US mosquito spraying backfires as insects evolve resistance across 84% blocks

In many parts of the eastern United States, getting rid of mosquitoes has become just another household service. Homeowners hire private companies to spray their yards, or they buy insecticides and foggers themselves, hoping to reclaim summer evenings from constant bites. But new research from North Carolina State University suggests those choices may be quietly helping mosquitoes adapt and survive the very chemicals meant to kill them.Two linked studies, one on how resistance emerges and spreads and another on the social patterns behind it, focus on the Asian tiger mosquito (Aedes albopictus). This invasive species is now common across much of the eastern U.S., and it’s a major daytime biter in the Southeast. It thrives in urban and suburban settings, breeding in small containers and any place that can hold water: flower pots, clogged gutters, toys left outside, even bottle caps.The key finding is unsettling: by 2023, genetic markers for insecticide resistance were present across 84% of sampled residential blocks in one North Carolina county, and those hotspots were more common in wealthier neighbourhoods where spraying is more frequent.Backyard spraying and an unintended evolution experiment In North Carolina, households often turn to pyrethroids, a widely used class of insecticides, to control adult mosquitoes. Pyrethroids are relatively inexpensive and are considered safer than many alternatives for use around homes, pets and people. In regions with serious mosquito-borne diseases, they can play an important role in limiting transmission. In much of North Carolina, though, they’re used mainly to reduce nuisance biting.The studies suggest that backyard spraying, repeated across many homes, can act as a strong local selection pressure as homeowners with more disposable income are likelier to pay for regular mosquito spraying or to buy chemical products. Also, mosquitoes that happen to survive those treatments, often because of slight genetic advantages, are the ones that reproduce. Over time, those survival traits spread through local mosquito populations, leading to insecticide resistance.“Homeowners with money will pay to spray for mosquitoes, which selects for localized resistance… Over time, as more and more resistance builds up in the population, it will become harder to control populations. The insecticides will no longer be as effective,” explained Martha Burford Reiskind, an associate professor of biological sciences at NC State and the corresponding author on both papers, according to Phys.org.How resistance spread across Wake County The research focused on Wake County, North Carolina, and specifically on resistance to pyrethroids. The team looked for mutations in mosquito DNA that are known to reduce sensitivity to these insecticides.One study analysed archived mosquito samples from 2016 to 2024 to track how quickly a particular resistance mutation appeared and spread. At first, the picture looked reassuring, as in 2016–2017, no resistant mosquitoes were found in the samples. In 2018, the first resistant mosquitoes appeared, but only in a small cluster of locations.Then things changed rapidly by 2023, as genetic markers associated with pyrethroid resistance were found in 84% of the sampled residential blocks. And 39% of sampled mosquitoes carried the resistance mutation.In just a few years, resistance had gone from undetectable to widespread. This pattern is familiar in places like South America and Asia, where large-scale, continuous spraying to combat diseases like dengue and malaria has driven resistance. But Burford Reiskind noted that seeing similar dynamics driven largely by private, backyard mosquito control is new in the mainland United States.Wealth and resistance: a neighbourhood pattern The second study asked a simple but important question: Where is resistance most common, and does that correlate with social or economic factors?Researchers used total property value in each area as a proxy for homeowner wealth. While they did not directly measure how much each household sprayed, the assumption was that wealthier residents are more likely to pay for private mosquito control services or to purchase chemical treatments regularly.The results showed a clear pattern: Resistance was higher in neighbourhoods with higher property values. These areas likely see more frequent use of insecticides, creating consistent pressure that favours resistant mosquitoes.To detect resistance, the team used genetic screening to look for a mutation associated with “knockdown resistance” (kdr), as per the report. This mutation affects the mosquito’s nervous system, making pyrethroids less effective at paralysing and killing the insects. “This mutation is often the first to show up when there’s insecticide resistance,” Burford Reiskind said, reported Phys.org.The tiger mosquito isn’t just a nuisance. It’s an aggressive daytime biter capable of transmitting dengue, Zika, chikungunya and dog heartworm. While these particular studies focused on resistance, not disease, the implications are clear: when mosquitoes become harder to kill, the tools public health officials rely on during outbreaks may lose their power right when they’re needed most.Michael Reiskind, a professor of entomology at NC State and co-author of both papers, said the next step is to expand sampling to a broader mix of neighbourhoods, which is varying in age, wealth, housing density and layout, to see how widely these patterns hold.Why spraying alone is a problemThe core message from the researchers is not that insecticides should never be used, but that relying on chemical sprays alone is a risky strategy. Overuse or misuse can push mosquito populations to evolve resistance, making those products less and less effective over time.Instead, they advocate for integrated pest management (IPM) approaches that focus first on preventing mosquitoes from breeding, especially by removing standing water (where larvae develop); should also use physical and biological controls where possible; and reserve chemical insecticides as one tool among many, applied carefully and strategically rather than constantly.For individual homeowners, that means regularly checking and emptying containers that collect rainwater (flowerpots, buckets, toys, gutters, birdbaths, etc.); covering or treating water features that can’t be drained; cleaning clogged gutters and yard drains; and working with neighbours to reduce breeding sites across the whole area, since mosquitoes can fly between yards.A wider question for communitiesThese findings raise a broader question: when people make choices that feel sensible in isolation, like paying to spray the yard, what happens when thousands of people do the same thing?In Wake County, the answer seems to be that mosquitoes have adapted, and the resistance that began in a small cluster in 2018 is now common across most sampled blocks, especially in wealthier areas. That suggests communities may need to rethink how they approach mosquito control, balancing comfort, cost and long-term effectiveness.Thinking about your own environment, what do you think is the most practical step people could take, beyond spraying, to reduce mosquitoes while also avoiding the kind of resistance these studies are warning about?Catch the latest World News and Live updates. Download the TOI app.