Neonicotinoids are designed to target insect pests, but their extensive use and long persistence in soils mean that non-target soil organisms such as earthworms are likely to be chronically exposed to them. The common use of neonicotinoids in agricultural systems makes them highly likely to come into contact with other agro-chemicals that may give rise to synergistic effects. Chronic exposure of common earthworm (Lumbricus terrestris) to neonicotinoids in single or combined use with inorganic NPK fertiliser may pose problems that are not accounted for in most biological risk assessments under laboratory conditions. To assess the impacts of such chronic exposure on earthworms, different concentrations of a neonicotinoid, in single or combined use with granulated NPK fertiliser were applied under controlled field conditions in three separate experiments, to mesocosms each containing 10 worms. Response variables measured were nocturnal activity and copulations, survival, change in mass and cocoon production. The neonicotinoid used was Thiacloprid, in the domestic formula most readily available to the public from 2017 to 2019 (‘Provada Ultimate Bug Killer Concentrate 2’ manufactured by Bayer (BPUBKC2)). Soils used included either standardised test soil ‘LUFA2.2’ or ‘Levington’s Organic Blend Topsoil’ (OBT). Two experiments used LUFA2.2 and one used OBT. The former was used to investigate whether LUFA2.2 is a suitable test soil for adaptation of the OECD earthworm reproduction test (ERT) for controlled field conditions. Each experiment applied different concentrations of Thiacloprid in single or combined use with NPK to assess effects of the chemicals in isolation and if mixtures caused synergistic effects. Applications of Thiacloprid in single and combined use with NPK can cause high levels of mortality to L. terrestris at a range of concentrations. All Thiacloprid treatments caused 100 % mortality to L. terrestris when accommodated in LUFA2.2. As such, it was not conclusive that NPK gave rise to synergistic effects on mortality. Mortality in OBT was not significantly affected by exposure to combined treatments. However, NPK alone can have great effect on mortality of L. terrestris. Survival was significantly reduced by the presence of NPK in LUFA2.2 but not in OBT.