Two of the most extensive field studies conducted to date in Europe and Canada have confirmed the hypothesis that neonicotinoid insecticides are harmful to bees and other pollinating species.
The results, published Friday (30 June) in the peer-reviewed journal Science, also reveal that local environments can mitigate the impact of the pesticide, which is widely used in farming despite being dubbed a “bee killer”.
The chemical – which acts on the nervous system of insects – had a “largely negative” effect on the pollinating insects that are essential to many crops. It reduced their reproductive success and boosted mortality rates, according to the research that was partly funded by the German chemicals company Bayer and Switzerland’s Syngenta.
“These findings point to neonicotinoids causing a reduced capacity of bee species to establish new populations in the year following exposure,” according to the study.
“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,” said David Goulson, a biology professor at Britain’s University of Sussex who did not participate in the research.
The two studies are important because they were field studies that sought to examine the real-world exposure of bees to pesticides in nature.
Scientists who conducted the European research – in Britain, Hungary and Germany – told reporters their overall findings suggested neonicotinoids are harmful to honeybee and wild bee populations and are “a cause for concern”.
But scientists representing companies who funded the work – Germany’s Bayer AG and Switzerland’s Syngenta AG – said the results showed “no consistent effect”.
Several independent experts said the findings were mixed or inconclusive.
The first experiment, conducted over a total of 2,000 hectares (5000 acres) in the UK, Germany and Hungary exposed three bee species to winter oilseed rape crops with seeds coatings containing either clothianidin from Bayer CropScience or Syngenta’s thiamethoxam.
The coatings were temporarily banned by the European Union in 2013 due to concerns regarding their impact on bee health, though there are now plans to ban them completely in fields but not in greenhouses.
Decline in reproduction rates
The researchers found exposure to the pesticide reduced winter survival rates in Hungary, where the colony population fell by 24% and in Britain where survival rates were “very low”.
Germany did not see a dramatic decline and even showed a positive effect on bees exposed to neonicotinoids, although researchers said this was temporary and the reasons behind it were unclear. Lead author Ben Woodcock put down the surprising German result to the availability of alternative flowering resources.
“This represents the complexity of the real world,” said Richard Pywell, a professor at the UK’s Centre for Ecology and Hydrology who co-led the work. “In certain circumstances, you may have a positive effect … and in other circumstances, you may have a negative effect”
More importantly, all three countries saw a decline in reproduction rates, linked to residual neonicotinoid in nests.
The second study conducted in Canada showed that worker bees and queens exposed to the insecticide died earlier, while the overall health of colonies was also weakened.
Worker bees exposed to the treated pollen during their first nine days had their lifespans cut short by 23%, were unable to maintain a healthy laying queen and had poor hygiene.
The researchers were surprised to learn the contaminated pollen the honeybees collected did not belong to corn or soybean plants originally treated by the insecticides.
“This indicates that neonicotinoids, which are water soluble, spill over from agricultural fields into the surrounding environment, where they are taken up by other plants that are very attractive to bees,” said researcher Nadia Tsvetkov.
Several British specialists with no direct involvement in the study who were asked to assess its findings said they were mixed.
Rob Smith, a professor at the University of Huddersfield, said the results were “important in showing that there are detectable effects of neonicotinoid treatments on honeybees in the real world”, but added: “These effects are not consistent”.
Lynn Dicks at the University of East Anglia said the findings “illustrate the complexity of environmental science”.
“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,” she said.
Norman Carreck, an insect expert at Sussex University, said: “Whilst adding to our knowledge, the study throws up more questions than it answers.”