The fogging van clears your lane for an hour. What is breeding on your balcony refills it by morning.

The fogging machine that comes down the lane in dengue season is the most visible thing a municipality does about mosquitoes, and close to the least effective. It kills adult mosquitoes that happen to be flying in the open when it passes. It does not reach the ones resting inside cupboards and behind curtains, it does nothing at all to larvae, and by the following morning the containers in the neighbourhood have produced replacements. The exercise is not useless in an outbreak, but it is a way of buying hours, and it is treated as though it were a cure.

Understanding why control fails means starting with the insect rather than the machine.

The mosquito that matters most is a domestic animal

Not all mosquitoes are the same problem. Anopheles, the malaria vector, tends to bite at night and breed in larger natural water bodies. Culex, the common nuisance biter and the vector of Japanese encephalitis and filariasis, is happy in dirty, organically rich water — open drains and sewage.

Aedes aegypti, which carries dengue, chikungunya and Zika, is a different animal in every respect that matters for control. It bites in the daytime, with peaks in the morning and late afternoon. It lives inside and around homes rather than in the wild. And it breeds in small volumes of clean water: overhead tanks left open, water stored in drums and buckets, plant saucers, money plants in bottles, discarded tyres, air-cooler trays, fridge drip pans, coconut shells, the plastic sheeting on a construction site, and the rim of a flowerpot.

Three consequences follow immediately, and each of them undoes a standard intervention.

A bednet protects a sleeping person from a mosquito that bites at night. Against a daytime biter it does very little. Spraying drains is aimed at Culex and misses the container in the balcony. And the breeding site of the mosquito that infects your household is, statistically, inside your household or within a very short flight of it — this species does not travel far.

Aedes eggs add one more difficulty. They are laid on the damp wall just above the waterline and can survive drying out for months, hatching when the container is refilled. Emptying a container is therefore not enough; the sides have to be scrubbed. A dry pot that has not been cleaned is a mosquito outbreak waiting for the next rain.

Why the insecticide stopped working

Insecticide resistance is the second half of the failure, and it has been building for decades in plain sight.

Resistance is not a mosquito "getting used to" a chemical. It is ordinary selection: a population contains rare individuals with a trait that lets them survive a dose, those individuals breed, and after enough generations — and a mosquito generation is a matter of weeks — the trait is the norm. What is unusual about mosquito control is how relentlessly single-minded the selection pressure has been. Pyrethroids have carried the great bulk of public-health vector control for decades, including being for a long time the only class permitted on insecticide-treated bednets, and a single class used everywhere is an efficient way of breeding resistance to that class.

Four mechanisms show up, and they behave differently:

• Target-site resistance. Mutations in the voltage-gated sodium channel — collectively known as knockdown resistance — alter the exact protein pyrethroids and DDT bind to. The insecticide arrives and no longer fits its lock. Related mutations in the acetylcholinesterase gene do the same for organophosphates and carbamates.
• Metabolic resistance. The insect over-produces detoxifying enzymes — cytochrome P450 monooxygenases, esterases, glutathione S-transferases — and dismantles the compound before it acts. This one is dangerous because it is broad: an enzyme that clears one chemical often clears related ones the population has never been exposed to.
• Cuticular resistance. A thicker or altered cuticle slows how much insecticide gets in, buying the insect time to detoxify.
• Behavioural resistance. The population shifts to biting earlier in the evening, or outdoors, or resting on untreated surfaces — avoiding the intervention rather than surviving it. This is the hardest to measure and the easiest to miss, because a resistance bioassay in a laboratory tests whether a mosquito survives contact, not whether it now avoids contact altogether.

Public-health spraying is not the only source of pressure. Agricultural insecticides act on the same targets, and larvae developing in field-adjacent water are selected long before an adult ever meets a fogging machine — a reminder that vector control and crop protection are drawing on one shared pool of chemistry.

A fogging machine treats the mosquito you can see. The population is in the water you have not emptied, and the chemistry that used to kill it is being used up faster than it is being replaced.

What the evidence supports

The interventions that hold up are less satisfying to watch and considerably more effective.

Source reduction is the foundation, and for Aedes it is mostly domestic: cover stored water, empty and scrub containers weekly, invert anything that holds rain, deal with the cooler tray and the fridge tray, and remove standing water from construction sites, which are among the most productive breeding grounds in Indian cities. The weekly rhythm matters — the cycle from egg to adult runs about a week to ten days in warm weather, so a genuinely weekly reset breaks it.

Larviciding attacks the same stage chemically or biologically. Bacillus thuringiensis israelensis is a bacterial larvicide that is specific to mosquito and blackfly larvae and safe in drinking-water containers, which makes it usable exactly where source reduction is hardest.

Adulticiding — fogging — has a narrow, real role: knocking down infected adult mosquitoes fast during an active outbreak, to interrupt transmission while other measures take effect. It works best indoors and applied properly, which is not what a truck driving past a closed window achieves. Used as routine reassurance, it wastes insecticide and adds selection pressure for no epidemiological gain.

Biological control has a mixed record worth knowing honestly. Larvivorous fish are effective in the right water bodies, but the widely introduced Gambusia has caused ecological damage to native fish and amphibians in many places, and using native larvivorous species instead is now the better-supported approach.

The most interesting recent result is Wolbachia. This is a bacterium that lives naturally in many insects but not in Aedes aegypti; introduced into the mosquito, it interferes with the ability of dengue virus to replicate inside it, and it spreads through the wild population on its own because of how it distorts mosquito reproduction. A randomised controlled trial in Yogyakarta, Indonesia, reported a roughly 77% reduction in dengue cases in treated areas, which is a striking figure for any vector intervention, and deployments have followed in several countries. Indian work on establishing Wolbachia-carrying Aedes colonies has been under way at ICMR's vector control research centre. It is not a switch anyone can flip nationally, but it is the first approach in a long time that does not depend on a chemical the mosquito is learning to survive.

Why it matters for students and researchers

Medical entomology in India is a field where routine, unglamorous data collection is genuinely scarce and genuinely decisive. Resistance monitoring — standard bioassays run on local populations, repeated over seasons — tells a programme which insecticide will still work in a given district, and without it, spraying is guesswork with a budget. Container surveys establishing which vessels actually produce most of the adults in a locality can redirect an entire municipal effort. Studies of biting-time shifts are among the few ways behavioural resistance becomes visible at all.

None of this needs exotic equipment. It needs consistency, local sampling and honest reporting of negative results, and it is thin in the published literature relative to India's disease burden — which means a well-designed district-level study here can say something the international literature does not already know.

That work sits squarely in the scope of the International Journal of Insects (ISSN 3049-1649), a peer-reviewed journal launched in 2024 publishing across insect biology, ecology, behaviour, physiology, genetics, evolution and systematics. For life sciences, zoology and public health students, mosquito control is a good illustration of a general rule: when an intervention stops working, the interesting question is usually not what to spray next, but what the organism has been doing while nobody was measuring.

Frequently asked questions

Does fogging actually control mosquitoes?

Only briefly. Fogging kills adult mosquitoes exposed to it at that moment and has no effect on larvae, so the population is replaced within days. It has a genuine role in interrupting transmission during an active outbreak, but not as a routine measure.

Where does the dengue mosquito breed?

Aedes aegypti breeds in small containers of clean water in and around homes — stored water, plant saucers, bottles, tyres, cooler trays, fridge drip pans and construction sites — not in drains or dirty water.

Why do bednets not prevent dengue?

Because Aedes aegypti bites during the day, with peaks in the morning and late afternoon. Bednets protect against night-biting mosquitoes such as the malaria vector Anopheles, and do little against a daytime biter.

What is insecticide resistance in mosquitoes?

It is the result of selection: mosquitoes carrying traits that let them survive an insecticide breed and become the norm. The main mechanisms are mutations in the insecticide's target protein, over-production of detoxifying enzymes, thicker cuticles, and behavioural shifts such as biting earlier or outdoors.

Is it enough to empty a water container?

No. Aedes eggs are laid just above the waterline and survive drying for months, hatching when the container refills. The sides need scrubbing, not just emptying.

What is the Wolbachia method?

Wolbachia is a bacterium introduced into Aedes aegypti that reduces the mosquito's ability to transmit dengue and spreads through wild populations on its own. A randomised trial in Indonesia reported around a 77% reduction in dengue cases in treated areas.