One of the most extensive field studies to date assessing the impacts neonicotinoid pesticides can have on honeybees and wild bees has found that they do cause harm. 

Spanning 2,000 hectares across the U.K., Germany and Hungary, the large-scale experiment exposed three bee species to winter oilseed rape crops that had been treated with seed coatings containing neonicotinoid clothianidin, from Bayer CropScience, or Syngenta’s thiamethoxam.

In trials took place across 33 sites.

Researchers from the U.K.'s Centre for Ecology & Hydrology (CEH) published results in the peer-review journal Science this week.

The mass study found exposure to treated crops reduced the overwintering success of honeybee colonies, which is a measure of year-to-year viability, says CEH. This was the case for the U.K. and Hungary.

Overwintering involves bees building underground nests to wait out the winter period before emerging in the spring to start new colonies.

Colony numbers fell by 24% in Hungary, while the U.K. honeybee colony survival was generally very low, according to the research. The lowest was when bees fed on clothianidin treated oilseed rape in the previous year.

Meanwhile the picture was very different in Germany. No harmful effects on overwintering honeybees were found.

Created to kill pests including the cabbage stem flea beetle, neonicotinoids continue to split opinion and cause controversy.

The CEH says the effective ban in the European Union in 2013 came about because of concerns regarding their impact on bee health.

Findings from the study will now be passed onto the European Food Standards Agency (EFSA), which is currently preparing a comprehensive report on the subject due for release in November.

Meanwhile the European Commission is drafting proposals to extend the neonicotinoid ban to be voted on by member states at a later date.

Lower reproductive success – reflected in queen numbers (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.

“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,” CEH lead author Dr Ben Woodcock says. 

Differing impacts

Woodcock suggests the impacts could differ between countries because of interacting factors such as how many other flowering resources are around for the bees to feed on in the farmed landscape.

Other factors could include general colony health, with U.K. and Hungarian honeybees tending to be more diseased.

The German hives were larger, showed little evidence of disease and there was a broad range of wild flowers for the bees to feed on - which could explain why Germany showed no real negative effect of neonicotinoids on honeybees.

“Neonicotinoid seed dressings do have positive attributes: they target insects that damage the plant, can be applied to the seed at low dosage rates but protect the whole plant and reduce the need for broad spectrum insecticide sprays,” adds Dr Woodcock.

“Their use as an alternative chemical control option is also useful in controlling pests where insecticide resistance to other pesticides is already found, so play an important role in food production.”

He adds that more research is needed to get a clearer picture of how to reduce or alleviate negative consequences.

““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," he says.

“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 CropScience and Syngenta funded the research assessing the impact of neonicotinoids on honeybees, while the Natural Environment Research Council funded the analysis of the impact on the wild bees.

The experiment, including design, monitoring and analysis, were scrutinised by an independent scientific advisory committee chaired by Professor Bill Sutherland of Cambridge University.

“Neonicotinoids remain a highly contentious issue with previous research on both honeybees and wild bees inconclusive,” adds co-author Professor Richard Pywell.

“This latest field study was designed, as far as possible, to reflect the real world due to its size and scope. We therefore believe it goes a considerable way to explaining the inconsistencies in the results of past research, as we were better able to account for natural variation in factors like exposure to the pesticide, bee food resources and bee health for different bee species."

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