Delhi civic bodies switch to new insecticides to tackle mosquito breeding

Civic agencies in the national Capital have for the first time in a decade switched to new types of insecticides to tackle mosquito breeding, officials said, explaining that the old ones may have become ineffective because they have been in use for so long.

The decision comes amid rising concerns that this summer — which began sooner than usual and is likely to be warmer than normal — may see a spurt in mosquito breeding, which could trigger more serious spread of dengue and chikungunya. Last year, around 5,000 people were infected by dengue in Delhi, which can turn fatal in extreme cases.

“The continued use of the old chemicals every year ostensibly resulted in widespread resistance in mosquitoes, prompting us to look for alternatives,” said Neena Valecha, director of Indian Council of Medical Research’s Institute of Malaria Research.

The new insecticides — pyri-proxifen and diflybenzuron — are colourless, odourless powders that belong to a class of chemicals known as insect growth regulators (IGRs). They work by stopping a larva from maturing into a pupa, and the New Delhi Municipal Corporation and the South Delhi Municipal Corporation have begun using them in recent months in certain areas.

The results were encouraging. One of the prominent features of IGR is that it spreads from one place to other with the help of mosquitoes. When mosquitoes sit on water that has absorbed the insecticide, they carry it to other places,” said BK Hazarika, medical health officer of South Delhi Municipal Corporation.

But the colourless and odourless nature of the new insecticides has made people sceptical of its use -- specifically whether it has been sprayed at all, according to some field officers.

The new chemicals also need to be used in much lesser quantity —0.1gm for 50litre of water — compared to the conventional chemicals that were mixed in a ratio of 5gm in 20 litres.

Valecha said the IGRs are highly toxic to insects, larvae or pupae but not harmful to humans and aquatic organism. “They are hormonal retardant and hamper the growth of mosquito into adults and ultimately kills them after 3-4 days,” she said. An SDMC official said the switch will come with some challenges. “The quantity to be added is so less (0.10 gm) that field officers get confused and end up adding more than prescribed dose,” he said. “For their convenience, spoons (marked with quantities) are provided but chances of error will remain high for first few weeks .Also though there is no study about the side effects of IGR, we suspect that its use may affect the flora and fauna of water bodies.”

But Valecha dismissed such fears and said that slight excesses would not affect humans or life at water bodies.

The second challenge could be of optics. “The previous insecticide (BTI) was grey in colour and had an odour so people believed us that something had been done to control mosquito breeding. But now, even if we spray the solution, people doubt us,” said an NDMC spokesperson.

Also, the IGR retards the growth of larvae and takes at least 3-4 days to kill mosquitoes. “Our field workers have already received complaints from residents about the solution being ineffective in killing the mosquitoes, and then we need to explain them the procedure in detail,” said NDMC spokesperson.

The civic agencies have also started training field officers to ensure that the new medicines are not overused or misused.