5 of 6 insecticides used to kill Aedes mosquitoes no longer effective

Most of the widely used insecticides for killing Aedes mosquitoes are no longer effective. The dengue-carrying Aedes mosquito has developed resistance to these chemicals.

Researchers tested six insecticides from three chemical classes in Dhaka and Chattogram and found evidence of resistance to five of them. Even after increasing the concentration of some insecticides tenfold, up to 84 per cent of Aedes mosquitoes survived.

These findings emerged from a study led by Professor Kabirul Bashar of the Department of Zoology at Jahangirnagar University, conducted in collaboration with researchers from Japan’s Institute of Health Security. The researchers also identified two genetic mutations associated with insecticide resistance in the mosquitoes.

Malathion, an organophosphate insecticide, was found to be effective. Within 24 hours of application, 98 per cent of the mosquitoes collected from the field in both Dhaka and Chattogram died.

The research began in 2023 and is still ongoing. It was presented earlier this month at the Asia Pacific Mosquito and Vector Control Conference in Bangkok, Thailand.

In this context, the country has been observing the World Mosquito Day today, 20 August.

Touhid Uddin Ahmed, former chief scientific officer of the government’s Institute of Epidemiology, Disease Control and Research (IEDCR) and a noted entomologist, said the development of resistance to insecticides among Aedes mosquitoes could have serious consequences.

There is now a risk that even newly introduced insecticides could become ineffective. In science, this is known as “cross-resistance.” The situation could increase the risk of severe dengue infections.

Five of the six are no longer effective

Researchers collected 1,450 samples of Aedes mosquito larvae and adult mosquitoes from various locations in Dhaka and Chattogram. In addition to collecting larvae directly, they used different types of traps. The species were then identified in the laboratory and their sensitivity to insecticides was tested.

The results for two types of mosquitoes—the first generation collected from the field and the subsequent generation bred from them in the laboratory—were presented separately. Mortality rates were calculated one hour and 24 hours after the insecticides were applied.

The study tested six insecticides from three classes: pyrethroids, carbamates and organophosphates. Resistance was found in Aedes aegypti to all four pyrethroid insecticides tested: deltamethrin, alpha-cypermethrin, permethrin and etofenprox. The mosquitoes are not dying from these insecticides.

Bendiocarb, a carbamate insecticide, showed the same pattern. The mosquitoes remain sensitive only to malathion, an organophosphate insecticide.

The study concluded that Aedes aegypti mosquitoes in Bangladesh have developed resistance to pyrethroids and bendiocarb. In contrast, malathion remains effective.

How resistant are the mosquitoes to each insecticide?

The most concerning findings came from tests involving permethrin. Twenty-four hours after applying permethrin at a concentration of 0.75 per cent, only 10 per cent of the mosquitoes in Dhaka had died. In other words, 90 per cent survived. The mortality rate was even lower in Chattogram, at 7 per cent.

Even after increasing the concentration of the chemical tenfold to 7.50 per cent, there was little change. In Dhaka, 79 per cent of the mosquitoes survived, while in Chattogram the survival rate was 84 per cent.

Etofenprox produced almost the same results. At a concentration of 0.50 per cent, 93 per cent of the mosquitoes in Dhaka and 90 per cent in Chattogram were still alive 24 hours later. At a concentration of 0.05 per cent deltamethrin, 83 per cent of the mosquitoes in Dhaka and 84 per cent in Chattogram survived.

However, mortality increased as the concentration of the chemical increased. With alpha-cypermethrin at a concentration of 0.05 per cent, 68 per cent of the mosquitoes in Dhaka and 83 per cent in Chattogram survived. Even at 0.50 per cent, 57 per cent in Dhaka and 34 per cent in Chattogram survived.

At a concentration of 0.10 per cent bendiocarb, 44 per cent of the mosquitoes in Dhaka and 35 per cent in Chattogram were still alive after 24 hours.

Of the six insecticides tested, malathion produced the best results. Twenty-four hours after applying malathion at a concentration of 5 per cent, 98 per cent of the field-collected mosquitoes in both Dhaka and Chattogram had died.

In tests on the next generation of mosquitoes bred in the laboratory from field-collected mosquitoes, the mortality rate was 99 per cent. However, researchers believe it is not safe to rely entirely on malathion simply because one chemical remains effective. If the same chemical is used continuously and indiscriminately, resistance to it could eventually develop as well.

Public health expert Mushtuq Husain considers the study highly significant. He said chemicals alone would no longer be enough to control mosquitoes. Biological methods are also needed.

More importantly, surveillance is needed to monitor mosquitoes, patients and the development of insecticide resistance. Such surveillance is currently absent. Having this information would make dengue prevention considerably easier.