The pros and cons of spraying pesticides to keep disease-carrying mosquito populations down

Researchers are trying to find ways to quell growing mosquito populations that spread disease without putting recovering populations of important pollinators like bees and butterflies at risk.

Pesticides are an important management tool for mosquito control as well as for other pests that impact agriculture, Laura Melissa Guzman, assistant professor of biological sciences and quantitative and computational biology at the University of Southern California, told ABC News. The sprays, which contain low concentrations of pesticide, have been developed to target the insect physiology to kill them, Jeff Riffell, a professor of biology in the Department of Neuroscience and Biology at the University of Washington, told ABC News.

"The only really acute way to control these diseases, such as West Nile, Zika, are actually through some of these insecticide sprays," Riffell said.

The strategies in which the spraying occurs are intentional and well-planned, Riffell said, describing mosquito control efforts in the U.S. as "fantastic." Health officials target only areas that are found to have high levels of mosquito activity, Riffell said.

There are various insecticides -- some target the larvae stage, while others target adults, the stage in which the mosquitoes are actually biting people, Riffell said.

New research conducted by Riffell's team studied the mating habits of mosquitoes and how targeting the mating "swarms," in which females fly into large gatherings of males to mate, according to a paper published Friday in the scientific journal Current Biology.

"If you spray where these swarms are occurring, you actually really decrease, or decimate, the mosquito populations in these kind of urban areas," Riffell said.

On Monday, the New York City health department began spraying pesticides to kill mosquitoes and reduce the risk of West Nile virus, which has infected six people in the city so far. The infection rate typically peaks in August, according to the health department.

At least 33 states have reported cases of mosquito-borne illnesses so far this year, according to the Centers for Disease Control and Prevention.

One person in New Hampshire has died after possibly contracting Eastern Equine Encephalitis virus, according to the state's Department of Health and Human Services, and at least six cases of EEE have been reported across the country, according to the CDC and state health departments.

Three deaths -- two in Wisconsin and one in Illinois -- have been attributed to West Nile, according to state health officials.

Former National Institute of Allergy and Infectious Diseases Director Anthony Fauci revealed over the weekend that he was recently hospitalized after contracting West Nile.

About 135,000 storm drains, nearly 7,500 miles of sewer tunnels and 4,000 acres of wetlands make New York City a "mosquito haven," according to the Health Department, which advised people to stay indoors during the spraying and to wash skin and clothing exposed to pesticides with soap and water.

While the pesticides pose little risk to humans and pets, they could pose a danger to pollinators, many of which have seen dwindling populations in the last decade, Guzman said.

Agricultural use of pesticides is associated with declines in bee populations, according to Guzman, who recently completed research on the topic. Counties with high pesticide use tend to see bee declines between 19% to 45%, according to the paper, published on Tuesday in Nature Sustainability.

Overall, butterfly populations in the U.S. are declining about 1.6% per year, a 2021 study published in Science found. The Western bumblebee bombus occidentalis has declined by more than 90% over its historic range, while the rusty patched bumblebee is now federally listed as endangered it's under the Endangered Species Act.

However, the pesticides are rigorously tested before they are implemented on a wide scale, Riffell said.

"The United States has strict guidelines testing these insecticides -- that they can use in kind of our urban areas and on other pollinators," he said.

Most of the research has been done on honey bees, the experts said, which the low concentrations of insecticide have shown to have little impact on, the experts said.

However, the extent to which pesticides impact other pollinators -- which are crucial to global plant growth and crop production -- is still unclear.

Health experts predict the spread of mosquito-borne illness to increase as climate change continues to warm global temperatures, creating warmer regions in which mosquitoes can thrive.

Between 2004 and 2018, the number of reported illnesses from mosquito, tick and flea bites more than doubled, with more than 760,000 cases reported in the U.S., according to the CDC.

By 2070, an additional 4.7 billion people may be at risk of malaria and dengue due to climate change, according to a 2022 study published in the Lancet Planetary Health journal.

At a church in the Inwood neighborhood of upper Manhattan, those who created and care for a butterfly sanctuary on church grounds are worried that the pesticides will kill the butterflies that lay their eggs and develop amid the sanctuary's milkweeds.

"We need them as pollinators, and we need them for the beauty and the joy they bring," Keith De Cesare, founder of the Inwood Butterfly Sanctuary, told ABC New York station WABC.

De Cesare said that he was worried about the spraying planned to occur in Inwood.

"No one really wants these sprays in their backyard, but it's been shown to be an incredibly effective strategy at controlling these mosquitoes that are causing these pathogens of disease," Riffell said.