Effects of mild winter conditions and two pyrethroid insecticides on the development and survival of juvenile common toads (Bufo bufo)

Combined effects of pollutants and climate change can have long-term negative effects on Earth's biodiversity. We tested how amphibians are affected by exposure to pesticides around metamorphosis in combination with current winter conditions or those projected to become the norm by 2100. We applied two pyrethroid insecticides, cypermethrin or deltamethrin, to newly metamorphosed common toads (Bufo bufo) via spraying, thereafter reared animals in outdoor enclosures, and finally exposed them either to a long&cold winter (1.5 degrees C for 91 days) or to a short&mild one (4.5 degrees C for 61 days). After hibernation, we examined changes in body mass, size and gross morphology of parotoid glands and visceral organs, and determined the phenotypic sex of the surviving toadlets. In the long&cold winter scenario, we observed high mortality rate, regardless of the chemical treatment (control: 82%, cypermethrin: 78%, deltamethrin: 70%). At the same time, mortality was significantly lower in the short&mild winter scenario (control: 33%, cypermethrin: 41%, deltamethrin: 26%). The decrease in body mass during overwintering did not differ among treatment groups. Our results contradict the hypothesis of sex-biased mortality caused by chemical pollutants, harsh or mild winter conditions or by their combination. No significant deformations of the internal organs were found in either group. Our results suggest that winter conditions becoming ever milder in the temperate zone may prove beneficial for amphibian overwinter survival, and that exposure to the tested pyrethroids at ecologically relevant concentrations after metamorphosis does not interfere with the fitness of juvenile common toads. ### Competing Interest Statement The authors have declared no competing interest.