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Neonicotinoids, a widely used class of insecticide, are suspected of harming honeybee populations. But there is growing evidence that they might also have detrimental effects on other important pollinators.
Researchers believe neonicotinoids might at least in part be responsible for the mass deaths of honeybees around the world. Now two studies show that this class of insecticide could also harm wild bees and butterflies.
Ben Woodcock and colleagues from the Centre for Ecology and Hydrology in the UK examined changes in the occurrence of 62 wild bee species across England between 1994 and 2011 - the period when neonicotinoids became widely used.
They found "evidence of increased population extinction rates" of wild bees in places where neonicotinoids were applied to oilseed rape crops.
Farmers treat oilseed rape seeds with neonicotinoids before planting them. This way, the whole plant including pollen and nectar will contain neonicotinoids in later stages of growth.
According to a study published in "Nature Communications", the decline of wild bees was on average three times higher among species that regularly feed on these oilseed rape crops compared to species that feed on other plants.
"As a flowering crop, oilseed rape is beneficial for pollinating insects," says Ben Woodcock. "This benefit however, appears to be more than nullified by the effect of neonicotinoid seed treatment on a range of wild bee species."
He stresses, however, that wild bee population declines might also have other causes, including habitat loss, pathogens and other insecticides.
Unlike honeybees, most wild bees live a solitary life and do not form colonies.
Worldwide, several thousands of wild bee species exist. They all prefer different kind of plants and are economically important pollinators, just like honeybees.
Neonicotinoid insecticides might also harm butterflies, US researchers report in another study which was published in "Biology Letters".
Matthew Forister of the University of Nevada and colleagues found a "dramatic decline in the number of butterfly species" in lowland Northern California.
The butterfly decline in this region started in the late 1990s. Neonicotinoid use in the region began in 1995. Over time, this insecticide class replaced other substances used before.
The higher the amount of neonicotinoids applied, the fewer the number of butterflies, the researchers found. Butterfly species with smaller bodies and which produce fewer generations per year seem to suffer particularly badly.
Bad news for reproduction
Researchers have already demonstrated that neonicotinoids probably harm honeybees - even in small doses.
German researchers in Mainz found that the insecticides interfere with bees' reproduction rates by reducing the amount of an important chemical in the royal jelly bees produce to feed larvae. With lower amounts of this substance, acetylcholine, more larvae are prone to die.
Neurobiologist Randolf Menzel, one of the leading bee researchers in Germany, showed that neonicotinoids make bees lose their orientation and their memory so that the animals don't find their way back to their colony.
The chemicals might also lower the sperm count of male bees, the Institute of Bee Health at the University of Bern, Switzerland, reported just a few weeks ago.
Insectide producers disagree
Neonicotinoid manufacturers, however, deny their products has any dentrimental effects on bees, as long as users keep to the appropriate dose.
"For the regulatory approval of an insecticide, its safe use in relation to bees has to be demonstrated and product authorisations specify how insecticides should be used," the //www.bayercropscience.co.uk/tools-and-services/stewardship-food-and-environment/bee-health-and-neonicotinoids/:website of chemical company Bayer reads. "Unauthorised use of such products is rare. This gives us the confidence to discount insecticide use as a major contributor to poor bee health."
They blame other factors for worldwide mass deaths of bees and other pollinators: habitat loss, air pollution and, above all, varroa mites, a parasite that spreads viral deadly diseases in bee colonies.
Companies like Bayer insist neonicotinoids are essential to maintain modern agriculture.
According to a study commissioned by neonicotinoid producers Bayer, Syngenta and Valent, neonicotinoid insecticides increased crop yields in farms across North America by up to 70 percent.
A UN report on biodiversity, however, warned earlier this year, that the worldwide loss of pollinators could threaten human food supplies and cost hundreds of billions of dollars.
A widespread pesticide
Neonicotinoids, "neonics" for short, kill sucking insects such as aphids and soil insects such as root weevils. Compared to other registered classes of insecticides, neonics are less toxic to birds and mammals, as they attack the insects' neurosystem.
The first neonicotinoid insecticide was developed in the 1980s. Neonics became widely used in the late 1990s and early 2000s. They are registered in 120 countries and applied to combat soil pests, especially on corn, canola, cotton and sugar beet crops as well as on the vast majority of fruit and vegetables.
The EU put a temporary partial ban on neonics in 2013, prohibiting their non-commercial use as well as their use on certain crops. As an additional precaution, these insecticides may only be applied after blossom time so that pollinators come into contact with the substances as little as possible. Some countries like France are planning to ban neonicotinoids in the future.
In the US, there are no restrictions on the use of these substances. The US Environmental Protection Agency says it will complete its periodic review of neonicotinoids in 2018.