Unpublished field trials by pesticide manufacturers show their products cause serious harm to honeybees at high levels, leading to calls from senior scientists for the companies to end the secrecy which cloaks much of their research. The research, conducted by Syngenta and Bayer on their neonicotinoid insecticides, were submitted to the US Environmental Protection Agency and obtained by Greenpeace after a freedom of information request. Neonicotinoids are the world's most widely used insecticides and there is clear scientific evidence that they harm bees at the levels found in fields. Neonicotinoids were banned from use on flowering crops in the EU in 2013, despite UK opposition. The newly revealed studies show Syngenta's thiamethoxam and Bayer's clothianidin seriously harmed colonies at high doses, but did not find significant effects below concentrations of 50 parts per billion (ppb) and 40ppb respectively. Such levels can sometimes be found in fields. However, scientists said all such research should be made public. "It is hard to see why the companies don't make these kinds of studies available," said Prof Dave Goulson, at the University of Sussex. "It does seem a little shady to do ... the very studies the companies say are the most important ones - and then not tell people what they find." Syngenta had told Greenpeace in August that "none of the studies Syngenta has undertaken or commissioned for use by regulatory agencies have shown damages to the health of bee colonies". Goulson said: "That clearly contradicts their own study."