Wild populations of the Asian tiger mosquito in Darjeeling district have acquired resistance to popular household insecticides and could raise the risk of dengue outbreaks, scientists at the University of North Bengal have said.

The researchers have documented — for the first time in wild mosquitoes in north Bengal — the presence of a genetic mutation known to help the Aedes albopictus mosquito develop resistance to an insecticide called deltamethrin.

“The presence of this mutation suggests that mosquitoes are exploiting multiple pathways to resist common insecticides,” said Dhiraj Saha, professor of zoology at the insect biochemistry lab at the University of North Bengal, Siliguri, who led the study.

The Aedes mosquito can transmit multiple viral infections, including dengue, chikungunya and Zika, and health experts believe that the temperature and relative humidity in northern Bengal have been conducive for the growth of mosquito populations.

Saha and research scholar Manas Pratim Modak collected 200 mosquitoes, 50 from various sites in each of four towns. They were collected from discarded containers and flowerpots in Bagdogra, discarded tyres in Matigara, bamboo stumps and plastic tanks in Naxalbari, and cement tanks and discarded tyres in Sukna.

The mosquitoes sampled from Matigara were found to possess the so-called knock-down resistance (kdr) mutation, F1534C, that induces genetic changes in the insect allowing it to evade deltamethrin, an insecticide that belongs to a class of compounds called synthetic pyrethroids.

Saha and Modak have published their findings in Scientific Reports, a peer-reviewed research journal.

Several anti-mosquito products for use in households contain synthetic pyrethroids.

Since 2011, scientists have detected similar kdr mutations in mosquito specimens in Singapore, China, France, Italy, Japan and the US. The National Institute of Malaria Research, New Delhi, had identified kdr mutations in mosquitoes in Bangalore in 2020.

In an earlier study four years ago, Saha and his colleagues had shown that Aedes populations in north Bengal had acquired resistance to insecticides through another pathway – by increasing their levels of detoxifying enzymes.

“The kdr mutation is an additional route of resistance, more powerful than insecticide-detoxifying enzymes,” Saha said. “Future strategies to control mosquito populations should focus not just on adult-stage mosquitoes but also on the larval stages, which are easier targets than adult mosquitoes.”

The scientists believe that the emerging resistance is the consequence of the application of large amounts of insecticides and the repeated use of the same insecticide over many years. They have called for “alternative” mosquito control tactics.