Charlotte Smith explores threats to the survival of bees. One in three mouthfuls of food comes from insect, principally bee, pollinated crops. Yet last year a third of British bee colonies collapsed. One controversial theory is that bees are being weakened by exposure to a type of pesticide called neonicotinoids. These are derived from nicotine and are used to kill insects like aphids. They've been banned for use on some crops in France, Germany, Italy and Slovenia, but the UK government insists there's no case for doing so in this country.