In order to prevent a large population of mosquitoes from invading cities, wetlands are studied and the quantity of larvae regulated throughout the year.
«We are devoured», «c’est infernal», «we left after three days“, could we read in the columns of the local press in recent weeks. In the South, and especially in Occitania, tourists complain of an invasion of mosquitoes “exceptional“. Some holidaymakers have gone so far as to shorten their stay, exasperated by the nuisance. Faced with these complaints and the prospect of economic loss, a “emergency mosquito controlwas organized in one of the municipalities concerned, Grau-du-Roi, after a call from the mayor.
On Monday, July 11, at 5 a.m., teams from the Interdepartmental Mosquito Control Agreement (EID) spread a product to kill mosquitoes in the city. A rare operation, which had not occurred in this part of the territory since 2015, according to EID. “Exceptional phenomenon, exceptional consequences», justifies Jean-Claude Mouret, one of the operational coordinators of the public body. Mosquitoes need aquatic areas to lay their eggs, and the stormy showers in June, accompanied by tidal winds, provided insects with many places to lay their eggs. After this meteorological episode, the EID counted thousands of new larvae.
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The product used against adult mosquitoes was a “natural pyrethrum in aqueous solution“, explains Jean-Claude Mouret. Before and during its application, the organization took precautionary measures: “the operation was done early in the morning, we warned the beekeeping authorities, we moved away from waterways and Natura 2000 sites» (natural or semi-natural sites in the European Union, with exceptional flora and fauna).
Year-round mosquito control
If the mosquito control of Grau-du-Roi has made people talk, it is not uncommon to control the mosquito population. Throughout the year, wet places, such as marshes, are studied and the amount of larvae there is regulated. The goal? Prevent species – which can travel tens of kilometers to reach cities – from invading urban areas. Using aerial (helicopters) and ground (pick-up) equipment, teams locally distribute a bio-insecticide called Bti. It is a bacterium intended for the larvae and 70 to 80% of them are eliminated after administration of this product.
According to Jean-Claude Mouret, its application is harmless for the climate and other species. “We work in line with environmental issues“, he says, “the EID conducts incidence studies to have the least significant possible impact on the environments“. An assertion qualified by the scientific community. Fanny Guillet, researcher at the CNRS, denounces an opacity around the incidence of the bacterium. A few studies have been carried out over the years and one of them, carried out in the Camargue, highlights impacts on the biodiversity of the area tested.
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Bti, supposed to be selective – acting only on mosquito larvae – also attacks a family of insects called chironomids. The researchers observed a total disappearance of these diptera, and with it an imbalance in the food chain. The population of bats and birds, especially swallows, has declined considerably, these species feeding largely on chironomids. “Very few studies have been done, I imagine there is still much more to discover», Regrets the researcher. She also mentions a study conducted in Germany which underlines the consequences for amphibians, which, after exposure, would undergo atrophy leading to a decline in the population in the medium term.
What alternatives?
To date, several scientific innovations offer effective mosquito control that respects fauna and flora. Among them, CO2 traps. In recent years the invention has become widespread on the territory and some municipalities have invested to install hundreds of devices, like Hyères (in the Var), which had more than 400 CO2 traps in 2021 An investment according to Fanny Guillet – the purchase being at the expense of the municipality.
The operation of these machines is simple: they diffuse a small quantity of C02, which reproduces human breathing, and emanates an olfactory lure to attract mosquitoes, which are then sucked in. According to the results, the impact on other species is less, “only female mosquitoes are attracted to the terminalsays the company that sells them, adding that the device would eliminate up to 88% of nearby nuisances.
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