Interactive effect of glyphosate-based herbicides and organic soil layer thickness on growth and reproduction of the tropical earthworm Pontoscolex corethrurus (Müller, 1857)

Abstract Glyphosate-based herbicides (GBHs) have negative effects on the fitness parameters of earthworms; however, these effects may vary from one commercial product to another and between different recommended doses. Additionally, frequent applications of GBHs may decrease the thickness of the topsoil layer due to increased soil erosion through vegetation reduction. To investigate the effects of frequent applications of GBHs, and their interactive effect on the performance of P. corethrurus, we performed a two-factor experiment exposing in vitro the earthworm to two commercial GBHs (factor I, FAENA FUERTE (FF, Monsanto Co.) and GLYFOS (GLY, Cheminova Co., with three levels)) and soil thickness (factor II, with two levels); having six treatments in total, each with four replicates. Both herbicides were applied at their recommended doses; a total of 11 times at intervals of 14 days in soil layers 2.5 and 5 cm deep. Earthworm biomass and cocoon production were measured every 12 days for 168 days. Application of GLY resulted in significant biomass decreases of between 51 ± 11.54% (here and further S.E. is provided) and 100 ± 0.00% compared to a control. For FF, the final biomass decreased significantly by between 19 ± 7.33% and 24 ± 8.44% at both doses, independently of soil thickness. Additionally, a significant biomass reduction of 41 ± 7.93% was detected when soil thickness was low at highest dose. At a soil depth of 5 cm, GLY significantly decreased cocoon production by between 69 ± 7.86% and 83 ± 10.53% at each dose compared to the control. In contrast, the different doses of FF did not affect the number of cocoons produced, although there was a significant decrease of 72 ± 7.18% when the soil layer was reduced. The growth rate increased significantly under exposure to both GBH at all tested doses. Our results suggest that cumulative doses of GLY are more detrimental for P. corethrurus than those of FF and that soil thickness may help buffer some of the negative effects of GBHs. In order to reduce negative impacts on earthworms in the field, we therefore suggest maintaining the topsoil layer thickness and adjusting the doses of GBHs to the lowest end of the recommended range.