Two major studies into how bees are affected by a group of pesticides banned in Europe gave mixed results on Thursday, fueling a row over whether the chemicals, called neonicotinoids, are safe.
The studies, one conducted across three European countries and another in Canada, found some negative effects after exposure to neonicotinoids in wild and honeybee populations, but also some positives, depending on the environmental context.
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 Switerland's Syngenta AG - said the results showed "no consistent effect."
Several independent experts said the findings were mixed or inconclusive.
The European Union has since 2014 had a moratorium on use of neonicotinoids - made and sold by various companies including Bayer and Syngenta - after lab research pointed to potential risks for bees, crucial for pollinating crops.
But crop chemical companies say real-world evidence is not there to blame a global plunge in bee numbers in recent years on neonicotinoid pesticides alone. They argue it is a complex phenomenon due to multiple factors.
A spokesman for the EU's food safety watchdog EFSA, said the agency is in the process of assessing all studies and data for a full re-evaluation of neonicotinoids, expected in November.
EFSA's scientific assessment will be crucial to a European Commission decision in consultation with EU states on whether the moratorium on neonicotinoid use should remain in place.
The two studies published on Thursday, in the peer-reviewed journal Science, are important because they were field studies that sought to examine the real-world exposure of bees to pesticides in nature.
Researchers who led the Canadian study concluded that worker bees exposed to neonicotinoids - which they said often came from contaminated pollen from nearby plants, not from treated crops - had lower life expectancies and their colonies were more likely to suffer from a loss of queen bees.
On the findings of the European study, researchers told a briefing in London that exposure to neonicotinoid crops harmed honeybee colonies in two of the three countries and reduced the reproductive success of wild bees across all three.
They noted, however, that results from Germany showed a positive effect on bees exposed to neonicotinoids, although they said this was temporary and the reasons behind it were unclear.
"This represents the complexity of the real world," said Richard Pywell, a professor at Britain's Center of 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"
Overall, however, he said: "We are showing significant negative effects on [bees'] critical life-cycle stages, which is a cause for concern."
Several specialists with no direct involvement in the study who were asked to assess its findings said they were mixed.
Rob Smith, a professor at Britain's 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 Britain's Sussex University, said: "Whilst adding to our knowledge, the study throws up more questions than it answers."