Insects may not be the only collateral damage from a controversial group of insecticides.
The class of chemicals, called neonicotinoids, is used in agricultural fields worldwide to reduce crop-eating pests. Since neonicotinoids were introduced in the 1990s, researchers have reported unintended harm to bees and other pollinators (SN Online: 4/5/12). But a new study suggests that the deadly chemicals may also cause declines in bug-eating bird populations, a possible sign of cascading effects in the environment.
“The effects could be more widespread in the ecosystem than we thought,” says ornithologist Ruud Foppen of Sovon, the Dutch Center for Field Ornithology in Nijmegen.
Foppen and colleagues analyzed Dutch data on water pollution, which can act as a proxy for pesticide usage because neonicotinoids travel to waterways in farm runoff. The researchers focused on concentrations of imidacloprid, the most commonly used neonicotinoid, which had been measured in 3,947 waters samples from 555 locations across the Netherlands between 2003 and 2009. The researchers compared those pollution levels with bird surveys from 2003 to 2010 of 15 farmland species, such as barn swallows and common starlings. All of the bird species the researchers analyzed eat insects, nine exclusively.
The researchers found a general pattern: the more imidacloprid in waterways, the greater the downturn in bird numbers over time. In areas with more than 20 nanograms of the chemical per liter of water, bird populations shrank by 3.5 percent each year. The researchers found no link between the drops and other factors, such as land-use changes including changes in crops. The team also saw no link between the drops and bird population declines that had started before imidacloprid use.
The team speculates that when high levels of the pesticide spread beyond farms, they kill off swaths of insects, cutting into birds’ food supply. But the researchers acknowledge that the pesticide could also kill birds directly.
Although the Netherlands is unique in having such complete datasets of both bird populations and water pollution, the country is not alone in its use of neonicotinoids, the authors say. Researchers have found similar concentrations of the insecticides in other countries, suggesting a widespread threat to birds, the Dutch team says July 9 in Nature.
Ecologists have suspected that neonicotinoids cause ecological problems for species beyond pollinators, but this is the first study to find direct evidence, says ecologist Dave Goulson of the University of Sussex in Brighton, England. “This moves the debate on quite a bit,” he says.
In 2013, the European Commission restricted the use of three neonicotinoids, including imidacloprid, for two years because of concern about harm to bees. The U.S. Environmental Protection agency has begun reviewing neonicotinoids’ effect on pollinators. On July 7, the Natural Resources Defense Council petitioned the U.S. Environmental Protection Agency to ban the use of these chemicals.