Wall brown butterflies have declined by 37 per cent over 10 years but are neonicotinoids to blame?
Scientists have discovered a strong correlation between the declines of some of the UK’s most widespread butterflies and the use of a group of pesticides called neonicotinoids.
For common species such as the small tortoiseshell, gatekeeper and wall brown, the expanse of farmland where neonicotinoids have been applied to crops such as oilseed rape is the best predictor of their declines, according to models developed by researchers.
Neonicotinoids – or neonics as they are commonly known – are pesticides specifically developed to kill crops pests by attacking nervous systems. They were introduced in the mid-1990s, but their use increased significantly in the first decade of the 21st century.
For example, total useage across Britain nearly doubled in just three years – from 42,300kg over 841,000ha in 2007 to 80,000kg over 1.27m ha in 2010.
Though a strict causal link with butterfly declines was not demonstrated in the study, data from the UK Butterfly Monitoring Scheme (UKBMS) shows negative trends for many butterfly species over a 10-year period, 2000-2009.
In that time, the small tortoiseshell declined by 64 per cent, the wall brown by 37 per cent, the gatekeeper by 23 per cent and the small white by 26 per cent.
But declines in other species, such as the orange-tip, marbled white and meadow brown, were only very weakly correlated with the use of neonics. Populations of two species, the comma and ringlet, increased in that time period.
Richard Fox, head of recording for Butterfly Conservation, said the research did not provide any proof that neonics were behind the negative population trends, and that the apparent relationship between the two could be a ‘proxy’ for other factors relating to intensive agriculture, such as loss of hedgerows or increasing field sizes.
“No work has been done to prove that neonics are poisonous to butterflies, but it would be very odd if they weren’t, because they are nerve agents for invertebrates and butterflies are invertebrates with nerves,” he said.
“Is it the neonics that are killing off our butterflies or is it part of a wider problem,” he added. “We urgently need more research to find out the causes of butterfly declines”
The EU imposed a two-year temporary moratorium on the use of neonicotinoids in December 2013, though the ban was lifted for farmers in some parts of Britain in 2015.
Read the research for yourself in PeerJ
Find out more about the UK Butterfly Monitoring Scheme