Controversial pesticides blamed for the loss of bee colonies may also be having much wider environmental effects and damaging wild bird populations, research has shown.
Scientists in the Netherlands linked declines in farmland bird species, such as starlings and tree sparrows, with a neonicotinoid chemical used to protect crops from insect pests.
Nine of the bird species studied fed exclusively on insects, and all relied on insects to feed their young.
The findings, published in the journal Nature, suggest that the harmful environmental effects of neonicotinoids go far beyond their alleged impact on bees.
They echo the claims made in Rachel Carson’s influential book Silent Spring, published in 1962, which led to a nationwide ban on the pesticide DDT in the US.
The book, which brought fears about the environmental hazards of indiscriminate pesticide use to the attention of the American public, focused mainly on the fate of birds.
The Dutch scientists showed that local bird populations were significantly lower in areas where there was more water contamination by the pesticide imidacloprid.
All but one of the species studied, which also included the mistle thrush, yellow wagtail and Eurasian skylark, were affected between 2003 and 2010.
Six populations saw significant declines of 3.5 per cent a year in areas where surface water concentrations of imidacloprid were higher than around 20 nanograms per litre.
The evidence points to a depletion of the birds’ primary food source – insects – rather than any direct effect due to ingestion of the chemical.
Imidacloprid is one of three neonicotinoids affected by a two-year Europe-wide ban that came into force last December amid controversy about their impact on bee populations.
But the ban, imposed to protect bees, only applies to flowering crops and those sown in the spring and summer. The chemicals continue to be used on other major crops such as winter wheat and barley, and sugar beet, and neonicotinoids can still legally be sprayed on garden and park plants.
The authors of the new study, led by Caspar Hallmann from Radboud University in Nijmegen, concluded in their paper: “Our results on the decline of bird populations suggest that neonicotinoids pose an even greater risk than has been anticipated.
“Cascading trophic (food web) effects deserve more attention in research on the ecosystem effects of this class of insecticides and must be taken into account in future legislation.”
Usually applied as ‘seed dressings’ to arable crops the seeds are coated with the chemical so that it is absorbed and spread through the tissues of the growing plant. All parts of the plant then become toxic to insect pests feeding on it.
But during sowing, a little of the chemical is lost as dust that blows away to be deposited on other vegetation.
Some also leaches into the soil and is washed into waterways.
The fear is that by these indirect routes the pesticides can affect aquatic insects and non-targeted plants eaten by beetles, grasshoppers and caterpillars.
These invertebrates provide food not only for birds but also some mammals.