French scientists say neonicotinoid insecticides can harm individual honeybees although whole colonies are able to recover in the wild.
They say this explains the “missing link” that laboratory trials often report that pesticides harm honeybees, but field surveys show no deleterious effects.
The European Commission has banned three neonicotinoids, which are largely used as seed dressings in oilseed rape, and this ban is to be reviewed towards the end of this year.
The French work monitored tagged honeybees in the wild and showed that bees foraging around treated crops die off at a faster rate than normal, but colonies are able to compensate by boosting the number of worker bees in the hive.
Lead researcher Dr Mickael Henry of the French National Institute for Agricultural Research (Inra) in Avignon, says the work led to two main conclusions.
First, the researchers found that field exposure to the banned neonicotinoids is linked to significant excess mortality in free-ranging bees.
Second, colonies appear to be able to compensate for the excess mortality so as to preserve an unaltered performance in term of population size and honey production.
Instead, the most exposed colonies modified the timing of their reproduction, delaying drone brood production in favour of increased worker-bee production.
“We have now reconciled the conflicting laboratory and field assessments of neonicotinoid toxicity. It is thus urgent that risk assessors take into account the scientific evidence for behavioural disorders triggered by trace level of neonicotinoids,” the reports says.
Last month, a review led by two Oxford University professors of all the academic literature showed there was no evidence of significant damage to honeybees from neonicotinoids in the field.
The EC imposed its ban in December 2013 on three neonicotinoids that were used as seed dressing on bee-attractive crops, such as oilseed rape, due to their perceived harmful effect on bees, and will review the ban next month.
The EC ban covers three neonicotinoids – clothianidin, used in Bayer CropScience’s Modesto; thiamethoxam, used in Syngenta’s Cruiser; and imidacloprid.
The seed treatments were used to control cabbage stem flea beetle, and without the dressings, 3.5% of the nation’s oilseed rape crop was lost to the pest in autumn of 2014, according to an AHDB survey.
This season, growers in four flea beetle hotspot counties – Suffolk, Cambridgeshire, Bedfordshire and Hertfordshire – were allowed to use neonicotinoids under emergency legislation on up to a maximum of 30,000ha, or about 5% of the national crop.
The UK government has always opposed the EC ban, as has the agrochemical industry, which has agreed to fund an independent trial run by the Centre for Ecology and Hydrology, due to be published before Christmas.
The French work is reported in Royal Society journal Proceedings B.