Global warming could make insecticides less effective

Washington: Global warming may impair the effectiveness of an important insecticide against mosquitoes that can transmit dengue, chikungunya, yellow fever, and other diseases, new study has found.

Researchers from Montana State University in US showed that permethrin becomes less effective at killing the yellowfever mosquito (Aedes aegypti) as temperatures increase.

These mosquitoes, which are found in the tropics and the subtropics, can transmit viruses that lead to dengue, chikungunya, yellow fever, and other diseases.

"Many of the areas where these insecticides are employed have varying drastic temperature changes," said Shavonn Whiten, graduate student at Montana State University.

In their study, the researchers exposed adult mosquitoes to varying concentrations of permethrin at a range of temperatures.

They found an inverse relationship between death and temperature from 16 degrees Celsius to 30 degrees Celsius, which showed the highest negative correlation.

From 30 to 32, there was, however, a positive correlation between mortality and temperature. And from 32 to 34, the negative correlation resumed, the researchers said.

"It probably has something to do with variability and heat stress," said Robert Peterson, from Montana State University.

"Once you get to those higher temperatures, there are other things going on regarding stress on the mosquito that cancel out the effect of the pyrethroids (a class of pesticides to which permethrin belongs) working better at lower temperatures and worse at higher temperatures," Peterson said.

The researcher gave a number of possible reason for this effect. They said that lower temperatures may make the mosquito neurons more sensitive to permethrin, which is a neurotoxin.

The permethrin may persist longer and remain active at lower temperatures or lower temperatures may enhance the ability of the insecticide to bind to its target site, they said.

People involved in mosquito-control efforts should take temperature into account when choosing a pest-control product, according to Peterson.

"If we are applying at higher and higher ambient temperatures, we could have a reduction in control," he said.

"Therefore you need to pick something that's going to be efficient and not be a waste of time and money in controlling mosquitoes," he said.

The study was published in the Journal of Medical Entomology.