Mosquitoes are evolving to survive common backyard sprays

Mosquitoes in Wake County are rapidly developing resistance to some of the most common insecticides used to kill them, forcing backyard-spraying companies to confront whether their own treatments may be contributing to the problem.

Two new studies from North Carolina State University tracked the emergence of a genetic mutation associated with resistance to pyrethroids, a class of insecticides commonly used by homeowners and pest-control companies.

Researchers found no mosquitoes carrying the mutation in samples collected in 2016 and 2017. They detected the first resistant mosquitoes in a small cluster in 2018. By 2023, resistance-associated mutations appeared in 84% of the residential blocks sampled, and 39% of the mosquitoes carried the mutation.

Preliminary data from 2025 show another slight increase, according to Martha Burford Reiskind, an associate professor of biological sciences at NC State and a corresponding author of the studies. Researchers are continuing to collect mosquitoes this year.

“When we did that study, we thought, well, we’ll check,” Burford Reiskind said. “We did not expect to see it so widespread and at such high prevalence.”

The mutation does not mean pyrethroids have stopped working entirely. Researchers describe it as an early warning that mosquitoes are building resistance. Mosquitoes with two copies of the mutation can withstand pyrethroids at certain levels, and additional mutations could produce stronger resistance.

That creates a dilemma for an industry built around helping people enjoy their yards without being bitten — and for public health officials who may need the same chemicals during a disease outbreak.

What spray companies are doing

Keith Brown, owner of Mosquito Shield of Wake County, said his company uses pyrethroids but does not rely on one treatment throughout the mosquito season.

The company generally treats a property every 12 to 21 days, adjusting the schedule based on heat, rainfall, and mosquito activity. Brown said technicians alternate among pyrethroids, a proprietary blend containing natural oils, and other products.

Mosquito Shield also uses insect growth regulators that prevent larvae from developing, mosquito-control stations that target egg-laying females, and larvicides for standing water. Technicians inspect properties for breeding sites and alert homeowners to water collecting in gutters, toys, drainage pipes, and other easily overlooked areas.

“Less use of pesticides, the better, and we can still maintain that control,” Brown said.

Brown said the company has not changed its chemicals or application schedule specifically because of the NC State findings. "It has not observed an increase in treatments failing," he said.

Mosquito Shield typically receives callbacks from about 2% to 3% of customers following treatments, a rate Brown said has remained stable. "Weather and overlooked breeding sites are usually responsible when mosquitoes return quickly," he said.

Still, Brown acknowledged that repeated residential spraying could be contributing to resistance.

“There may be some resistance coming,” he said. “What we would stress is integrated pest management, using the growth inhibitors, using the larvicides in combination with the insecticides. You’re not just going out and spray, spray, spray.”

Brown said the company tries to establish control with pyrethroids before reducing their use and relying more heavily on its natural-oil blend. The company also avoids treating too soon after a previous application and may use a non-pyrethroid product if a customer requests another treatment.

Researchers say the important distinction is not simply whether a company changes products but whether it uses different modes of action and targets multiple stages of the mosquito life cycle. Repeatedly switching among products that affect mosquitoes in the same way may still allow resistance to build.

Burford Reiskind said researchers want to work with private companies, public health agencies, and mosquito-control officials to determine which chemicals are becoming less effective and how to preserve pyrethroids for emergencies.

“If their product is no longer working because insecticide resistance is rising, that’s going to be problematic for them from a financial perspective,” she said. “We really need to see how we might be able to change the different types of insecticide use.”

Why resistance matters

Pyrethroids are relatively inexpensive, effective, and considered among the safer insecticides for use around homes, people, and pets. They can quickly reduce adult mosquito populations, making them an important defense when mosquitoes are spreading disease.

The studies focused on the Asian tiger mosquito, an invasive, aggressive daytime biter that can transmit dengue, Zika, and chikungunya viruses, as well as dog heartworm. Those diseases are not currently spreading through tiger mosquitoes in Wake County, but the mosquito capable of carrying them is already established here.

“Our main arsenal for bringing down mosquito populations rapidly during a disease outbreak is really pyrethroids,” Burford Reiskind said. “What we want is a population that’s susceptible.”

The researchers found the highest rates of resistance in older neighborhoods with higher property values. They used property value as a proxy for homeowners’ ability to purchase mosquito-control treatments.

The studies did not track pesticide applications at individual homes, meaning they do not directly prove that private spraying produced the mutations. Researchers found that rising resistance coincided with increased residential insecticide use; however, they said the geographic pattern suggests repeated backyard treatments are creating selection pressure.

When a yard is sprayed, susceptible mosquitoes die, while those carrying resistance mutations have a greater chance of surviving and reproducing. Their offspring can then carry the same traits beyond the property where the chemicals were applied.

Researchers are now studying how resistant the mosquitoes are in practice, whether they carry multiple mutations, and how directly residential applications are driving the change.

Climate change expands the mosquito’s reach

The resistance concern comes as the tiger mosquito expands into places where colder conditions once limited its survival.

The species arrived in the continental United States in the 1980s and spread rapidly. Unlike some tropical mosquitoes, its eggs can survive the winter, helping it establish populations in more temperate climates.

Burford Reiskind said rising temperatures are allowing tiger mosquitoes to move farther north, including into major metropolitan areas along the northeastern coast. Continued warming could open additional territory that was previously too cold for the species.

Brown said Mosquito Shield is also seeing mosquitoes emerge earlier and remain active later, although rainfall and short-term weather conditions create significant differences from one season to another.

This year’s early drought initially suppressed mosquito activity in Wake County. More recent rainfall has created new breeding habitat and triggered hatches.

“The warmer it is, the faster they reproduce,” Brown said.

Protecting pollinators

Broad insecticide use can also raise concerns about bees, butterflies, and other insects that are not the intended targets.

Brown said Mosquito Shield trains technicians to monitor wind to reduce drift, apply treatments early or late in the day when pollinators are less active, and avoid flowering plants, fruits, vegetables, and other areas where pollinators are likely to land.

Technicians focus instead on shaded, damp vegetation where adult mosquitoes rest, he said.

“We’re not just spraying everything to spray everything,” Brown said. “We just treat the areas where the mosquitoes are going to land.”

Customers can ask a company to identify the active ingredients it uses, explain how it rotates treatments, and describe what precautions it takes around pollinator habitat.

How to reduce mosquitoes without fueling resistance

Researchers are not telling homeowners they must abandon mosquito control. They recommend using several strategies instead of repeatedly relying on a single insecticide.

Homeowners can:

• Empty and scrub containers that hold water at least once a week. Check flowerpot saucers, toys, buckets, tarps, trash cans, and birdbaths.
• Inspect less obvious breeding sites, including clogged gutters, uncapped fence posts, corrugated drainage pipes, and areas beneath decks.
• Use mosquito dunks or bits containing Bti in water that cannot be emptied. The larvicide targets mosquitoes before they become biting adults. Follow the product label.
• Ask pest-control companies whether they rotate products with different modes of action, use larvicides, and inspect for breeding habitat.
• Use EPA-registered insect repellent and wear loose-fitting, long-sleeved clothing when mosquitoes are active.
• Keep window and door screens in good repair.
• Remember that larval controls take time. They prevent the next generation from emerging but do not immediately kill adult mosquitoes already flying.

Burford Reiskind said reducing reliance on pyrethroids could potentially reverse the trend if carrying the mutation creates a biological disadvantage for mosquitoes when the chemical is no longer present. Researchers are now studying whether that is the case.

“We’re actually very optimistic that we’ve caught it early,” she said.

The question is whether homeowners and the mosquito-control industry will change their practices before an early warning becomes a widespread failure.