Effects of the Fungicide Benomyl on Earthworms in Laboratory Tests Under Tropical and Temperate Conditions

Soil organisms play a crucial role in the terrestrial ecosystem. Plant protection products (PPPs) are known to affect soil organisms and might have negative impacts on soil functions influenced by these organisms. Little research has been performed to date on the impact of PPPs on tropical soil ecosystems. Therefore, in this study it was investigated whether the effects of the fungicide benomyl (chosen as a model substance) differ between tropical and temperate regions and whether data generated under temperate conditions can be used for the Environmental Risk Assessment (ERA) in tropical regions. The effect of benomyl on earthworms was evaluated in acute and chronic laboratory tests modified for tropical conditions. These tests were performed at two temperatures (20°C and 28°C) and with two strains (temperate and tropical) of the compost worm Eisenia fetida. The fungicide was spiked in two natural and two artificial soils. In addition to the organization for economic cooperation and development (OECD) artificial soil, a tropical artificial soil (TAS), containing a tropical fern product (xaxim) instead of peat, was developed in this study. The results from the laboratory tests and a literature review showed that the effects of benomyl were, on average, lower under tropical conditions (LC₅₀: 450–630 mg active ingredient (a.i.)/kg; EC₅₀: 0.8–12.9 mg a.i./kg) than under temperate conditions (LC₅0: 61–67 mg a.i./kg; EC₅₀: 1.0–1.6 mg a.i./kg) by a maximum factor of 10.3 (acute tests) and 12.9 (chronic tests). This result might be caused by an increased degradation of benomyl, and/or its first metabolite carbendazim, at higher temperatures, but a different sensitivity of the two worm strains cannot be ruled out. Despite the lower toxicity under tropical conditions and assuming comparable application rates, a preliminary assessment confirms the risk of benomyl to soil invertebrates under both conditions.