NEONICOTINOID pesticides have once again been implicated in the long-term decline in wild bee populations.

An 18-year study of bee numbers across England, cross referenced with data on the acreage of neonic-treated oilseed rape crops, has suggested that the chemical is responsible for declines of up to 30%.

Published this week in the journal Nature Communications, the results from the Centre for Ecology and Hydrology looked at 62 species of wild bees – excluding purpose-bred honeybees and bumblebees – and concluded that since the commercial licensing of neonics for UK oilseed crops in 2002, and the concurrent increase in the planted acreage, there had been "long-term, large-scale" detrimental impacts.

"Neonicotinoids are harmful, we can be very confident about that and our mean correlation is three times more negative for foragers than for non-foragers," said Dr Nick Isaac, a co-author of the new paper

"Historically, if you just have oilseed rape, many bees tend to benefit from that because it is this enormous foraging resource all over the countryside," added lead author Dr Ben Woodcock. "But this co-relation study suggests that once its treated with neonicotinoids up to 85%, then they are starting to be exposed and it's starting to have these detrimental impacts on them.

"What we can't say is what these detrimental impacts are, but what it does suggest is you can have these population declines and they can be big – I mean 30% is a big decline."

Chemical manufacturer Bayer Crop Science described the CEH work as "an interesting statistical study", but argued that intensive farming might be the real cause of the decline.

"Since most of the oilseed rape grown in the UK was treated with a neonicotinoid seed treatment during the years that this study looked at, we believe its findings would be more correctly headlined that intensive agriculture is causing some issues with pollinators," said Bayer's Dr Julian Little.

"Whether this is due to the use of insecticides is not clear; a lack of nesting sites and pollen and nectar sources in these areas may also be critical factors."