Do mosquito sprays still work? An honest guide to insecticide resistance

Reach for a spray when the mosquitoes arrive and you expect them to fall. Often they still do. But in a growing number of places, mosquitoes now survive doses of insecticide that once killed them. That change has a name, insecticide resistance, and it is not a rumour or a marketing line. It is measured in laboratories, tracked by health agencies and rising. This explainer sets out what resistance is, what a new measurement of it found, and what it means for anyone deciding how to keep mosquitoes down this summer.

What insecticide resistance actually means

Insecticide resistance is a heritable ability to survive a dose of insecticide that would kill an ordinary, susceptible mosquito. It is not the mosquito getting used to a chemical during its own short life. It is a change in the population across generations, as the few individuals carrying a survival trait live to breed while their neighbours die.

Two mechanisms do most of the work. In target-site resistance, a mutation changes the part of the mosquito the insecticide is meant to attack, so the chemical no longer binds well. The best-known example is knockdown resistance, often shortened to kdr, which blunts pyrethroids, the insecticide class used in most household sprays and bed nets. In metabolic resistance, the mosquito produces more of the enzymes that break the insecticide down before it can act. A single population can carry both.

Resistance builds fastest where the same chemical is used heavily and repeatedly. Every application that kills the susceptible mosquitoes and spares the resistant ones tilts the next generation further toward survival. That is ordinary natural selection, running at the speed of a mosquito's life cycle.

What the new measurement found

Research indexed on 23 July 2026 in the journal Evolutionary Applications put hard numbers on how strong this can get. Scientists at Arizona State University and the United States Department of Agriculture measured three knockdown-resistance mutations, V410L, V1016I and F1534C, in Aedes aegypti mosquitoes from Florida. Aedes aegypti is a primary vector of dengue, chikungunya, Zika and yellow fever.

Mosquitoes carrying all three mutations in double copy were more than 52 times as resistant to deltamethrin, a common pyrethroid, as a susceptible laboratory strain. Other genotypes were 8 to 19 times as resistant. It is, the authors note, the first detailed dose-response mapping across these genotypes, and it shows resistance is not a simple on-or-off switch but a graded scale set by which mutations a mosquito carries.

Two further findings matter for how resistance is managed. First, the advantage of being resistant is not fixed. A model in the study showed it rises and falls with the dose applied, how much insecticide the mosquitoes are exposed to, and how common the resistant gene already is in the population. Second, male and female mosquitoes had similar resistance once their body size was taken into account, which means males, which do not bite, could be used for cheaper resistance surveillance.

So have sprays stopped working?

No, and it is worth being precise here rather than alarmist. Resistance is local and variable. A product can still knock mosquitoes down well in one area and underperform against a resistant population a few hundred kilometres away. The Florida measurement describes Florida Aedes aegypti; it is not a verdict on every mosquito everywhere.

What the evidence does say is that you can no longer assume a spray is doing what its label promises against every population, and that leaning on chemicals alone becomes less reliable over time. The World Health Organization documents widespread pyrethroid resistance in the mosquitoes that carry malaria, and tracks confirmed resistance by country and insecticide class on its Malaria Threats Map. Because pyrethroids are the backbone of both bed nets and much space spraying, resistance to that one class has outsized consequences. And every extra round of the same chemical, applied to the same population, pushes resistance further along.

This is exactly why public-health agencies test before they spray. WHO publishes standard procedures for exposing local mosquitoes to a known discriminating concentration and counting how many survive, so control programmes know whether their chosen insecticide still works locally before they spend on it. Mosticare has covered how that global baseline is refreshed and monitored.

Why layered, non-chemical prevention holds up

If the chemical layer is losing reliability in places, the durable answer is not a stronger chemical. It is to stop depending on any single method. Public-health specialists call this integrated vector management, and for a household it comes down to a few things that resistance cannot switch off.

The most powerful is removing the water the next generation needs. Aedes mosquitoes breed in small containers of standing water, so emptying or covering plant saucers, buckets, watering cans, blocked gutters, toys and folded covers every week removes larvae before they ever fly. No insecticide is involved, so no resistance applies.

Physical barriers work the same way. An intact, well-fitted window or door screen and a properly hung mosquito net block bites by standing between the mosquito and the person. A physical net keeps working regardless of any chemical resistance, because it is a mesh, not a poison. Repellents such as DEET or picaridin, used according to the label, add personal protection when people stay exposed outdoors. And where professional control is used, rotating insecticide classes and monitoring resistance keeps the chemical tools working longer instead of burning them out.

None of these is a cure. A net or a spray reduces bites; it does not treat disease. For the mosquito-borne infections spreading this year, that distinction is the whole point. There is no specific antiviral for dengue and no human vaccine or specific treatment for West Nile virus, so preventing bites in the first place carries most of the load. When one prevention layer weakens, the honest response is to strengthen the others, not to spray harder.

What this means for you

• A household spray can still help, but do not treat it as a guarantee, and do not rely on it alone.
• Repeated fogging of the same area with the same product is the pattern that breeds resistance fastest, so it is the least durable strategy.
• The measures that resistance cannot defeat are physical: weekly removal of standing water, intact screens and nets, and repellents used correctly.
• If a professional service treats your area, ask whether they monitor resistance and rotate insecticides. That is a sign of control that will still work next season.

Medical disclaimer

This article gives general public-health information, not medical advice. Insecticide resistance varies by place and by mosquito population, and local guidance changes during each season. Follow all product labels and consult your national or local health authority for current advice. Seek medical care after mosquito exposure if you develop fever, rash, severe joint pain, confusion, neck stiffness or other worrying symptoms.

Sources cited

1. Evolutionary Applications, knockdown-resistance quantification in Florida Aedes aegypti, indexed 23 July 2026.
2. WHO, Global Malaria Programme, insecticide resistance.
3. WHO Malaria Threats Map.
4. WHO, standard operating procedure for testing insecticide susceptibility of adult mosquitoes in WHO tube tests.
5. WHO, Dengue and severe dengue.
6. WHO, West Nile virus.

Last updated 26 July 2026.