Researchers from the Centre for Ecology & Hydrology (CEH) have published results of a large-scale, field-realistic experiment to assess neonicotinoid impacts on honeybees and wild bees across Europe, in the peer-review journal Science today.
The experiment, undertaken in the UK, Germany and Hungary, exposed three bee species to winter oilseed rape crops treated with seed coatings containing neonicotinoid clothianidin, from Bayer CropScience, or Syngenta’s thiamethoxam.
Neonicotinoid seed coatings are designed to kill pests such as the cabbage stem flea beetle, but were effectively banned in the EU in 2013 due to concerns regarding their impact on bee health.
The researchers found that exposure to treated crops reduced overwintering success of honeybee colonies, a key measure of year-to-year viability, in two of the three countries. In Hungary, colony number fell by 24 percent in the following spring. In the UK, honeybee colony survival was generally very low, but lowest where bees fed on clothianidin-treated oilseed rape in the previous year.
No harmful effects on overwintering honeybees were found in Germany.
Lower reproductive success, reflected in queen number (bumblebees) and egg production (red mason bee), was linked with increasing levels of neonicotinoid residues in the nests of wild bee species buff-tailed bumblebee (Bombus terrestris) and the Red Mason Bee (Osmia bicornis) across all three countries.
According to the CEH lead author, Dr Ben Woodcock, “The neonicotinoids investigated caused a reduced capacity for all three bee species to establish new populations in the following year, at least in the UK and Hungary.”
He suggests the differing impacts on honeybees between countries may be associated with interacting factors including the availability of alternative flowering resources for bees to feed on in the farmed landscape as well as general colony health, with Hungarian and UK honeybees tending to be more diseased.
In contrast, the hives in Germany happened to be larger, showed little evidence of disease and had access to a wider range of wild flowers to feed on. Dr Woodcock suggests that this may explain why in this country alone there was no evidence of a negative effect of neonicotinoids on honeybees.
In response to the report, Bayer issued the following statement:
The CEH did not find consistent effects across Germany, Hungary and the UK on key indicators of honey bee health such as colony strength, forager mortality, overwintering success of the colonies, behavior or disease susceptibility in honey bees.
In the German part of the study, the researchers found a positive correlation between hive performance and neonicotinoid seed treatment, i.e. the honey bee colony strength increased when the bees foraged on treated oilseed rape. This is in line with the observation that the colonies at the treatment sites were on average stronger at study initiation than those at the control sites.
In contrast, a weaker colony performance was observed in the UK and partially in Hungary. Colony mortality in the UK was too high across all treatments to support robust scientific conclusions, particularly on overwintering colony strengths. Consequently, no reliable risk conclusions can be drawn from these locations.
In Hungary, the average strengths of the colonies at the treatment sites were slightly lower at study initiation than at the control sites and the diversity of the surrounding landscape was not the same between the control and treatment fields.
As Dr. Richard Schmuck, director of Environmental Safety at Crop Science, a division of Bayer, explains, “In order to understand such inconsistencies between the country results, it is important to consider that in the CEH study, honey bee colonies of different strengths were not equally distributed over the different treatment groups. We have recently completed a sensitivity analysis of a 20-year field study database which suggests that the development of bee colonies is highly influenced by the colony strength at study initiation.”
If these differences in hive size and the diversity of environmental surroundings are adequately considered, statistical analysis no longer shows the reported differences. “It is unfortunate that the reported statistical analysis was not corrected for differences in hive sizes or differences in environmental landscapes, but it does highlight the complexity of doing this sort of study on such a large scale,” Schmuck added.
“Therefore, we do not share the CEH’s interpretation that adverse effects of the seed treatments can be concluded from this study, and remain confident that neonicotinoids are safe when used and applied responsibly.”