Fera scentists found what they called an absence of any "clear relationship between observed neonicotinoid levels and measures of colony success" suggesting neonicotinoids are not a "major source of field mortality and morbidity to bumblebee colonies".
Others have disputed the findings.
The Government has supported continued use of the pesticides until its trials were finished, despite moves by the European Commission to ban them. An appeal against an inconclusive vote by EU agriculture ministers on banning the pesticides will go ahead this spring, probably on April 22/23.
Fera's Effects of neonicotinoid seed treatments on bumble bee colonies under field conditions (click here) concluded: "This study was not a formal statistical test of the hypothesis that neonicotinoid insecticides reduce the health of bumble bee colonies. Nevertheless, were neonicotinoids in pollen and nectar from treated oilseed rape to be a major source of field mortality and morbidity to bumblebee colonies, we would have expected to find a greater contribution of insecticide residues from nearby treated crops and for there to have been a clear relationship between observed neonicotinoid levels and measures of colony success. The absence of these effects is reassuring but not definitive. The study underlines the importance of taking care in extrapolating laboratory toxicology studies to the field, as well as the great need of further studies under natural conditions."
Defra chief scientist Ian Boyd said of analysis of this and other studies: "Decisions on the use of neonicotinoids must be based on sound scientific evidence. The analysis of laboratory studies published by Defra today demonstrates that while we cannot rule out the possibility of neonicotinoids affecting pollinators we cannot be clear as to the extent of their impact.
"Some of the studies analysed did not replicate the realistic dosage which bees and other pollinators would be exposed in the natural environment. The results of a study by Fera – also published today - into the foraging habits of bees on crops treated with neonicotinoids seems to support this assessment, as the Fera study suggests bumble bee forage over large distances.
"I therefore support the conclusions of the analysis that further data based on more realistic field trials is required."
Defra said previous studies showing neonics are harmful to bees is because of "overdosing of bees in the dosing studies".
Defra's assessment of key evidence about neonicotinoids and bees concluded: "This assessment cannot exclude rare effects of neonicotinoids on bees in the field, it suggests that effects on bees do not occur under normal circumstances. This assessment also suggests that laboratory based studies demonstrating sub-lethal effects on bees from neonicotinoids did not replicate realistic conditions, but extreme scenarios. Consequently, it supports the view that the risk to bee populations from neonicotinoids, as they are currently used, is low." (click here)
Commenting on the UK Government’s field trials on the effects of neonicotinoid seed treatments on bumble bee colonies, Professor David Goulson, of the Biological & Environmental Sciences department at the University of Stirling said: "The authors have admitted that the study was not robust.
"All we can really learn from it is that bumblebee nests placed on farmland, even on farms currently using no neonicotinoids, are likely to be exposed to a cocktail of these chemicals. And some negative effects on colonies were found in this study though not highlighted by the authors.
"Given the flaws in the study design, Defra should certainly not be basing any decisions on this."