Scientists behind the largest study so far conducted on neonicotinoid insecticides - have said each insecticide acts differently on bees’ brains. Neonicotinoids are a type of systemic insecticide, taken up by the plant and stored in the plant tissue - launched in the 1990s they are amongst the most widely used insecticides worldwide.  Three neonicotinoids were subjected to a partial ban by the European Commission in 2013, over growing evidence of their health impacts on bees.

However, this week scientists from the University of Dundee said one neonicotinoid product may have been ‘unfairly’ named amongst the offenders, at least as far as risks to bumblebees go. Much of the evidence of the impacts of these chemicals on bee health comes from studies conducted on domestic honeybees; as these come from managed populations, they are easier to study. Across the northern hemisphere, honeybees are in decline, as are many wild species of bee, and other pollinating insects. Although pesticide manufacturers blame these declines on habitat loss, disease and climate change, environmentalists and independent scientists have said certain agro-chemicals may be contributing to declines.

In research published this week, Dundee researchers found that one of the partially banned neonicotinoid insecticides – clothianidin – did not show the same detrimental effects on bee colonies as its close chemical relatives imidacloprid and thiamethoxam.

Dr Chris Connolly of Dundee’s School of Medicine led the study, working with colleagues from the University of St Andrews. Dr Connolly said, “There has been growing concern over the risk to bee populations from neonicotinoid insecticides and their long-term consequences to essential ecosystem services and food security.”

He claimed, “Our knowledge of the risk of neonicotinoids to bees is based on studies of imidacloprid and thiamethoxam and these findings have generally been extrapolated to clothianidin. However, in this study we have looked at the three neonicotinoids in parallel. What we have found is that imidacloprid and thiamethoxam, but not clothiandin, exhibit toxicity to bumblebee colonies when exposed at field-relevant levels.”

Dr Connolly continued, “There was also further variation in the effects on bees between the three insecticides. So we can clearly see that the banned neonicotinoids are not the same, so they should be considered independently when considering risk and legislation. From our findings, we consider that it is premature to place a permanent ban on the use of clothianidin. That said, a moratorium on its use should continue until the knowledge gaps are filled on its wider impact on other species.”

However, earlier this month researchers from the University of Sussex said there is evidence that, though it didn’t appear to affect bumblebees’ learning in studies, clothiandin still affects honeybees. They also said previous studies have suggested the impacts of clothiandin exposure may be even greater for bumblebees than honeybees.

Professor Dave Goulson from the University of Sussex said the Sussex findings served as a warning against extrapolating findings from one insect species to another.  

As part of their study, the Dundee researchers tested 75 bee colonies at five separate locations in Scotland, which the researchers said makes the study one of the largest of its kind, conducted in field-realistic conditions and with very few insecticides being used on neighbouring fields. The research was conducted over five weeks last summer. Dundee’s Dr Connolly said the research once again confirmed the threat to bumblebees from use of imidacloprid and thiamethoxam.

The lead author said, “We have seen further evidence to indicate the risk from these insecticides, including deficits in colony strength. Given these findings, small changes in the pesticide structure or its target site in insects are likely to be critical to risk assessment and each pesticide/insect combination needs to be considered independently.” Echoing the conclusions of the Sussex researchers, Dr Connolly said, “Evidence should not be extrapolated to similar chemicals or insects. Real risk must be determined empirically.”

The Dundee study was released as the NFU applied for another derogation to use partially banned neonicotinoids on oilseed rape crops again this year. After a delay last year, the union’s second request was granted, allowing partially banned seed treatments to be used in four English counties, amid claims that government advisors had been gagged.

The Expert Committee on Pesticides (ECP) is set to consider the request at its next meeting on 4th May, though the final decision will be taken by government. Environmental group Friends of the Earth has reacted strongly to the application, claiming that granting the NFU’s request would be “Reckless and unnecessary.”