Paris - A widely-used class of insecticides called neonicotinoids has contributed to the large-scale and long-term decline of wild bees, according to research unveiled on Tuesday.A study covered 62 species from 1994 to 2011, examining the impact of exposure to the pesticide, which was used to treat oilseed rape crops in England.In five of the species, including the spined mason and furrow bees, the chemicals accounted for at least 20% of local population extinctions, researchers estimated.And compared to bees that foraged on a wide range of flowers, decline was three times more pronounced among species - such as the buff-tailed bumblebee - that feed regularly on the rapeseed.The findings, published in the peer-reviewed journal Nature Communications, are timed to inform a review by the European Food Standards Authority (EFSA) on the overall risks associated with so-called neonic pestkillers. The review is scheduled for completion by January 2017.The results also bolster small-scale and short-term studies that have previously fingered neonics as a culprit in bee decline, especially species bred for pollination and honey production.Unlike contact pesticides - which remain on the surface of foliage - neonicotinoids are absorbed by the plant from the seed phase and transported to leaves, flowers, roots and stems.They have been widely used over the last 20 years, and were designed to control sap-feeding insects such as aphids and root-feeding grubs."Our results have implications for the conservation of not only bee communities in intensively farmed landscapes, but the capacity of these systems to maintain stable crop pollination services," said lead author Ben Woodcock, a researcher at the NERC Centre for Ecology and Hydrology in Oxfordshire.Neonics are deemed to be only one of several causes for the dramatic decline in bee colonies, especially in Europe and the United States."It is unlikely that they are acting in isolation," Woodcock said in a statement."Habitat loss and fragmentation, pathogens, climate change and other insecticides" are other likely factors, he said.Previous studies have found neonicotinoids can cause bees to become disorientated such that they cannot find their way back to the hive, and lower their resistance to disease.Recent experiments showed that the insecticides also diminish the viability of bee sperm.The European Union has placed a moratorium on the sale of neonicotinoids, though some European countries continue to use them under various exemptions.Wild bees provided crop pollination services worth billions. A recent study estimated that value at more than $3 250/ha each year.Bees of all kinds account for an estimated 80% of plant pollination by insects.