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There can be little doubt now that the world’s most widely used insecticides are bad for bees. Two new studies add to the mountain of evidence that neonicotinoids are harmful to pollinators, and add to the pressure for Europe, at least, to introduce a full ban.

The European Union has had a temporary moratorium on using three major neonicotinoids on bee-attractive crops since 2013, though farmers can apply for emergency authorisation to keep using them. The European Food Safety Authority (EFSA) is due to publish an assessment in November on whether to make the ban permanent, and legislators are already discussing whether to extend it to cover all uses outside greenhouses.

One of the studies was the largest field trial to date, involving honeybees, bumblebees and solitary bees at 33 oilseed rape sites in the UK, Germany and Hungary. The team were given a licence to use two banned neonicotinoid insecticides (NNIs), clothianidin and thiamethoxam. One of these, or no NNIs at all, was used at each site, with the allocation made at random.


Even where no chemical was used, bees’ hives and nests contained NNI residues, including traces of the banned imidacloprid, which was not used in the study. This shows that all three chemicals have remained in the environment even after the moratorium.

In wild bees, the study found a link between higher levels of NNI residues and negative effects on reproduction: fewer queens in bumblebee hives and fewer egg cells in solitary bee nests.

Diet and disease

In honeybees, the effects varied from country to country. During April to June flowering period, NNI exposure had negative effects on colony size in the UK and Hungary, but short-lived positive effects in Germany. In Hungary, the negative effects persisted over the winter, with worker numbers down 24 per cent the following spring.

It’s not clear exactly why there was this variation in effects, but diet and disease could be important factors. Bees in Germany had a more varied diet, getting less of their pollen from oilseed rape, and also had lower levels of parasites.

“The findings suggest that if we’re better at managing bee diseases, we may help mitigate these effects,” said study author Richard Pywell at the Centre for Ecology and Hydrology, UK.

Read more: Decline of wild bee species in England linked to pesticide use

The second study, by Nadejda Tsvetkov and colleagues at York University in Toronto, Canada, investigated levels of neonicotinoids that bees encounter in corn-growing regions in Canada. It found they were exposed for four months – almost the whole of their active season, and longer than had been suspected.

The researchers also tested the effects of exposure on honeybees in the lab. Worker bees exposed to realistic levels of clothianidin had shorter lifespans and their colonies were more likely to permanently lose queens. The toxicity of the insecticide doubled when it was present together with a commonly used fungicide.

In the field tests, most of the NNI-containing pollen collected by bees came from plants such as wildflowers that are not the targets of pesticide spraying. As in the European study, this raises concerns about the extent to which NNIs persist in the environment.

“Whatever you use the chemicals on, they are contaminating the environment and getting into bees’ food,” says Dave Goulson at the University of Sussex, UK. That suggests the EU’s current moratorium, which only applies to flowering crops such as corn and rape, won’t work, he says.

Although the field trial results varied between countries, the overall evidence points to harmful effects for bees, says Goulson. “I think you’d have to be pretty unreasonable at this point not to accept that, at least some of the time, these chemicals harm bees when used in normal farming practice.”

Goulson expects the EFSA to recommend at least that the moratorium on flowering crops be made permanent, and that it might raise the possibility of an extension to all crops. “I don’t think there can be any doubt that that’s what they’ll be recommending, but they might be suggesting going much further than that.”

Pywell said there needs to be more research on pesticides that might replace NNIs. “Whatever is decided, we’ve got to consider the alternatives,” he said.

Two makers of NNIs, Bayer and Syngenta, funded the European study but have rejected its conclusions, claiming the results were inconclusive. “We remain confident that neonicotinoids are safe when used and applied responsibly,” said Richard Schmuck of Bayer in a statement.

Journal reference: Science, DOI: 10.1126/science.aaa1190

Journal reference: Science, DOI: 10.1126/science.aam7470