More than half a million male mosquitoes carrying a parasite called wolbachia have been released in Barcelona. When these mate with females, the eggs don’t hatch, reducing the number of mosquitoes in the city and the risk of people getting mosquito-borne infections, such as dengue.

This approach is already being used in Singapore, but this is the first large-scale release in Europe. “In Barcelona, we are talking about the first example of a real introduction in the public spaces of the city,” says Rubén Bueno Mari at pest-control company Rentokil.

Globally, mosquito-borne infections are a growing problem because of the spread of invasive species, aided by global warming. In Europe, the main concern is the Asian tiger mosquito (Aedes albopictus), a daytime-feeding insect with stripy legs that has evolved to thrive in human environments.

The species, which can spread infections such as dengue and chikungunya, is now established in 16 countries in Europe, including Spain, leading to record numbers of cases of these infections. In August, the European Centre for Disease Prevention and Control warned that more needs to be done to tackle mosquito-borne diseases.

The Asian tiger mosquito can breed in very small amounts of water, such as rainwater-filled pieces of rubbish. This means conventional methods, such as putting insecticides in large pools of water, don’t work well, says Bueno Mari. Now, in conjunction with a Google project called Debug, Rentokil is deploying a variant of the sterile insect technique.

This takes advantage of the fact that many female insects mate only once, so if they mate with a sterile male, their eggs won’t hatch. In the 1950s and 60s, this approach eliminated screwworm from the US and Mexico.

But sterile males are produced by zapping them with radiation, which means they don’t live long and makes them less likely to mate. To get around this, Debug is instead taking advantage of an extraordinary bacterial parasite called wolbachia that infects many insects. Wolbachia is passed down in eggs, and one of its many tricks is preventing infected males from fertilising female eggs that are uninfected or carry a different strain of the parasite.

In other words, wolbachia-infected male mosquitoes are effectively sterile. This approach is known as the incompatible insect technique. In very small earlier trials in Rome, the release of wolbachia-infected males reduced the number of viable eggs by nearly 90 per cent, according to Rentokil and Debug.

In Barcelona, releases began in mid-August after regulators gave the go-ahead, says Bueno Mari. Each week, 80,000 males are produced at Debug’s breeding facility in Miami and flown to Spain. The releases will cease in mid-November and continue from May. They are being funded by Rentokil and Debug for the moment.

One issue with this approach is that it requires highly accurate separating of male and female mosquitoes to prevent the release of fertile females and the potential spread of wolbachia locally, which would render the strategy ineffective. Debug is using an automated process based on AI image recognition to do this, but Bueno Mari couldn’t say what percentage of females get released. “This is part of the information that we would like to extract, thanks to these trials,” he says.

However, the mosquitoes being bred and released are the same strain of the insect that is already present in Spain, he says. This means that, even if some females survive and breed, there is no chance of introducing other strains.

Even if the parasite did spread, wolbachia-infected mosquitoes are less likely to spread infections – in fact, the parasite is being deliberately introduced to mosquitoes in some places as an alternative strategy for reducing the spread of mosquito-borne diseases. What’s more, using a different strain of wolbachia can restore the effectiveness of the incompatible male technique.

In Singapore, wolbachia-infected male Aedes aegypti mosquitoes have been released in some parts of the city, but not in others, to assess the method’s effectiveness. A study published earlier this year concluded that the technique reduced dengue cases by 70 per cent. Some people living in treated areas may have been infected when in non-treated areas, so protection might be even higher if entire cities are treated.

“There is no single magic solution to stop the spread of mosquitoes and mosquito-borne diseases,” says Bueno Mari. “We are always talking about integrated mosquito management, but this is a new solution that can help us in a different way, so it’s promising.”

Debug is seeking permission for releases in Florida and California. Others groups are developing so-called gene drives that would spread among mosquitoes and either wipe them out or block the spread of infections.