Plants and millipedes as terrestrial bioindicators of 2,4-D contamination: phyto-cytogenotoxicity in Allium cepa and Zea mays and feeding activity in Rhinocricus padbergi

Although 2,4-D is one of the most widely used herbicides worldwide, herbicidal soil-mediated toxicity to higher plants and detritivores remains underexplored. This study aimed to determine the phyto-cytogenotoxic potential of soil artificially treated with the commercial herbicide U 46 BR (active ingredient: 2,4-D) using Allium cepa and Zea mays. Further, the feeding activity of Rhinocricus padbergi exposed to leaf litter treated with 2,4-D was measured. Concentrations spanning environmentally realistic to critical contamination levels (0 to 3.68 mg/kg) were tested. Root growth of A. cepa was significantly inhibited at 0.92, 1.84, or 3.68 mg/kg, while Z. mays was affected only at 3.68 mg/kg, indicating higher A. cepa sensitivity. Across all concentrations, A. cepa exhibited cytotoxic (reduced mitotic index) and/or genotoxic (increased chromosomal aberrations) responses. In R. padbergi, exposure to 2,4-D reduced assimilation and assimilation efficiency and increased fecal egestion. Considering soil physicochemical traits (clay texture, pH 6.9, moderate organic matter), our findings indicate the role of bioavailability in modulating toxicity. Collectively, the effects observed in primary producers and key detritivores demonstrate that environmentally relevant 2,4-D levels in soil might impair plant performance and feeding activity of soil invertebrates.