Effects of four types of pesticides on survival, time and size to metamorphosis of two species of tadpoles (Rhinella and Physalaemus centralis) from the southern Amazon, Brazil

Pesticides have been implicated as one of the main factors responsible for amphibian population declines. Although Brazil is one of the countries that harbours the largest diversity of amphibians on the planet and is a leader in the use of pesticides, few studies have addressed the effects of these substances on amphibians in Brazil. We evaluated the effect of four herbicides (glyphosate, 2,4-D, picloram and a picloram and 2,4-D mixture) commonly used in the southern Amazon on tadpoles of Rhinella marina and Physalaemus centralis. To address the acute toxicity of each pesticide, we calculated LC50₉₆ values and compared them with values reported for several fish species provide by manufacturers, which are often used to infer toxicity of pesticides in Brazil. To address the chronic effects of each pesticide, we maintained tadpoles from Gosner stage 25 until stage 42 or metmorphosis and tested how fractions of LC50₉₆ (25%, 50%, and 75% of LC50₉₆) affected survival, time to metamorphosis and size of metamorphs of the tadpoles. Picloram and the mixture of picloram and 2,4-D showed the highest acute toxicity (LC50₉₆) among the pesticides tested, with a much higher value than those reported for fish. Survival was affected by different concentrations depending on the type of pesticide, without a standard for chronic toxicity. The time to metamorphosis was reduced only in P. centralis, with 2,4-D at 25 and 50% of the LC50₉₆ concentration. Therefore, with the other pesticides, the tadpoles were not able to accelerate their metamorphosis. The size of the metamorphs was increased or reduced depending on the concentration of the pesticide and the species, and in some cases, it was intermediate concentrations that had the greatest effect. These results indicate the need to reassess the current methods of estimating environmental risk because the effects on amphibian fauna are drastic and there is great expansion of agriculture areas in the Amazon.