Open the windows to let in some fresh air at night, or keep them closed to protect against mosquitoes? In some homes, without mosquito nets, the debate rages and points of view seem irreconcilable. During the June heatwave, the supporters of coolness often won. Their adversaries, tired of seeing the thermometer skyrocket in the rooms and unable to close their eyes during the night, agreed to open everything wide, resigned, ready to be devoured. With alert ears, they listened for the dreaded high-pitched whirring… but it never came.
Indeed, during the heatwave, many French people – from Paris to the Camargue – noticed a clear reduction in mosquitoes, while the end of June is usually their favorite period to come and spoil our nights and evenings. An impression confirmed by scientists specializing in the study of insects, who affirm that the high temperatures have had – temporary – repercussions on mosquitoes.
First of all, there is a simple and natural phenomenon: the increase in temperature accentuates evaporation and contributes to emptying stagnant water reservoirs but also ditches, ponds and certain small river arms. These are all areas usually favored by mosquitoes for breeding and depositing their eggs which will then become larvae. The eggs, dry, could not hatch. As for the larvae, they dry out very quickly and die within a few minutes to a few hours, once deprived of water.
A mechanical phenomenon is added. Certainly the French opened their windows during the night, but they sealed themselves off for the rest of the day and sometimes until late in the evening. Mosquitoes therefore had more difficulty entering homes. Furthermore, fans act as real repels for flying predators. Their average flight speed, 1 or 2 km/h, is no match for the blast of the machines. Disturbed, they are no longer able to aim at their prey and land peacefully on the skin.
The mosquito hidden but not defeated
Finally, and perhaps most importantly, mosquitoes do not tolerate extreme, dry heat. “Insects do not have thermal regulation, so their temperature is equivalent to that of the outside,” recalls Mathieu de Flores, entomologist at the Office for Insects and their Environment (Opie), on BFMTV. However, if a mosquito thrives fully in a climate of 25 to 30°C, its body suffers when the mercury passes the 35°C mark. “During this very hot weather, mosquitoes will try to hide in the vegetation, avoid becoming dehydrated and dying. They are therefore less active,” notes Mathieu de Flores.
This strategy allows them to survive heat peaks. This particularly applies to the tiger mosquito, now present in 83 French departments, according to Public Health France. Although it is generally more resistant and more adaptable than many other species, it does not better tolerate the extreme heatwave which combines a sharp rise in temperatures and drought.
However, the mosquito has not disappeared. He hid, protected, biding his time. His metabolism slowing down, he watches for the return of rain and humidity in the air, to resume his activity. The eggs are able to survive for up to several months if they are kept cool and in the shade. As soon as their pool fills again, they will be able to resume the cycle of their development and become adults in one to two weeks.
This delayed hatching, associated with the return of adult mosquitoes, may raise fears of a post-heatwave rebound effect. While the first days may appear calm, a sudden peak in activity can be observed one to two weeks after the heatwave, in the event of mild heat (between 20 and 30°C day and night) associated with the return of the rain.
For the moment, these conditions do not seem to be entirely met in France, with air remaining dry over the majority of the country, intense heat returning during the day in the southern half and temperatures falling further below 18°C at night in many departments. The rebound should remain limited. For now.
