Is there a risk to honeybees from use of thiamethoxam as a sugar beet seed treatment?
Authors: Thompson Helen , Vaughan Sarah , Mahlein Anne-Katrin , Ladewig Erwin , Kenter Christine 
Published: July 2021

The ban imposed by the European Union on the use of neonicotinoids as sugar beet seed treatments was based on the exposure of bees to residues of neonicotinoids in pollen and nectar of succeeding crops. To address this concern, residues of thiamethoxam (TMX) and clothianidin (CTD) were analyzed in soil collected from fields planted in at least the previous year with thiamethoxam-treated sugar beet seed. This soil monitoring program was conducted at 94 sites across Germany in two separate years. In addition, a succeeding crop study assessed residues in soil, guttation fluid, pollen, and nectar sampled from untreated succeeding crops planted in the season after thiamethoxam seed-treated sugar beet at eight field sites across five countries. The overall mean residues observed in soil monitoring were 8.0 ± 0.5 µg TMX + CTD/kg in the season after the use of treated sugar beet seed. Residue values decreased with increasing time interval between the latest thiamethoxam or clothianidin application before sugar beet drilling and with lower application frequency. Residues were detected in guttation fluid (2.0–37.7 µg TMX/L); however, the risk to pollinators from this route of exposure is likely to be low, based on the reported levels of consumption. Residues of thiamethoxam and clothianidin in pollen and nectar sampled from the succeeding crops were detected at or below the limit of quantification (0.5–1 µg a.i./kg) in 86.7% of pollen and 98.6% of nectar samples and, unlike guttation fluid residues, were not correlated with measured soil residues. Residues in pollen and nectar are lower than reported sublethal adverse effect concentrations in studies with honeybee and bumble bee individuals and colonies fed only thiamethoxam-treated sucrose, and are lower than those reported to result in no effects in honeybees, bumble bees, and solitary bees foraging on seed-treated crops. Integr Environ Assess Manag 2022;18:709–721. © 2021 SYNGENTA. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

KEY POINTS

Soil monitoring conducted at 94 sites across Germany in the season after sugar beet was grown from thiamethoxam-treated seed revealed overall mean residues of 8.0 ± 0.5 µg thiamethoxam + clothianidin/kg. Soil residue values decreased with increasing time interval between the latest thiamethoxam or clothianidin application before sugar beet drilling and with lower application frequency. Thiamethoxam and clothianidin residues in pollen and nectar sampled from untreated succeeding crops planted in the season after thiamethoxam seed-treated sugar beet at eight sites across five countries were at or below the quantification limit in 86.7% of pollen samples and 98.6% of nectar samples. Residues in pollen and nectar of succeeding crops were far lower than sublethal adverse effect concentrations in honeybees and non-Apis bees and, therefore, residues in soil resulting from use of thiamethoxam as a seed treatment on sugar beet are not likely to result in effects on pollinators.