Two of the world’s most widely used insecticides cause significant harm to bumblebee colonies, a new study has found, but a third had no effect.
The work shows the distinct effects of each type of neonicotinoid pesticide, from cuts in live bees and eggs to changed sex ratios and numbers of queens. Previously, the different types of neonicotinoids have often been treated as interchangeable.
Neonicotinoids and other pesticides have been implicated in the worldwide decline in pollinators, which are vital for many food crops, although disease and loss of habitat are also important factors. There is strong evidence that neonicotinoids harm individual bees but little evidence so far that colonies suffer as a result. The EU imposed a moratorium on the use of neonicotinoids on flowering crops in 2013.
The new study examined the effect of three neonicotinoids from the level of brain cells to colonies in the field. The latter involved 75 colonies across five sites in Scotland and included control colonies that were not given access to the pesticides.
The research found that both imidacloprid (made by Bayer) and thiamethoxam (Syngenta) at realistic levels of exposure harmed the bumblebee colonies. For example, imidacloprid cut the number of brood cells, which contain eggs, by 46%, while thiamethoxam reduced the number of live bees by 38%. But clothianidin (Bayer) had no effect other than increasing the number of queens produced.
“There is clear evidence that imidacloprid and thiamethoxam are harmful to bees but our evidence raises a question over clothianidin,” said Dr Christopher Connolly, at the University of Dundee and who led the research published in the peer-reviewed journal Science Reports.
“I think there is sufficient evidence for a ban on imidacloprid and thiamethoxam, but not for clothianidin although the moratorium should continue,” as more evidence is gathered.
A study published in November found that honeybee colonies exposed to thiamethoxam were able to compensate for bee deaths by producing more workers, so no overall impact was seen. However, honeybee colonies have far more individual bees than bumblebees, suggesting the latter have far less ability to compensate.
The only effect seen in the new study from clothianidin exposure was an increase in queens. “The number went through the roof,” said Connolly. “The purpose of a colony is to produce queens so this could be a really good thing, unless, say, all those queens turn out to be infertile.”
Connolly said the research did not show clothianidin was harmless to all pollinators: “It does not show it is safe, but it does show it may be less harmful to bumblebees.” He said the work showed the effects of one neonicotinoid could not be assessed from studies on another: “All three behave completely differently. You can’t predict the insect-insecticide relationship by extrapolation.”
“This is an important and timely study,” said Mike Garratt, an ecologist at the University of Reading and not involved in the new research.
Friends of the Earth’s Dave Timms said: “Allowing farmers to use banned bee-harming pesticides would be reckless and unnecessary. Oilseed rape yields have actually risen since the pesticide ban was introduced, while the evidence of the harm these chemicals pose to bees has increased.”