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Study Links Bee-Killing Pesticides to Decline in Bird Populations

In addition to previous research on the direct impacts of pesticides on pollinators and other beneficials, a recent study published by Dutch scientists establishes an additional indirect link between neonicotinoid use and insect-eating birds. The report, which came out on Wednesday, provides evidence that neonicotinoids, a class of systemic pesticides, are indirectly hurting larger creatures by reducing insect prey populations such as mosquitoes and beetles.

Barn swallow feeding young. Photo credit: Radboud University Nijmegen

Researchers found that in certain areas of the Netherlands where water is contaminated with high concentrations of imidacloprid, a commonly used neonicotinoid, bird populations tend to decline by an average of 3.5 percent every year. Further analysis found that this spatial pattern of decline appeared only after the introduction of imidacloprid to the Netherlands in the mid-1990s, even after correcting for land-use changes that have been known to affect bird populations in farmland.

“To our surprise we did find a very strong effect on birds”, said lead author of the study, Caspar Hallmann, a Ph.D. student from Radboud University in the Netherlands, to Reuters. In fact, according to the study, which was published in the journal Nature, nine of 15 bird species studied only eat insects and all feed insects to their young. Hallmann added, “We cannot say this is proof (that the pesticide causes the decline in bird numbers) but we cannot explain the… decline of birds by any other factors.” The study also looked into other possible causes like pollution. 

Bayer CropScience issued a speedy response expressing disagreement with the study findings. The company writes that the study did not “demonstrate that there is a causal link between the use of neonicotinoids and the development of bird populations in Europe.” The company went on to say that neonicotinoids “have gone through an extensive risk assessment which has shown that they are safe to the environment when used responsibly according to the label instructions.” The company, along with Syngenta, has been accused of forestalling attempts to ban neonicotinoids via the proposal of bee health plans that call for more research, implementing agricultural best management practices and planting new habitat. These solutions fail to address the real problem that their products are highly toxic to bees.

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The recent report titled, Worldwide Integrated Assessment (WIA), undertaken by the Task Force on Systemic Pesticidesindicates otherwise. Twenty-nine scientists representing multiple disciplines analyzed more than 800 peer-reviewed publications on the impacts of systemic pesticides. The report emphasizes that neonicotinoids and their metabolites are persistent and harmful, even at very low levels, and that the chemicals have far-reaching impacts on entire ecosystems, from direct exposure to persistence in soil and water. Bees, butterflies, worms and other pollinators and non-target organisms are also put at risk. Scientists concluded that even when neonicotinoids were used according to guidelines on their labels, the chemicals’ levels in the environment still frequently exceeded the lowest levels known to be harmful to a wide range of species.

The European Union (EU) began implementation of a two-year moratorium in April on neonicotinoids used on flowering crops stemming from scientific evidence that the chemicals are harmful to bees. The pesticides can still be used legally in the E.U. on non-flowering crops, such as barley and wheat, the scientists said. Germany’s Bayer and Switzerland’s Syngenta, the two main producers of the pesticides, have contested the moratorium. They suspect that “colony collapse disorder,” which has resulted in the large drop in bee populations in Europe, Asia, the Americas and the Middle East, are due to a virus spread by a parasitic mite. Opposition to neonicotinoid use remains strong, however. Syngenta recently withdrew its emergency application to allow the use of neonicotinoids on UK oilseed rape crops (known as canola in the U.S.) in face of public outcry. According to Reuters, more than 200,000 people protested against the request, with around 35,000 more writing to environment secretary Owen Paterson.

The higher the concentrations of imidacloprid in the surface water, the greater the decline in bird numbers. For the fifteen bird species that were included in the study, numbers decreased on average by 3.5 percent per year in areas with more than 20 nanograms of imidacloprid per liter. This concentration is greatly exceeded in many parts of the Netherlands.

The Dutch study recommends that future legislation consider and take into account the wider impact of pesticides on wildlife. Dave Goulson, Ph.D., of Sussex University, writes in a commentary in Nature that the study was “the first to provide direct evidence that the widespread depletion of insect populations by neonicotinoids has knock-on effects” on larger animals. Goulson has done work on the far-reaching effects neonicotinoids have on biodiversity and ecosystem health; a review of his from last year found that not only are neonicotinoids the most widely used insecticides in the world, but they persist and accumulate in soil, are prone to leaching into waterways, commonly exceed the LC 50 (the concentration which kills 50 percent of individuals) for beneficial organisms, and the consumption of small numbers of treated seeds presents a direct risk of mortality in birds and mammals.

Sound familiar? The link between pesticide use and birds is not a new one. Rachel Carson’s book, Silent Spring, chronicled the profligate use of pesticides and their effects on the environment and on birds in particular. While Carson wrote specifically about DDT, an organochlorine pesticide, the message is similar—neonicotinoid pesticides effects have been shown to have widespread consequences on beneficial insects, the environment and birds.

Read more about how neonicotinoids affect non-target organisms, or Pierre Mineau’s, Ph.D., in-depth presentation with the American Bird Conservancy on the impact of insecticides on birds. You can also visit our BEE Protective page to learn more about how honey bees and other pollinators are going through rapid population declines, and what you can do to help. 

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