Private Mosquito Spraying Drives Tiger Mosquito Resistance in Raleigh

Most mosquito spraying in the United States takes place without any involvement from a government body.

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

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

A recent study examines how these individual, scattered decisions have affected the city’s mosquito population.

Taken together at city scale, these choices have altered which mosquitoes are able to survive.

A city without public mosquito spraying

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

It reproduces in small collections of still water and bites during daylight hours.

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

The sampled blocks spanned a broad range of property prices, from lower-value homes to houses valued at more than two million dollars.

Neither North Carolina nor Wake County operates its own spraying scheme. The pressure on these mosquitoes therefore depends entirely on private, garden-by-garden decisions.

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

Trapping tiger mosquitoes across Raleigh

The researchers placed one specialised dry-ice-baited trap in each garden for roughly 24 hours. During about a month of sampling, they collected 987 tiger mosquitoes.

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

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

Wealth predicts resistance

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

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

The pattern becomes clear when the blocks are arranged according to property value: mosquitoes were more resistant in more expensive neighbourhoods.

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

At approximately $1.5 million, more than four out of five mosquitoes were resistant. Across all blocks combined, the gene appeared at a frequency of 39 percent.

One mutation to monitor

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

This mutation weakens that action, meaning the chemical has far less effect. Biologists refer to the characteristic as knockdown resistance, or kdr.

The particular change, known as F1534S, has been identified in tiger mosquitoes throughout much of the world. It was first detected 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 swift local increase

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

By 2023, it was carried by more than half of the sampled insects. As the gene first emerged in Raleigh around 2016, the rise has been 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 moves in clusters

The resistance gene was not distributed evenly or at random across Raleigh. Nearby blocks were likely to have comparable levels of the mutation.

This uneven pattern reflects the clustering of property values within cities. Affluent streets tend to be located close to other affluent streets, and resistant mosquitoes tracked that wealth.

Tiger mosquitoes also do not travel far: most move less than about 1 kilometre from their hatching site.

Because there is limited movement between blocks, local spraying can produce a distinct local effect.

Fewer mosquitoes, greater resistance

The data contained an unexpected detail. Better-off blocks had fewer mosquitoes in total, rather than more.

This dismissed the most straightforward interpretation. It might be assumed that spraying was heaviest where mosquito numbers were greatest, yet resistance was not associated with mosquito abundance.

Older neighbourhoods did have more insects, with numbers increasing by roughly 3.1 percent for every additional year of a home’s age. However, neither mosquito totals nor property age predicted resistance.

Wealth was the only factor with a clear association. It appears that money, rather than the severity of mosquito problems, influences how frequently residents spray.

Why tiger mosquito resistance matters

At present, the tiger mosquito is mainly a daytime nuisance in North Carolina.

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

In 2024, dengue infected more than seven million people across the Americas alone. Pyrethroids provide a frontline defence when outbreaks of this kind occur.

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

This makes resistance important even in places where disease remains unusual today. If sprays are ineffective in a year dominated by nuisance mosquitoes, they could also fail in a more serious outbreak.

“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.”