Mosquito repellents in Chattogram are less effective now: study

Repellents being used in Chattogram have lost effectiveness to kill mosquito significantly, a study has found.

A technical committee formed by Chattogram University at the bidding of Chattogram City Corporation submitted the report on Tuesday.

The authorities decided to conduct the study six months ago after suspicion had aroused over the effectiveness of the repellents.

The findings came as concerns over mosquito-borne dengue fever are growing amid the coronavirus pandemic.

Similar results had been obtained in studies in Dhaka North and South City Corporations two years ago as researchers pointed out mosquitos were becoming resilient to the repellents. The authorities later changed the mosquito repellents for the capital.

The researchers in Chattogram used samples of four adulticides and a larvicide from the city corporation in the study on mosquitos and larvae grown in laboratories from samples collected from the port city.

One of the adulticides was light diesel oil or black oil being used by the city corporation. The two others were chemicals for experimental use.

The fourth one, a herbal repellent, was trialled as both adulticide and larvicide.

The larvicide spray in use was found only 16 percent effective to kill mosquitos. The effectiveness rose to 84 percent when the level of repellent was raised 10 times.

The new herbal sample was able to produce 100 percent result in two hours as both adulticide and larvicide.

It was made of extracts from various plants, said Dr Rabiul Hasan Bhuiyan, an associate professor at the university’s biochemistry and molecular biology department who led the study.

Besides being less effective, the chemical repellents are harmful for other insects and the environment, he said.

The study found the adulticide spray being used by the city corporation to be 34 percent effective. Raising its density by 10 times increases the effectiveness to 76 percent.

Using only kerosene spray was found 84 percent effective in killing larvae, but it takes two hours for the repellent to start working.   

Using a fogger, however, was only 19 percent effective when both herbal and chemical adlticides were applied. Raising the amount did not yield expected results as well.  

The researchers made some recommendations, including efforts to keep water running in sewage lines, regular study on effectiveness of repellents, and use of herbal repellents instead of chemicals.

They recommended techniques like the one to kill mosquitos by farming fish that eat larvae, said Dr Mohammad Omar Faruque, an associate professor at the botany department.

“Mosquitos cannot stand the smell of lemon, mint, basil, neem and bay leaves. So, we’ve recommended planting these trees on road dividers,” he said.