Rising temperature increases methomyl (Lannate ^® BR) toxicity in Amazonian frog tadpoles: Impacts on survival and thermal limits

The increased use of pesticides in tropical regions, combined with rapid warming due to climate change, poses a critical threat to amphibian populations. However, the interactive effects of these stressors on tropical amphibians remain poorly understood. In this study, we assessed lethal and sublethal responses to methomyl in two Amazonian anurans, <i>Osteocephalus taurinus</i> and <i>Scinax ruber.</i> Tadpoles were exposed to methomyl at two temperatures (26.5 °C and 30 °C), simulating current and projected climate scenarios. Acute toxicity was quantified through LC₅₀, and thermal tolerance was assessed <i>via</i> critical thermal maximum (CT_max). Warming significantly increased methomyl toxicity in <i>O. taurinus</i>, lowering the LC₅₀ from 96.4 mg/L to 45.9 mg/L at 30 °C. S. ruber showed no such effect, with LC₅₀ values of 15.5 mg/L (26.5 °C) and 19.7 mg/L (30 °C). <i>S. ruber</i> was approximately six times more sensitive to methomyl than <i>O. taurinus</i> across temperatures. Methomyl exposure did not alter CT_max in either species, although variability increased in <i>O. taurinus</i> at higher concentrations. These findings provide novel evidence of temperature-modulated methomyl toxicity in Amazonian frog tadpoles, highlighting the need to incorporate native species, sublethal endpoints, and climate-relevant stressors in ecotoxicological risk assessments.