Environmental risk assessment of glyphosate and 2,4-D-based herbicides on earthworms in tropical agroecosystem

Herbicides are among the most widely used weed management methods worldwide, particularly glyphosate and 2,4-D, which rank among the top-selling active ingredients due to their broad-spectrum activity and cost-effectiveness. Glyphosate is the most extensively used herbicide globally, while 2,4-D remains one of the oldest and most frequently applied selective herbicides. Their recurrent use raises concerns about environmental risks, especially for non-target soil organisms such as earthworms, which are bioindicators of soil health. The objective of this study was to evaluate the risk posed by glyphosate and 2,4-D-based herbicides to earthworms in tropical agroecosystems, using Eudrilus eugeniae as a model species. Two experimental plots (15 × 15 m each) were treated with glyphosate or 2,4-D formulations. Residues in soil were quantified using high-performance liquid chromatography. Risk was characterized by the ratios between predicted environmental concentrations and measured soil concentrations, for both lethal and reproductive endpoints. Results showed that 2,4-D residues exceeded the predicted effect concentration for reproduction (PECrep) over the 35-days monitoring period, suggesting a risk of reduced cocoon production and hatching success. In contrast, glyphosate residues remained below PECrep, indicating a low risk for earthworm reproduction. Both herbicides remained well below acute lethal thresholds (LC50), but glyphosate maintained PEC50/CS ratios above 1 throughout the experiment. Overall, the findings indicate that while glyphosate residues did not pose a significant risk to earthworm reproduction, 2,4-D-based herbicides present a potential ecological risk. These results highlight the need to carefully consider herbicide type in integrated weed management strategies and to complement regulatory risk assessments with field-based evaluations.