The US Environmental Protection Agency’s decision last week to allow the release of billions of genetically engineered mosquitoes in California and Florida has several environmental and public health groups worried about the potential impacts of the experimental releases on public health and the environment.

The EPA move follows last year’s pilot field trial in the Florida Keys where, starting April 2021, the British biotechnology company Oxitec and the Florida Keys Mosquito Control released half a billion of these transgenic mosquitoes. The purpose was to test if they could help reduce populations of Aedes aegypti, a mosquito species that can carry viruses that cause deadly diseases like yellow fever, dengue, chikungunya, and Zika — none of which are major concerns in the United States.

The results from that controversial pilot trial — which took place in locations that were notified only just days beforehand — aren’t even public yet, but on March 6, the US Environmental Protection Agency gave Oxitec the go-ahead to extend the field trials, not just in Florida, but also in four central and southern California counties — Fresno, Tulare, San Bernardino, and Stanislaus.

If the proposal is approved by Florida and California state regulators, it would be the biggest release of GE insects in the world.

In California, billions of GE mosquitoes would be released in the four counties over a two-year period beginning as soon as this year. Additional mosquito releases would also continue in Florida’s Monroe County (which comprises Key West and parts of the Everglades National Park and Big Cypress National Preserve) for another two years.

Oxitec’s OX5034 GE mosquitoes are all male. They are mass-produced in a laboratory to carry two types of genes, a self-limiting gene that prevents their female offspring only from surviving beyond the larval stage, and a fluorescent-marker gene that glows under a special red light, enabling researchers to identify them in the wild. The idea is that when these male GE mosquitoes mate with the wild females in an area, all of their offspring inherit a copy of these two genes and the females among them die off before reaching adulthood, eventually leading to a population collapse. Since only female mosquitoes bite — they need blood to produce eggs — the company says the modified male insects, including both the initial releases and their male progeny, pose no danger to people. And, it says, unlike pesticides, its “technology does not use any chemicals that are harmful to the environment” or have any known harmful effects on beneficial insect species.

But despite field trials in the US, Brazil, Panama, and the Cayman Islands, the actual beneficial impact of these GE mosquitoes is still not very clear.

After the first field trials in Brazil between 2013 and 2015, Oxitec claimed a nearly 95 reduction in target mosquito populations, but these trials were not set up to assess whether the decline in mosquito numbers actually reduced prevalence of disease as well. Meanwhile, a 2019 study by Yale University researchers found that some female offspring of the GE mosquitoes did survive to adulthood and mated with their native counterparts. Oxitec disputed the Yale team’s findings, which were published in Nature, as “speculative” and “unsubstantiated,” and in 2020 the journal’s editors added a disclaimer to that report, citing similar concerns.

In 2020, ahead of the Florida Keys pilot trial, the EPA, too, evaluated the potential risk of releasing GE mosquitoes into communities and determined that “there will be no unreasonable adverse effects to humans or the environment.”

But these assurances haven’t allayed the concerns of several scientists and watchdog groups. They say the EPA simply hasn’t put adequate checks in place for this project and that such large-scale releases of a mobile, transgenic organism would be hard to monitor or control and thus pose unknown environmental and public health risks. Additionally, they say, the EPA and local authorities in Florida haven’t adequately consulted the public or been transparent about the project so far.

“We wanted [the EPA] to set up a scientific review panel as it does for other new pesticides (since the GE mosquitoes are being regulated as pesticides),” Jaydee Hanson, policy director the environmental and public health group Center for Food Safety, told the Journal. “That’s an impartial group of scientists that looks at the data and recommends how to proceed. It didn’t do that. We had suggested cage trials ahead of any environmental release, where the local environment these mosquitoes would be acting in would be replicated so that we could study how they behave… Instead the company has leap-frogged from cage trial in the United Kingdom to release in Brazil and then Florida.”

There is no publicly available data from Oxitec’s 2021 field trials in Florida (where there were a few dengue case in 2020) to support the company’s claims that their GE mosquitoes reduced local Aedes aegypti populations. And Hanson points out that key information about health effects, including possible allergenicity and toxicity to humans (from bites by transgenic females who might somehow survive to adulthood) or to animals that might feed on them, was redacted from the company’s application for a trial release permit, which is available to the public.

And though Aedes aegypti has been found in in cities across California, the state’s public health department says none of the viruses these mosquitoes carry “are currently known to be transmitted within California.” Which makes the EPAs decision to open the state up for trial releases rather confounding.

An Oxitec official told USA Today that the this second round of trials is actually planned to be much more limited — covering only the Florida Keys and expanding to one region in California — Visalia in Tulare County. Interestingly, Dr Mustapha Debboun, the general manager of the Delta Vector Control District, which includes Tulare County, used to be the director of the Mosquito and Vector Control division of Harris County Public Health in Texas at a time Harris County was considering a trial of Oxitec’s GE mosquitoes. (Though the EPA approved a trial release in 2020, Harris County didn’t approve the plan)

Bioethicists like Natalie Kofler, a molecular biologist and founder of Editing Nature, a group that advocates for inclusive and informed decision-making when using biotechnology, say that in general, outbreaks of most of the diseases the OX5034 mosquitoes are supposed to curb are relatively rare in the United States, so it’s unclear whether the potential benefits of releasing them in these states will outweigh the potential risks.

Yellow fever has been pretty much eradicated in the US — the last major outbreak occurred in 1905. Dengue outbreaks, while common in the US territories of Puerto Rico, the US Virgin Islands, and American Samoa, have been relatively small and limited to small areas, and nearly all cases reported in the 48 contiguous US states were in travelers infected elsewhere, according to the Centers for Disease Control. And while a large Zika outbreak in the Americas in 2015-16 led to “widespread transmission in Puerto Rico and the US Virgin Islands,” the CDC reports that the majority of the US cases outside these territories were associated with international travel.

Scientists like Kofler have been calling for the EPA to have a more rigorous and inclusive process for evaluating new biotechnologies. In 2019, she and three other scientists critiqued Oxitec’s Brazil trial in The Conversation, calling for further study ahead of field trials. “One significant worry is that a new breed of mosquito might emerge that is more difficult to control,” and that the “new genes could also potentially alter evolutionary pressures on viruses carried by mosquitoes, like dengue fever, in unpredictable ways,” they wrote. “This includes potentially increasing their virulence or changing their host-insect interactions.” (In 2021, Kofler told Undark magazine that Oxitec had pushed to have the article retracted.)

Meanwhile, the company’s own lab researchers have found that the GE mosquitoes are, in fact, sometimes creating viable female larvae. The OX5034 female mosquitoes are modified so that they can’t survive to become adults without tetracycline, a common antibiotic, but the company’s researchers found that exposure to “even small amounts” of tetracycline can act as an antidote to the female mosquito-lethal trait, leading to a 15 percent survival rate female offspring.

“An additional problem is that in the Central Valley [where some of the releases are planned] you have pear, apple, and orange plantations that are sprayed with tetracycline and CAFOS [Concentrated Animal Feedlots] where animals are fed tetracycline, and you are releasing [GM] female mosquitoes that are supposed to die, but they won’t because they will be able to find tetracycline in the environment,” Hanson says. He says when the Center for Food Safety brought this concern to the EPA, the agency responded by declaring that the mosquitoes wouldn’t be released within 500 meters of where anybody was spraying tetracycline. “That assumes that these insects can’t travel,” he says.

“Other than Brazil, we have the most lax regulations right now, which is why [Oxitec] is doing the release here,” Hanson says. “It’s not that we are forever and a day opposed to these technologies but want to make sure that it is done right.” Hanson cites another, non-GM strategy to curb Aedes aegypti populations — using mosquitoes deliberately infected with the common Wolbachia bacteria, which limits their ability to transmit pathogens, as more a viable and “elegant” solution that does not pose a risk to natural ecosystems or human health.

The final decision on whether or not to allow for this expanded testing now rests with the states. Oxitec will have to seek permits from Florida and California.

Andrea Leal, director of the Florida Keys Mosquito Control District, has already indicated an interest in expanding the experiment. “We made significant progress during the pilot project last year, we look forward to continuing this important work during this year’s mosquito season,” she is quoted as saying in an Oxitec news release in February. The proposal, however, is still pending approval from the Florida Department of Agriculture and Consumer Services.

In California, the Department of Pesticide Regulation, which would be in charge of providing the permit, said it was yet to receive and application from the company.

“If a research authorization application is submitted, the department’s review will entail the same rigorous scientific evaluation that all such applications are subject to, as well as consultation with government agencies and a process for public input and engagement,” Leia Bailey, the department’s assistant director of communications and outreach, told the Journal via email last week.