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Mosquito repellents, no longer effective: official

Ramya Kannan

They have developed resistance to larvicides, insecticides

CHENNAI: What you have suspected all along is true: mosquitoes are indeed cocking a snook at coils and mats used to keep them away! Or simply, these repellents and vapourisers are no longer useful at keeping the mosquitoes away.

While scientific papers are yet to be published to establish the fact, officials at the Directorate of Public Health confirm that practically, in studies conducted by them, it has been clear that mosquitoes are not responding to repellents.

“We have reason to believe that mosquitoes have developed resistance to larvicides, pesticides and insecticides conventionally used to keep them out of home. The studies will be published soon,” S.Elango, Director of Public Health, told The Hindu.

While a contingency plan has been prepared to cut down mosquito breeding in view of the monsoon season, it would also be prudent for residents to undertake their own “physical precautions” when chemicals fail. “The main thing is to keep the home free from sources of breeding – ensure that there is no stagnant water in pots, air conditioner vents and buckets,” Dr.Elango added. He also recommends the use of mosquito nets and pyrethroid-treated nets available in the market, to keep the insects out of home.

For their part, the directorate will continue with regular source reduction activities, in addition to completing minor engineering works to prevent water stagnation.

The civic and municipal administration would continue to carry on with fogging and spraying operations.

“We have detected, through studies conducted in the State, that mosquitoes have also become immune to conventionally-used larvicides. We have communicated this with the National Vector Borne Diseases programme officials in Delhi,” V.Rajagopal, zonal entomologist, Vellore, said.

He also went on to explain the process by which resistance sets in: A mosquito has three pairs of chromosomes, one of which contains the resistance gene. It remains dormant when there is no exposure to pesticides, repellants and vaporisers. However, with repeated exposure to the chemicals the dormant gene is triggered and the mosquitoes are activated, Dr.Rajagopal said.

Chief Entomologist J.Madhavan said the common chemical used in insecticides, allethrin, along with the ash and charcoal combination that makes up coils and mats, have lost their effect.

“We have been using these chemicals and compounds for over one and a half decades. That is sufficient time for the insects to develop resistance,” he explained.

The repellents, which are meant only to repel them not even kill, are at best “mildly effective.” Internationally, researchers have suggested a combination of a non-pyrethroid insecticide combined with a repellent to beat resistant mosquitoes.

Tips

His tips for residents are: To close all windows and doors between 5 p.m. and 7-30 p.m. – This is the time for mating. It is only after mating that the female mosquito comes in search of blood to help the maturation of her ova or egg. The blood also partly forms a meal.

Use nets with 16 holes per inch – Mosquitoes cannot squeeze in through this.

Have the fan running at full speed – It will partly help repel mosquitoes.

Have the air-conditioning on – Mosquitoes will be almost inactive when the temperature goes below 20 degrees C.

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