Size matters: Significant negative relationship between mature plant mass and residual neonicotinoid levels in seed-treated oilseed rape and maize crops

•Larger crop plants have lower neonicotinoid concentrations.•Concentrations roughly halved with a four-fold increase in plant weight.•Growing larger crop plants has the potential to reduce the exposure of bees to neonicotinoids. Neonicotinoid insecticides have been under scrutiny in recent years due to their potential to harm bees. The European Union recently imposed a two year moratorium (2014–2015) on their application as a seed-treatment for certain bee-attractive crops. In this study we investigated the effect of mature plant size on residual neonicotinoid concentration in two widely grown, bee-attractive crops: oilseed rape (Brassica napus) and maize (Zea mays). Plants were collected from four commercial farms in Sussex, United Kingdom, three growing oilseed rape and one maize. All were grown from seeds treated with the neonicotinoid thiamethoxam. For both crops there was a significant negative relationship between mature plant mass and residual neonicotinoid (thiamethoxam and its metabolite clothianidin) concentrations (p