Effects were not restricted to a specific class of pesticides and seem to be influenced not only by the active substance but also the formulation additives. Both pyraclostrobin formulations contained the same amount of active chemical but differed in the content of the main formulation additive of solvent naphta (67% versus <25%). Mortality dropped from 100% in the high naphta product to 20% in the formulation with the lower solvent naphta content, indicating that other chemicals in the formulation may play a major role in the effect size of pesticides. Our test species, the European common frog Rana temporaria, is more sensitive to Headline compared to Bufo cognatus an American toad species that revealed a mortality of 70% after 72 h18 presumably due to skin properties that differ between toads and frogs.

Unlike in larval stages, where development is affected and deformities are observed14,15, environmentally relevant pesticide exposure of terrestrial life stages resulted in direct mortality in our study. The observation of acute mortality in a vertebrate group caused by commercially available pesticides at recommended field rates is astonishing since 50 years after the publication of Rachel Carson's Silent Spring19 one would have thought that the development of refined risk assessment procedures and our growing understanding of environmentally effective chemicals would make such effects virtually impossible. Differences in the formulation additives revealed a great influence on toxicity, indicating the need to expand the evaluation from active chemical ingredients to entire products. However, it will be difficult to understand the role of additives since in many formulated products the exact chemical composition is not declared and percentages of additive chemicals are only indicated as ‘proprietary ingredients’.

We studied a ‘realistic worst-case’ exposure scenario for single pesticides with direct overspray on a natural soil substrate. Although interception by crops may reduce exposure, repeated pesticide applications in a growing season might result in multiple contacts with a variety of products. Also, modeling approaches suggest that factors influencing juvenile and adult survival are most significant for the persistence of amphibian populations and a 40% annual mortality is expected to cause a substantial decline20.

The most toxic compound in our study, the top-selling fungicide Headline, is currently applied on 90 different crops from wheat in Canada to soy-beans in Argentina and its use may increase further due to proclamations of increase in crop yield and attractive discounts21. At present, several thousand pesticide products are registered globally and more than 2.3 million tons are applied on a major proportion of the land surface each year22. The demonstrated toxicity at recommended label rates of the few tested registered products resulting in mortality from 40% to 100% after seven days for juvenile European common frogs is alarming. Additionally, 40% mortality was observed for three registered products at 10% of the recommended label rate and therefore large-scale negative effects of terrestrial pesticide exposure on amphibian populations seem likely. Thirty-two of the 75 amphibian species occurring in Europe are associated with arable land according to the IUCN13 and for some species movements in this landscape coincide with pesticide applications13,16. Especially amphibians that migrate to aquatic spawning habitats reveal high population declines5,23 and pesticides might be a major threat for these species when crossing agricultural areas. Pesticide effects on terrestrial life stages of amphibians are so far not accounted for in amphibian conservation strategies where currently disease is discussed as a key factor24. We suggest that pesticides effects in cropped areas should be incorporated in landscape scale analyses for conservation management of amphibians.

Our results also indicate that existing risk assessment procedures for pesticide regulation are not protecting amphibians. Since amphibians are considered sentinel species for environmental and human health25, our results might even have implications for other taxa or entire ecosystems. It is therefore imperative to understand the underlying mechanisms of the toxicity of pesticides for amphibians to obtain a realistic estimate of the extent of their impact and to reconcile agricultural practice and amphibian conservation efforts.