Results from the first pan-European field study of neonicotinoid insecticides add to evidence that the controversial compounds are harming bees and pollinating insects.

The study was led by the Centre for Ecology and Hydrology (CEH) and was published in the journal Science on Thursday. Researchers in the UK, Hungary and Germany looked at the health of honeybees and wild bees exposed to oilseed rape crops treated with two neonicotinoids (Bayer’s clothianidin and Syngenta’s thiamethoxam).

They found that exposure to treated crops reduced the overwintering success of honeybees in Hungary and the UK (where honeybee colony survival was generally very low, but lowest where bees fed on clothianidin-treated oilseed rape in the previous year), though no noticeable effects were recorded in Germany. The researchers also found lower reproductive success (lower numbers of queens in bumblebees and egg production in red mason bees), associated with higher levels of neonicotinoid residues in the nests of wild bee species across all three countries.

On Thursday, CEH scientist Dr Ben Woodcock said differences in the observed impacts on honeybees between countries may relate to other factors thought to be having an impact on bee decline, which is going on across the northern hemisphere. For example, he said the relative success of German bees could be linked to more readily available alternative flowering plants to forage amongst, or to general colony health; this was better in Germany, which had larger colonies with less disease compared to the UK and Hungary.

The results of the CEH study come as the EU Commission prepares to extend its partial restrictions on three neonicotinoids (including two studied in the field experiment) to a full ban on outdoor use. Despite the negative impacts observed in the study, Dr Woodcock said, "There may be opportunities to mitigate negative impacts of neonicotinoid exposure on bees through improved honeybee husbandry or availability of flowering plants for bees to feed on across non-cropped areas of the farmed landscape. Both these issues require further research.

"The negative effects of neonicotinoids on wild bees may also be the result of diverse mechanisms of exposure that include persistent residues of neonicotinoids in arable systems due to their widespread and often very frequent use.”

Bayer contests CEH conclusions

The CEH study was funded by the National Environment Research Council (NERC), and by neonicotinoid manufacturers Bayer CropScience and Syngenta. Reacting to its publication on Thursday, Bayer said the “Field study delivers inconsistent results, highlighting the need to appropriately consider colony strength and environmental conditions.”

Bayer’s response highlighted that “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, behaviour or disease susceptibility in honey bees” and claimed colony mortality was too high in the UK to draw reliable scientific conclusions.  

Bayer’s environmental safety spokesperson Dr. Richard Schmuck said, “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.

“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.”

Dr Lynn Dicks, an NERC Research Fellow based at the University of East Anglia, noted that the study is only the second experiment to look at neonics’ impacts on free-living bees, foraging as they normally would in agricultural landscapes.

Dr Dicks highlighted the differing results returned by these two studies (the first of which was conducted in Sweden in 2015), and said, “These two studies should be interpreted together as setting boundaries for what the real impacts of neonicotinoid use on bees might be. They illustrate the complexity of environmental science effects. If there was a really big effect of neonicotinoids on bees, in whatever circumstances they were used, it would have shown up in both of these studies. Instead, the studies show there are potentially large impacts in some circumstances.”

She also said, “Importantly, neither of the studies experimentally tested imidacloprid,  which is the more toxic of the neonicotinoids previously most commonly used. Yet both studies found residues of imidacloprid in the bee nests or bee-collected nectar and pollen. This indicates that imidacloprid has contaminated the farmed environment and persists years after its use was discontinued.”

Also assessing the findings, Prof. David Goulson, who has done work on honeybees and neonics at the University of Sussex, said, “Exposure of bees [in this study] was entirely field-realistic; indeed, farmers simply followed normal farming practice. The findings are in agreement with a number of earlier studies; field exposure to neonicotinoids has clear negative impacts on bumblebees and solitary bees. Effects on honeybees were also predominantly negative but more variable.

“Interestingly, analysis of residues of neonicotinoids in bee nests suggests that much bee exposure was not from the treated crop adjacent to the colony but was coming from other sources in the landscape, suggesting widespread contamination of the environment. For example, the neonicotinoid imidacloprid was frequently detected in bee nests but was not used on the farms in the year of the study. This is in accordance with previous studies showing that neonicotinoids are persistent in soils and frequently contaminate wildflowers.

“In the light of this new study, continuing to claim that use of neonicotinoids in farming does not harm bees is no longer a tenable position.”

In a companion article to the study in Science, Nadia Tsvetkov, from York University in Canada, noted that field work in Canada has suggested that neonicotinoids may interact with other pesticides applied by farmers, especially fungicides, to harm honeybee colony health. This, combined with evidence from the research that neonicotinoids are persisting in the environment presents a worry, researchers said.

Zoology Professor Robert Paxton, from Martin Luther University, Halle-Wittenberg in Germany, said, “A worrying synergistic interaction [between neonicotinoids and other agrochemicals] had already been highlighted in a previous semi-field study; the detection of the potential long-term persistence of neonicotinoids in the soil by both [CEH and Tsvetkov’s] studies raises the spectre of a reprise of Rachel Carson’s Silent Spring.”

Reacting to the study, Soil Association policy officer Louise Payton said, “This long-awaited major study was funded chiefly by industry to end this debate. This should now be the final, fatal blow to neonicotinoids. With neonics widely polluting farms, and with UK farmers still treating most wheat, an outright ban is needed immediately. But a ban isn’t enough - neonics never should have been cleared for use. We also need a rethink of how we regulate all pesticides. Vulnerable wildlife is not being protected. A good next step would be to learn from organic farms, which support around 50% more species of wild pollinators than non-organic farms.”