In early July, New York health officials conducting routine tests on the city’s mosquito population found that an alarmingly large number were carrying the West Nile virus. The virus, which originated in the Eastern Hemisphere and is spread by Culex mosquitoes, was first detected in New York in 1999. In the decades since, the city has honed its response to a science. Officials considered data on the concentration of mosquitoes, along with the neighborhood’s vulnerability to infection, to decide what to do next. On the evening of July 15trucks rolled into neighborhoods in the borough of Queens for the first time this summer, clouding the air with a mixture of pesticides meant to kill the mosquitoes before they could spread the virus to people.
Spraying pesticides to kill adult mosquitoes, a technique known as adult killing, is a central pillar of cities’ public health strategy as mosquito populations expand, migrate to new areas and emerge earlier in the season, driven in part by by a changing climate. Some of them spread diseases previously confined to tropical areas, such as West Nile, malariaand dengue. An outbreak of the rare but deadly eastern equine encephalitisor EEE, is currently underway in the North East; one person in New Hampshire and one more in New York have died of the disease.
But the use of toxic chemicals to control mosquito populations — which officials say is necessary to protect public health — has long drawn opposition from environmental and community groups, which say the strategy endangers the very neighborhoods it’s meant to protect. set. They argue that the potential health effects of these substances, especially on the endocrine system, are not taken into account when planning mosquito control strategies, and urge public agencies to focus more on prevention and public education. Jay Feldman, director of the environmental group Beyond Pesticides, called the increase in mosquito-borne diseases “a concern that needs to be taken seriously,” especially as climate change increases pressure on governments to protect vulnerable people.
“But like other decisions to use toxic chemicals across broad sections of the population, those decisions must be made with transparency,” Feldman said. “And that’s where I think we’ve failed the public.”
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Americans have long tried to combat the nuisance — and public health threat — posed by mosquitoes through spraying. In the 1950s and 60s, trucks proliferated dichloro-diphenyl-trichloroethane – an insecticide developed in the 1940s and more commonly known as DDT – over farmland and neighborhoods, with the aim of combating diseases such as malaria and typhus. It was banned nationwide in 1972 after Rachel Carson exposed its harmful effects on wildlife in her book. Silent springwhich started the environmental movement. But even after DDT was phased out, adult kills with other chemicals remained common, both by public agencies and by pest control companies like Orkin and Terminix.
City and county public health departments and mosquito control agencies across the country use adult kill in combination with other tools. These include larvicides — chemicals that kill mosquito larvae before they have a chance to develop into adults, and are typically less toxic to other organisms than adults — and eliminating mosquito habitat, such as pools of standing water. The New York City Department of Health sprayed adulticides 137 times between 2018 and 2023, according to city dataand another 20 times this year. There is more than 1,100 vector control agencies across the country, and many of them use adults, including in California, Floridaand Texas.
The main aim of mosquito spraying programs is to prevent outbreaks of diseases such as West Nile virus, which more than 2,300 people across the United States over the past 25 years. The CDC has reported so far 748 cases of West Nile virus this year in 43 states, while deaths have occurred in states ranging from Illinois to Mississippi to New Jersey.
Climate change is now accelerate the spread of disease such as West Nile, as warmer temperatures repel mosquitoes develop faster, bite more oftenand become better incubators for viruses. Milder winters allows disease-carrying mosquitoes to survive into the following summer, while rainfall has increased – as recently across the south unleashed by Hurricane Helene — creates standing pools of water that serve as a breeding ground for the insect. Earlier hurricanesmeanwhile, drive outbreaks in damaged areas. Other factors also play a role; growing urbanization also put mosquitoes in more frequent contact with humans, while the decay of residual amounts of DDT in the environment caused populations of the insect to rebound.
“We need to be more aggressive,” New York City Health Commissioner Ashwin Vasan told Grist about the city’s mosquito control efforts this year, when officials had to increase spraying as well as other measures in response to higher-than-normal rates from Wes. Nile virus in the mosquito population. “This is now the new normal in terms of what public health looks like in the face of a changing climate.”
But as the need to deal with deadly mosquitoes grows more urgent, advocates are calling on officials to take a closer look at the enforcement of adult mosquitoes, raising concerns about their potential harm to human health and the environment. The main adulticides used by the New York City Health Department are Anvil 10+10 and Duet, both of which contain synthetic pyrethroids, a class of chemicals that kill insects by targeting their nervous system. Pyrethroids such as sumithrin, the active ingredient in both Anvil 10+10 and Duet, are also endocrine disruptorswhich can and are mimicking hormones in the body especially dangerous for unborn children. A study published in May in the journal Frontiers in Toxicology found that while data on the health impacts of endocrine-disrupting pesticides are scarce, pyrethroids are associated with lower sperm count in men.
The US Environmental Protection Agency does not investigate pesticides for their potential effects on the endocrine system. Feldman of Beyond Pesticides said that means compounds like Anvil 10+10 shouldn’t be considered safe just because they’re approved by the federal government. Other chemicals found in the insecticides have also been linked to health problems; the cancer-causing “forever chemicals” known as PFAS was found in pesticides including Anvil 10+10mainly from its storage in shipping containers covered with the substances. Anvil also contains piperonyl butoxide, an additive used to increase the potency of the pesticide, which the EPA is considering a possible human carcinogen.
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New York’s health department says mosquito spraying occurs at low enough concentrations that it does not pose a danger to human health, although the agency recommends people stay indoors while their neighborhoods are sprayed and warns that people with respiratory conditions or others “who are sensitive for spray ingredients may experience short-term eye or throat irritation, or a rash.” An environmental impact statement conducted by the city in 2001 concluded that any adverse effects on the public health of adults would not be considered significant when compared to the public health risks of allowing mosquitoes to proliferate.
Clarke, the maker of Anvil 10+10 and Duet, told Grist that its products have been reviewed by the EPA and that “adult mosquito control—used in conjunction with larval control and source reduction—is the best tool for reducing adult mosquito populations in areas that are experiencing an outbreak.” A Clarke spokesperson too told Politifact last year that drops of the company’s pesticides are specifically designed to work on mosquitoes, and that they break down as soon as they hit the ground.
But advocates say adults are a temporary solution at best because of the tendency of mosquitoes to develop resistance to these substances. Recent research from Arizona State University found that some mosquitoes are becoming resistant to the main pesticides used to control them. This creates a “treadmill effect,” Feldman said, where greater amounts of chemicals, as well as new types of pesticides, are needed to kill increasingly tolerant insects.
In his 2024 Comprehensive mosquito control and surveillance planNew York City said it applies adulticides only as a last resort. It reflects best practices in the mosquito control industry, said Dan Markowski, the technical adviser for the American Mosquito Control Association, a professional association of mosquito control workers, public agencies and private mosquito control applicators across the country, which receives funding from pesticide makers. including Clarke. The organization is working to build a nationwide database for mosquito surveillance, detect pesticide resistance and develop a model for spraying based on real-time weather data, with the goal of helping its members target and reduce their pesticide use.
“No one wants to apply pesticides over a wide area, but you have to very often because none of the other methods are 100 percent effective,” Markowski said. “And when you have an outbreak … at that point, you don’t have a lot of other options.”
Some governments are also experimenting with release genetically modified mosquitoes in the wild to breed sterile offspring, which reduces mosquito populations. Nano pesticideswhich is less toxic to mammals but still affects mosquitoes is also a promising area of research. However, advocates say the most proven way to deal with mosquitoes is by reducing their ability to breed – by clearing pools of standing water, and using larvicides – and educating the public to protect themselves by wearing long clothing and to use repellents.
Feldman pointed to the success of programs in cities such as Boulder, Coloradoand Washington, DCas proof that adults need not be a major part of mosquito control efforts. For example, the agency responsible for detecting and preventing the spread of West Nile virus in the nation’s capital does not employ adults; instead, the DC Department of Health is concentrating its efforts on larvicide, even handing out free larvicides for residents to apply in their own neighborhoods. Bouldermeanwhile uses an explicit “ecological” approach; boosting biodiversity, local officials have found, can lower populations of disease-carrying mosquitoes by forcing them to compete for resources with other species of mosquitoes as well as other types of insects.
“Until we start thinking systematically about these problems,” Feldman said, “we’re going to be chasing chemical after chemical, disease after disease, insect after insect, as we see increasing pressure on society to find the silver bullet that exists. not.”
