Pesticides used in farming are also killing worker bumblebees and damaging their ability to gather food, meaning colonies that are vital for plant pollination are more likely to fail when they are used, a study showed on Sunday. The United Nations has estimated that a third of all plant-based foods eaten by people depend on bee pollination and scientists have been baffled by plummeting numbers of bees, mainly in North America and Europe, in recent years. British scientists said they exposed colonies of 40 bumblebees, which are bigger than the more common honeybee, to the pesticides neonicotinoid and pyrethroid over four weeks at levels similar to those in fields. Neonicotinoids are nicotine-like chemicals used to protect various crops from locusts, aphids and other pests. “Chronic exposure … impairs natural foraging behavior and increases worker mortality, leading to significant reductions in brood development and colony success,” the scientists wrote in the report in the journal Nature on Sunday. Exposure to a combination of the two pesticides “increases the propensity of colonies to fail”, according to the researchers at Royal Holloway, University of London. A 2011 U.N. report estimated that bees and other pollinators such as butterflies, beetles or birds do work worth 153 billion euros ($200 billion) a year to the human economy and are in decline in many nations. The findings underscored the importance of wider testing of pesticides to ensure they do not also target bees, it said. France banned a neonicotinoid pesticide made by Swiss agrochemicals group Syngenta in June, citing evidence of a threat to the country’s bees.