(CN) - Tying insect repellants to a decline in salamanders, a study out Wednesday reports that bug sprays are giving a counterintuitive boost to mosquito populations.
Salamanders are considered a natural mosquito predator because they forage on the bug’s larvae during their aquatic juvenile phase. While they help keep populations of these nuisance insects in check, mosquito-borne diseases like dengue, chikungunya, Zika, and West Nile virus have sent consumers looking for additional firepower.
Publishing their findings today in Biology Letters, researchers from the Cary Institute of Ecosystem Studies in Millbrook, New York, are now the first to suggest that environmentally realistic concentrations of picaridin-containing repellents in surface waters may increase the abundance of adult mosquitoes due to a decrease in survival of aquatic salamanders.
"Use of insect repellents is on the rise globally,” said study co-author Emma Rosi, a freshwater ecologist at the Cary Institute, in a statement.
The research team tested the effects of two of the most widely used insect repellents - DEET (Repel 100 Insect Repellent) and picaridin (Sawyer Premium Insect Repellent) - on larval salamanders and mosquitoes. In a lab, they exposed mosquito larvae and just-hatched spotted salamander larvae to three environmentally relevant concentrations of these chemicals, as well as a control treatment.
Describing the concentrations used in the experiments as conservative, Rosi noted that they were prepared “based on unadulterated commercial formulations, not concentrations of pure active compounds."
While the mosquito larvae survived the treatments with no effect, salamanders in all of the treatment groups began to display tail deformities and other signs of impaired development after four days of exposure to repellent with picaridin. By day 25, more than half of picaridin-exposed salamander larvae died.
"The expediency of salamander mortality was disconcerting,” said study co-author Barbara Han, a Cary Institute disease ecologist, in a statement. “When studying the effects of a chemical on an amphibian, we usually look for a suite of abnormalities. We couldn't collect these data because the salamanders died so quickly.”
LC50 tests are used to define a chemical's environmental toxicity. These standard tests are based on one life stage of a single species. LC50 measures how long it takes for half of a test population to die with increasing exposure to a chemical in a lab over a four-day period.
Co-author Alexander Reisinger, an assistant professor at University of Florida, Gainesville, expressed concern over relying on the LC50 test to determine the ultimate safety of a chemical.
"We observed heavy salamander mortality with picaridin, but not until after the fourth day of exposure,” Reisinger said in a statement. “By the LC50 measure, picaridin would be deemed 'safe', but clearly, this is not the case. If a substance doesn't kill organisms within the first few days of exposure, it can still be toxic and have ecological impacts."
Rosi added that, while picaridin is lethal in a controlled lab setting, it may cause greater mortality in a natural environment, where organisms are exposed to multiple stressors.
"Animals don't exist in isolation,” she said. “In nature, competition, predation, resource limitation, and social interactions make it difficult for an organism to tolerate the added stress of exposure to a harmful substance, even in small amounts.”
The timing of both repellent use and amphibian reproduction is also key. Many amphibians breed only once, while mosquitoes have an extended breeding season, and reproduce multiple times.
Rafael Almeida, a visiting doctoral student from Cornell University, served as lead author of the study. "The amount of repellents entering waterways peaks seasonally,” Almeida said in a statement. “If amphibians are exposed during a sensitive life stage, entire cohorts could perish.”
He added that more research on the effects of repellents containing DEET and picaridin is needed “to determine the extent to which these chemicals disrupt aquatic ecosystems and potentially increase mosquito-borne disease risk worldwide.”
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