Quantitative weight of evidence assessment of higher tier studies on the toxicity and risks of neonicotinoids in honeybees. 4. Thiamethoxam

ABSTRACT

A quantitative weight of evidence (QWoE) methodology was used to assess several higher-tier studies on the effects of thiamethoxam (TMX) on honeybees. Assessment endpoints were population size and viability of commercially managed honeybee colonies and quantity of hive products. A higher-tier field toxicology study indicated a no-observed-adverse effect concentration (NOAEC) of 29.5 µg TMX/kg syrup, equivalent to an oral no-observed-adverse-effect-dose (NOAED) of 8.6 ng/bee/day for all responses measured. For exposures via deposition of dust, a conservative no-observed-adverse-effect-rate at the level of the colony was 0.1 g TMX/ha. There was minimal risk to honeybees from exposure to TMX via nectar and pollen from its use as a seed-treatment. For exposures via dust and dust/seed applications, there were no concentrations above the risk values for TMX in nectar and pollen. Although some risks were identified for potential exposures via guttation fluid, this route of exposure is incomplete; no apparent adverse effects were observed in field studies. For exposures via dust/seed and dust/foliar applications, few adverse effects were observed. Considering all lines of evidence, the quality of the studies included in this analysis was variable. However, the results of the studies were consistent and point to the same conclusion. The overall weight of evidence based on many studies indicates that TMX has no adverse effects on viability or survival of the colony. Thus, the overall conclusion is that the treatment of seeds with thiamethoxam, as currently used in good agricultural practices, does not present a significant risk to honeybees at the level of the colony.

Acknowledgments

The authors wish to thank Ms. Jennifer Miller (Miller Environmental Science Inc.) for QA services and Mr. Spencer West for assistance with literature searches and obtaining the references and associated SI. The authors wish to thank Syngenta, LLC for providing access to the unpublished reports and for funding the study. The decision to publish this assessment was the authors’, and they were solely responsible for the content and the opinions herein. We are indebted to Dr EJ Marshall and Prof Keith Walters for editing these papers and to the reviewers for the helpful and constructive comments.

Declaration of interest

No potential conflicts of interest were reported by the authors.

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Notes

¹ The NOAEC and LOAEC were derived from the characterization of the data in the report of the study (Syngenta 2015) in terms of the scoring guide for effects (SI Table 2). Our NOAEC and LOAEC were less than those derived by the authors of the report (measured concentrations of 39.7 and 73.8 µg TMX/kg, respectively) and provide a measure of conservatism in this assessment. However, the difference in NOAEC had almost no influence on the outcome of the assessment of risk.