Compilation and statistical analysis of pesticide residue levels in pollen and nectar: refined Residue Unit Doses (RUDs) for Tier 1 dietary bee risk assessment in North America

Current Tier 1 bee risk assessment in North America (US EPA, 2014) relies on an exposure estimation and risk assessment model called BeeREX. This model uses a Residue Unit Dose (RUD) approach to estimate residues in nectar and pollen. The RUD is the parameter expressing the residue concentration of a pesticide in pollen and in nectar for a standardized application rate of 1 kg/ha or 1 lb/A. For foliar spray applications, the current approach involves the use of the tall grass residue value from the T‐REX model (v.1.5) as a surrogate for pesticide concentrations in nectar and pollen. For soil treatments, the Tier I method involves the use of the Briggs’ soil‐plant uptake model, which is designed to estimate pesticide concentrations in plant shoots, and these are used as a surrogate for concentrations in pollen and nectar. For seed treatments, the Tier I exposure method is based on 1 mg a.i./kg concentration as an upper‐bound for pesticides in nectar and pollen. In comparison, the European Union (EU) Tier 1 risk assessment uses a database of nectar and pollen residue data (Kyriakopoulou et al., 2017). The US EPA has received in recent years residue studies from several applicants that can be used to adequately describe the distribution of pesticide residues that occur in pollen and nectar relative to application rate, method of application, and crop. By combining the US EPA and EFSA nectar and pollen databases a statistically refined estimation of RUD values can be calculated. The calculated nectar and pollen RUD values will then inform the BeeREX model with dietary exposure data relevant to the bee risk assessment.