India’s donation of 500 litres of technical grade malathion comes at an important moment in Sri Lanka’s fight against dengue. Health authorities have acknowledged that the insecticide currently being used in fogging operations is proving less effective in some parts of the country, prompting the Ministry of Health to seek an alternative chemical for testing.

We must acknowledge that the donation deserves appreciation. It reflects the close cooperation between Sri Lanka and India and provides public health officials with another weapon in a year that has already seen more than 82,000 dengue infections and over 60 deaths.

But the arrival of malathion should also force Sri Lanka to confront a difficult question. Have we placed too much faith in the thick white clouds of mosquito fogging?

For decades, fogging has been the most visible symbol of dengue control. When cases rise, residents demand that the mosquito fogging machines arrive in their neighbourhoods. 

Unfortunately, fogging was never intended to be the primary weapon against dengue. It is designed as an emergency measure to rapidly reduce the number of adult mosquitoes in a limited area during an outbreak. It is not a long-term mosquito control strategy, nor does it eliminate the source of the problem.

The Aedes aegypti mosquito spends most of its life cycle as eggs, larvae and pupae in stagnant water. Fogging does not kill these immature stages. Eggs attached to the sides of water containers can survive for months until rainwater allows them to hatch. Within days of spraying, new mosquitoes can emerge if breeding sites remain untouched.

Even its effectiveness against adult mosquitoes has limits.

Also, Aedes mosquitoes are daytime biters that often rest inside homes, behind curtains, beneath furniture and in other sheltered locations where fogging smoke may never reach. Outdoor spraying can therefore miss a significant proportion of the mosquito population. Timing also matters. Spraying at the wrong time of day can substantially reduce its impact.

Now another challenge has emerged. Resistance.

Around the world, mosquito populations have gradually developed resistance to insecticides that have been used repeatedly over many years. We appear to be experiencing the same phenomenon, with public health inspectors reporting that the chemical currently used for fogging is no longer producing satisfactory results in some areas.

That makes the decision to test malathion entirely sensible.

Malathion is hardly a new discovery. It has been used for decades in mosquito control programmes worldwide and played an important role in Sri Lanka’s successful malaria eradication efforts. Whether it proves effective against today’s dengue vectors remains to be seen. The National Dengue Control Unit is right to evaluate its performance before deploying it on a larger scale.

However, replacing one insecticide with another cannot become the country’s long-term strategy.

On the other hand, history suggests that every insecticide eventually loses effectiveness if it is used indiscriminately. Mosquitoes adapt remarkably quickly. Without careful resistance monitoring and rotation of different classes of insecticides, today’s solution can easily become tomorrow’s problem.

Therefore, eliminating these breeding sites remains vastly more effective than attempting to kill adult mosquitoes after they have already emerged. This is where we continue to fall short.

Public health inspectors conduct inspections, awareness campaigns are held and clean-up programmes are organised. Yet the same breeding sites are discovered repeatedly. Many households still assume dengue prevention is solely the Government’s responsibility, while some local authorities continue to struggle with waste management and drainage maintenance.

The result is a predictable cycle. Cases increase. Fogging intensifies. Numbers decline temporarily. Then the rains return, breeding sites reappear and another outbreak begins. Breaking this cycle requires a shift in thinking.

While fogging should be viewed as a targeted public health intervention, it should be regarded as a cure. Scientific surveillance should determine which insecticides remain effective and where they should be used. More importantly, equal emphasis must be placed on environmental management, community participation and sustained source reduction throughout the year, not just during outbreaks.

India’s donation of malathion is therefore both timely and valuable. It provides Sri Lanka with an opportunity to strengthen its mosquito control programme using evidence rather than assumption.

But if the country continues to believe that clouds of insecticide smoke alone can defeat dengue, it will be fighting yesterday’s battle with tomorrow’s mosquitoes.

Therefore, the most effective weapon against dengue has never been inside a fogging machine. It has always been in our own backyards.