Thiamethoxam: Long-term effects following honey bee colony-level exposure and implications for risk assessment.

Neonicotinoid insecticides have been used in a wide range of crops through seed treatment, soil and foliar applications and a large database exists on both their lethal and sub-lethal effects on honey bees under controlled laboratory conditions. However, colony-level studies on the effects of neonicotinoids in field studies are limited, primarily due to their complexity and the resources required. This paper reports the combined results of two large-scale colony-feeding studies, each with 6 weeks of continuous dosing of 12 colonies per treatment (24 control) to 12.5, 25, 37.5, 50 or 100 ng thiamethoxam/g sucrose solution. Exposure continued beyond dosing with residues present in stored nectar and bee-bread. The studies were conducted in an area with limited alternative forage and colonies were required to forage for pollen and additional nectar The studies provide colony-level endpoints: significant effects (reductions in bees, brood) were observed after exposure to the two highest dose rates, colony loss occurred at the highest dose rate, but colonies were able to recover (2-3 brood cycles after the end of dosing) after dosing with 50 ng thiamethoxam/g sucrose. No significant colony-level effects were observed at lower dose rates. The data reported here support the conclusions of previous colony-level crop-based field studies with thiamethoxam, in which residues in pollen and nectar were an order of magnitude below the colony-level NOEC of 37.5 ng thiamethoxam/g sucrose. The feeding study data are also compared to the outcomes of regulatory Tier 1 risk assessments conducted using guidance provided by the USA, Canada, Brazil and the EU regulatory authorities. We propose an adaptation of the European chronic adult bee risk assessment that takes into account the full dataset generated in laboratory studies while still providing an order of magnitude of safety compared with the colony feeding study NOEC.

Thiamethoxam honey bee colony feeding study: Linking effects at the level of the individual to those at the colony level.

Neonicotinoid insecticides have been used globally on a wide range of crops through seed treatment as well as soil and foliar applications and have been increasingly studied in relation to the potential risk to bees because of their detection in pollen and nectar of bee-attractive crops. The present article reports the results of laboratory studies (10-d adult and 22-d larval toxicity studies assessing the chronic toxicity of thiamethoxam to adult honey bees and larvae, respectively) and a colony feeding study, with 6 wk of exposure in an area with limited alternative forage, to provide a prewintering colony-level endpoint. The endpoints following exposure of individuals in the laboratory (10-d adult chronic no-observed-effect concentration [NOEC] for mortality 117 µg thiamethoxam/kg sucrose solution, 141 µg thiamethoxam/L sucrose solution; 22-d larval chronic NOEC 102 µg thiamethoxam/kg diet) are compared with those generated at the colony level, which incorporates sublethal effects (no-observed-adverse-effect concentration [NOAEC] 50 µg thiamethoxam/L sucrose solution, 43 µg thiamethoxam/kg sucrose solution). The data for sucrose-fed honey bee colonies support the lack of effects identified in previous colony-level field studies with thiamethoxam. However, unlike these field studies demonstrating no effects, colony feeding study data also provide a threshold level of exposure likely to result in adverse effects on the colony in the absence of alternative forage, and a basis by which to evaluate the potential risk of thiamethoxam residues detected in pollen, nectar, or water following treatment of bee-attractive crops. Environ Toxicol Chem 2018;37:816-828. © 2017 SETAC.

Effects of exposure to winter oilseed rape grown from thiamethoxam-treated seed on the red mason bee Osmia bicornis.

There has been increasing interest in the effects of neonicotinoid insecticides on wild bees. In solitary bee species the direct link between each individual female and reproductive success offers the opportunity to evaluate effects on individuals. The present study investigated effects of exposure to winter oilseed rape grown from thiamethoxam-treated seed on reproductive behavior and output of solitary red mason bees (Osmia bicornis) released in 6 pairs of fields over a 2-yr period and confined to tunnels in a single year. After adjustment to the number of females released, there was significantly lower production of cells and cocoons/female in tunnels than in open field conditions. This difference may be because of the lack of alternative forage within the tunnels. Under open field conditions, palynology of the pollen provisions within the nests demonstrated a maximum average of 31% oilseed rape pollen at any site, with Quercus (oak) contributing up to 86% of the pollen. There were no significant effects from exposure to oilseed rape grown from thiamethoxam-treated seed from nest establishment through cell production to emergence under tunnel or field conditions. Environ Toxicol Chem 2018;37:1071-1083. © 2017 SETAC.

A four-year field program investigating long-term effects of repeated exposure of honey bee colonies to flowering crops treated with thiamethoxam.

Neonicotinoid residues in nectar and pollen from crop plants have been implicated as one of the potential factors causing the declines of honey bee populations. Median residues of thiamethoxam in pollen collected from honey bees after foraging on flowering seed treated maize were found to be between 1 and 7 µg/kg, median residues of the metabolite CGA322704 (clothianidin) in the pollen were between 1 and 4 µg/kg. In oilseed rape, median residues of thiamethoxam found in pollen collected from bees were between <1 and 3.5 µg/kg and in nectar from foraging bees were between 0.65 and 2.4 µg/kg. Median residues of CGA322704 in pollen and nectar in the oilseed rape trials were all below the limit of quantification (1 µg/kg). Residues in the hive were even lower in both the maize and oilseed rape trials, being at or below the level of detection of 1 µg/kg for bee bread in the hive and at or below the level of detection of 0.5 µg/kg for hive nectar, honey and royal jelly samples. The long-term risk to honey bee colonies in the field was also investigated, including the sensitive overwintering stage, from four years consecutive single treatment crop exposures to flowering maize and oilseed rape grown from thiamethoxam treated seeds at rates recommended for insect control. Throughout the study, mortality, foraging behavior, colony strength, colony weight, brood development and food storage levels were similar between treatment and control colonies. Detailed examination of brood development throughout the year demonstrated that colonies exposed to the treated crop were able to successfully overwinter and had a similar health status to the control colonies in the following spring. We conclude that these data demonstrate there is a low risk to honey bees from systemic residues in nectar and pollen following the use of thiamethoxam as a seed treatment on oilseed rape and maize.