Field application doses of fipronil and 2,4-D and toxic effects on non-target soil organisms: a microcosm-level approach

Many studies on risk assessment of pesticides to non-target soil organisms have been performed based on single-species laboratory tests using a reference soil according to standard test guidelines. Our study aimed to simulate realistic exposure scenarios for Brazilian sugarcane crop fields. Pesticides 2,4-dichlorophenoxyacetic acid (2,4-D) and fipronil were sprayed on natural soil at recommended doses (234.5 g/ha and 400 g/ha, respectively) and the experiments were maintained at 25 ± 2 °C with a 12:12-hr light:dark photoperiod for 21 days. Rain events were simulated twice a week (Days 1, 4, 8, 11, 15, and 18). For the ecotoxicity evaluation of these pesticides, isolated and in mixture, Japanese turnip (Raphanus sativus var. acanthioformis) and onion (Allium cepa) seeds were introduced into the microcosms, and soil samples were collected at the beginning and end of the experiment for tests with the springtail Folsomia candida. Results showed that both plant species were negatively affected by exposure to 2,4-D. However, R. sativus was more sensitive than A. cepa, showing greater reductions in biomass (75.3%) and growth (55.4%). Fipronil and its metabolites (fipronil-sulfide and fipronil-sulfone) were toxic to the springtail F. candida, reducing up to 40.12% in reproduction. There were no differences between the toxicity of the mixture and fipronil for springtails and between the mixture and 2,4-D for both plants. Our results suggest adverse effects of 2,4-D and fipronil on soil ecosystems even at concentrations derived from the recommended dosages for sugarcane crops.