Private Mosquito Spraying Drives Resistance in Raleigh

Most mosquito spraying in the United States takes place without the involvement of any government agency.

Raleigh, North Carolina, has no public scheme to reduce the biting insects that swarm suburban gardens each summer.

Instead, residents are responsible for the task. They can treat their own lawns or hire private firms to do it for them.

A recent paper examines how these individual, dispersed choices have affected local mosquito populations.

Taken together across an entire city, those decisions have altered which mosquitoes are able to survive.

A city without spraying

The study focuses on the tiger mosquito, an invasive species that is now widespread throughout the eastern United States.

This mosquito reproduces in small pools of still water and bites during daylight hours.

Scientists from North Carolina State University (NC State) gathered mosquitoes from 31 residential blocks distributed around Raleigh.

The selected blocks represented a broad spread of property values, from lower-priced homes to houses valued at over two million dollars.

Neither Wake County nor the state operates its own mosquito-spraying programme. The pressure on these mosquitoes therefore depends on private, garden-by-garden decisions.

When homeowners do use sprays, they generally choose pyrethroids. The United States Environmental Protection Agency (EPA) describes this group as the country’s most commonly used mosquito-control chemical.

Trapping across the map

The researchers placed specially designed dry-ice-baited traps in gardens, using one trap per garden for approximately one day at a time. During around a month of sampling, they collected 987 tiger mosquitoes.

Numbers varied dramatically between gardens, from no mosquitoes at all to 200 in one trap. Each trap contained nearly 20 insects on average.

In the laboratory, the team sequenced the DNA of every mosquito twice, removing samples when the two readings did not match. The final count included only dependable results.

Wealth predicts mosquito resistance

Martha Burford Reiskind, an associate professor of biological sciences at NC State, is a corresponding author of the paper.

“Homeowners with money will pay to spray for mosquitoes, which selects for localized resistance,” said Professor Burford Reiskind.

The pattern becomes clear when neighbourhood blocks are ranked by house prices: mosquitoes were more resistant in more expensive areas.

For a property valued at about $350,000, the model estimated that one in five mosquitoes possessed the resistance gene. At roughly $720,000, that proportion rose to one-half.

At around $1.5 million, over four in five mosquitoes were resistant. Across all blocks combined, the gene occurred at a frequency of 39 percent.

One mutation to watch

The alteration occurs within a gene controlling the movement of nerve signals through a mosquito’s body. Usually, pyrethroids disrupt those signals and paralyse the insect.

This mutation weakens that impact, leaving the poison far less effective. Biologists refer to the characteristic as knockdown resistance, or kdr.

The particular change, known as F1534S, has appeared in tiger mosquitoes in many parts of the world. It was first identified in Singapore in 2009 and has since been associated with intensive spraying.

“This mutation is often the first to show up when there’s insecticide resistance,” said Professor Burford Reiskind.

A rapid local rise

The researchers were surprised by how quickly the change had spread. A 2020 survey in the same area detected the mutation in just 4.3 percent of the mosquitoes examined.

By 2023, it was present in more than half of the insects sampled. The gene was first seen in Raleigh around 2016, making its increase especially rapid.

“We see the rapid rise of pesticide resistance in places like South America and Asia where there is continuous spraying in some countries,” noted Professor Burford Reiskind.

“But resistance driven by private mosquito control is novel in the contiguous U.S., to our knowledge.”

Resistance travels in clusters

The resistance pattern was not randomly distributed around Raleigh. Nearby blocks generally had comparable levels of the gene.

This patchy distribution reflects how property values cluster within cities. Affluent streets tend to be close to other affluent streets, and resistant mosquitoes appeared to follow that pattern of wealth.

Tiger mosquitoes are also relatively sedentary, with most travelling less than six-tenths of a mile from their hatching site.

Because there is little movement between blocks, local spraying can produce a pronounced local effect.

Fewer mosquitoes, more resistance

The data contained an unexpected detail: wealthier blocks had fewer mosquitoes overall rather than more.

That finding dismissed the most straightforward explanation. It might have seemed that heavier spraying happened in places with the greatest mosquito numbers, yet resistance was not associated with mosquito abundance.

Older neighbourhoods did contain more insects, increasing by approximately 3.1 percent for every year of a home’s age. However, neither mosquito totals nor home age predicted resistance.

Only wealth had a clear association. It appears that money, rather than the severity of mosquito problems, influences how often residents choose to spray.

Why mosquito resistance matters

At present, the tiger mosquito is chiefly a daytime pest in North Carolina.

Elsewhere in the world, though, it transmits viruses causing dengue, West Nile and chikungunya, as well as the parasite that causes dog heartworm.

In 2024, dengue infected more than seven million people throughout the Americas alone. Pyrethroids are a frontline measure when outbreaks of this kind occur.

The mosquito has already brought dengue to places where the illness was previously uncommon. Some of these outbreaks happened in the United States.

Resistance therefore matters even in locations where disease is currently rare. If sprays stop working in a year dominated by nuisance mosquitoes, they could also fail in a more dangerous situation.

“A reliance on just chemical insecticides is never the best way to control a pest,” Burford Reiskind said.

“We really want to integrate pest management practices that look at the whole lifecycle of the pest.”