By Harvey Black
The effectiveness of an important mosquito-fighting insecticide may be a casualty of global climate change, according to a recent study in the Journal of Medical Entomology. Two researchers from Montana State University, graduate student Shavonn Whiten and Dr. Robert Peterson, have shown that permethrin becomes less effective at killing the yellowfever mosquito (Aedes aegypti) as temperatures increase. These mosquitoes carry and transmit viruses that are responsible for a number of diseases, including dengue, chikungunya, and yellow fever. They are found in the tropics and subtropics, and they breed in objects that contain water.
The new research helps shed light on an aspect of insecticide resistance that has only been sparsely examined over the years, but needs to be studied, according to Whiten.
“Many of the areas where these insecticides are employed have varying drastic temperature changes,” she said.
The areas where insecticides are used to control mosquitoes have been experiencing increasing minimum, maximum, and late afternoon temperatures, the researchers note, citing research on climate change. That translates into the possibility of decreased mosquito control by insecticides, said Peterson, but this research is a step in grasping the phenomenon.
In their lab study, the researchers exposed adult mosquitoes to varying concentrations of permethrin at a range of temperatures. The temperatures ranged from 16-34 °C. They found an inverse relationship between death and temperature from 16 degrees to 30, 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 reason for the reversal between 30 and 32?
“It probably has something to do with variability and heat stress,” said Peterson. “Once you get to those higher temperatures, there’s 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. But we just don’t know at this point. I wish we wouldn’t have seen that, but that’s biology.”
Also uncertain is the precise mechanism for the overall temperature mortality relationship. The authors point to such possibilities as the lower temperatures making neurons more sensitive to excitation, since permethrin is a neurotoxin. Another possibility may be that at lower temperatures the chemical persists longer.
Whiten also points to a third hypothesis.
“I think the inverse relationship between temperature and mortality is due to the ability of the insecticide to bind to its target site,” she said. “Temperature can greatly affect the gating kinetics.”
However, confirming that will requires in-depth molecular study.
Some researchers have examined the relationship between temperature and insecticide effectiveness. Katey Glunt and her colleagues at Penn State report that Anopheles stephensi, a major vector of malaria in southeast Asia, becomes less susceptible to malathion as temperatures decrease. And a 1999 study by MH Hodjati and CF Curtis found a negative temperature-mortality relationship for Anopholes stephensi with permethrin between 16 and 22 °C and a positive one between 22 and 37 °C.
But such studies as yet are few and far between, and Glunt and her colleagues find that worrisome.
“Without information about their action at different temperatures, we may deploy chemicals that will be less efficient than we expect under actual field conditions,” they wrote in an article in PLOS Pathogens.
“If we are applying at higher and higher ambient temperatures, we could have a reduction in control,” Dr. Peterson 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.”
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Harvey Black is a freelance science writer. A long-time resident of Madison, Wisconsin, he has written for numerous publications including Environmental Health Perspectives, Scientific American Mind, New Scientist, The Scientist, and the Milwaukee Journal Sentinel.