It sounds like the plot of a horror movie; a swarm of bacteria-infested mosquitoes unleashed on a town. But it isn’t fiction – it’s actually about to happen in Fresno, a city in California's San Joaquin Valley.

However, residents won’t be rushing to swat these mosquitoes. They are being deliberately released into the wild to help stop the spread of disease.

Verily, the life sciences arm of Google’s parent company Alphabet, plans to release 20 million mosquitoes in Fresno this summer, in a bid to stamp out one of the most troublesome species – the aedes aegypti.

Disease carriers

The aedes aegypti carries diseases such as Zika, dengue and chikungunya, which are spread through mosquito bites.

Although these diseases are not present in Fresno, officials worry that it may only be a matter of time. The aedes aegypti is not native to the area: it arrived in in 2013, and since then has rapidly enlarged its area of infestation. “Everybody was being bit,” says one resident.

Image: Fresno Mosquito & Vector Control District

The project, aptly called Debug, rears millions of mosquitoes treated with a bacteria called wolbachia, which makes the males sterile. They will mate with wild female mosquitoes and, though these females will go on to lay eggs, no offspring will be born as a result. Their technique doesn’t involve chemicals, toxins or genetic modification.

“Over time, there will be fewer and fewer bad mosquitoes,” says Debug.

Because male mosquitoes don’t bite, residents shouldn’t feel too adversely affected by the release.

Verily has partnered with MosquitoMate and Fresno County’s Consolidated Mosquito Abatement District (CMAD) to release the mosquitoes in two neighbourhoods, each approximately 300 acres in size.

Is it safe?

The idea of releasing wolbachia-infected mosquitoes into the wild is not new, but this is the first time it has been trialled on such a large scale.

In the past, raising huge numbers of mosquitoes was tricky, but the team of scientists and engineers at Debug have come up with technology to make rearing them much easier. The company also uses a combination of sensors, algorithms and novel engineering to quickly and accurately sort males from females.

Though the prospect of an invasion of bacteria-infested mosquitoes may sound alarming, scientists are confident that it is safe. Wolbachia is a natural bacterium present in up to 60% of all the different species of insects around us, including some mosquitoes. And it is harmless to humans.

Other uses

Other programmes using wolbachia bacteria are already underway.

An international coalition including governments, philanthropic organizations, and the Eliminate Dengue Programme (EDP), has poured millions into research into mosquito control using wolbachia.

Image: eLife

Research by EDP has shown that when wolbachia is introduced into the aedes aegypti mosquito population, it can prevent these viruses being transmitted to people.

Their approach is slightly different than that of Debug. Rather than trying to reduce the overall number of mosquitoes, they aim to spread wolbachia into wild mosquito populations to reduce the ability of these mosquitoes to transmit disease.

“We release a smaller number of male and female mosquitoes with wolbachia over a number of weeks and these mosquitoes then mate with the wild mosquito population. As the bacteria is passed on from generation to generation and over time, the percentage of mosquitoes carrying wolbachia grows until it remains high without any further releases. Mosquitoes with wolbachia are less able to transmit viruses to people, so the risk of outbreaks in those areas is reduced,” explains EDP.

The issue with Debug’s approach, says EDP, is that, like insecticides, their technique needs reapplying over time as the population of mosquitoes gradually returns.

Meanwhile, back in Fresno, while the release of the mosquitoes will mean an increase of mosquito activity in the area, it is hoped it will ultimately prevent disease.

Aedes aegypti is found throughout the world, with devastating consequences. If the project is successful, it could lead to other trials in diseased areas and help communities around the world affected by mosquito-borne viruses.