Impact of herbicide application intensity in relation to environment and tillage on earthworm population in sugar beet in Germany

The application of herbicides in sugar beet (Beta vulgaris L.) is essential to prevent yield loss due to weed competition. According to German regulations, herbicides can be applied in mixtures with variable intensities. The ecological impact of the resulting strategies is still poorly understood. However, it was hypothesized that the influence of herbicide strategies on earthworm abundance, biomass, and diversity is minor compared to environment and tillage intensity in sugar beet. Therefore, additional specific factor variation seemed to be a prerequisite for getting valuable results. The herbicide strategies were applied in a ploughing system and a mulching system in 19 environments (site × year) in Germany in 2008 and 2009. Earthworm expulsions were carried out in spring and autumn with 2204 samples in total. The earthworm population was determined by environment and tillage system rather than by herbicide strategies. The environments displayed the largest variability in earthworm abundance, ranging from 12 to 195 individuals m−2, and a considerable variation in the occurrence of earthworm species. In spring, the deleterious impact of ploughing, with 80% lower mean earthworm abundance compared to the mulching system, was observed across all environments. During vegetation, the stronger increase in earthworm population in the ploughing system did not compensate for the initial differences. Regardless of intensity, the herbicide strategies were not accompanied by corresponding detrimental effects on earthworms between each other. In conclusion, the earthworm population was subjected to a multiplicity of influencing factors and the results markedly demonstrated for the first time the negligible effect of herbicide application intensity in sugar beet.