Insect repellents containing picaridin can be lethal to salamanders. So reports a new study published in Biology Letters that investigated how exposure to two common insect repellents influenced the survival of aquatic salamander and mosquito larvae.

Insect repellents are a defense against mosquito bites and mosquito-borne diseases like dengue, chikungunya, Zika, and West Nile virus. Salamanders provide natural mosquito control. During their aquatic juvenile phase, they forage on mosquito larvae, keeping populations of these nuisance insects in check.

The paper is 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 predation pressure on mosquitoes at the larval stages.

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.

Mosquito larvae were not impacted by any of the treatments and matured unhindered. After four days of exposure to repellent with picaridin, salamanders in all of the treatment groups began to display signs of impaired development such as tail deformities. By day 25, 45-65% of picaridin-exposed salamander larvae died.

LC50 tests are used to define a chemical’s environmental toxicity. These standard tests, based on one life stage of a single species, measure how long it takes for 50% of a test population to die with increasing exposure to a chemical in a lab over a four-day period.

Lethal in a controlled setting, picaridin may cause greater mortality in a natural context, where organisms are exposed to numerous stressors.

Timing – of both repellent use and amphibian reproduction – is also key. Many amphibians breed in a single seasonal pulse, putting all their eggs in one basket, so to speak. Mosquitoes have an extended breeding season, and reproduce multiple times.

Lead author Rafael Almeida, a postdoctoral researcher at Cornell University, conducted the research as a visiting PhD student at the Cary Institute.

Future work is needed to explore the relationship among mosquito repellents, amphibians, and other ecological factors, and to better assess the severity of repellents’ impact in the wild.

Investigators included:

· Rafael Almeida – Cornell University
· Barbara Han – Cary Institute of Ecosystem Studies
· Alexander Reisinger – University of Florida, Gainesville
· Catherine Kagemann – Indiana University
· Emma Rosi – Cary Institute of Ecosystem Studies

Funding for this work was provided in part by the Bentley Holden Fund Award, Brazil’s Programa 213 Ciências Sem Fronteiras – Doutorado Sanduíche, and the National Science Foundation (Research Experiences for Undergraduates grant).

Photo of Salamander by John Clare, PhD courtesy Cary Institute.