, thanks to its antennae equipped with receptors sensitive to different types of molecules.
“Humans emit around 400 different volatile organic compounds through the skin that the mosquito can potentially use to track us. Each human gives off their own unique bouquet of smells, [qui découle de l’activité métabolique de son corps ainsi que de celle du microbiote de sa peau]. This olfactory chemical signature of a person’s skin explains why mosquitoes prefer to bite certain individuals rather than others,” explains the researcher.
Variations in human body odor also depend on many other factors, such as a person’s genetic profile, gender, age, foods they eat, health status or the amount of exercise they get. she does. “It has been shown that when we drink beer, we increase our attraction to mosquitoes”, gives the example of the entomologist, who also cites a study which showed that “certain mosquitoes were attracted to cheeses which emit the same odors than human feet, which would explain why these mosquitoes mainly bite the ankles, guided by the volatile compounds emitted by our feet.
Perfumes and soaps that we apply daily to the body, in the form of shampoo, laundry detergent, body shower gel or hand soap, also modify our body odor. And, therefore, our attractiveness for mosquitoes, indicates a study carried out by Mme Lahondère and his colleagues at Virginia Tech published in iScience in 2023.
In this study, researchers discovered that before washing, the volunteers who participated in the study had an odor profile dominated by 75% by aldehydes and ketones. But once they washed with four different soaps, they smelled mostly of terpenes, plant-derived compounds that mosquitoes visit to obtain nectar. Researchers have also observed that applying plant scents increases the attraction of mosquitoes to our skin. “Only soap with a simpler formulation, composed of coconut derivatives, was less attractive than the unwashed arm,” the researchers found.
The heat our bodies give off also helps mosquitoes locate us. This is why pregnant women, whose body temperature is higher, tend to attract more biting insects.
With the small Johnston’s organs located at the base of its antennae, the mosquito also detects air movements caused by our movements, which make its antennae vibrate. And when the mosquito is closer to its prey, it uses its vision, which allows it to detect contrasts and colors.
Armed with this new knowledge about mosquito preferences, researchers are now trying to develop new mixtures to repel and eliminate mosquitoes.