Researchers in the UK report new evidence that the pesticide fipronil, not the neonicotinoid imidacloprid, caused a massive die-off of honeybees in France from 1994 to 1998 (Proc. Natl. Acad. Sci. U.S.A. 2018, DOI: 10.1073/pnas.1804934115). Both pesticides hit the market in the early 1990s. Philippa Holder and colleagues at the University of Exeter and Fera Science, a UK public-private agricultural science venture, quantified the toxicity of the two pesticides to honeybees and used bioassays to determine the likelihood that they would bioaccumulate in bees. They used the information to predict mass mortality in a honeybee population at environmentally relevant concentrations. The researchers determined that bees rapidly eliminate imidacloprid from their bodies, but they bioaccumulate fipronil. Over time, after prolonged exposure, fipronil becomes more lethal to them. Fipronil is associated with kidney, liver, and thyroid problems in humans. Although regulators in the European Union banned fipronil for use on crops in 2017, seeds treated with fipronil can still be used in most EU countries until March 31, 2019. Fipronil is also commonly used around the world to control ants, cockroaches, and termites in structures and fleas and ticks on pets.