New mosquito repellents work more than three times as long as DEET, currently the most widely used such chemical and the gold standard by which to judge alternatives. Researchers discovered them using an artificial neural network, which they first trained by supplying it with the molecular structures of 150 known repellents. The neural network then scanned a library of roughly 2,000 untested piperidines, compounds related to the active ingredient in black pepper that can ward off insects. Human volunteers then wore arm patches impregnated with the best candidates and held their arms motionless in cages filled with roughly 500 mosquitoes for one minute each day. While DEET repelled mosquito attacks for an average of 17.5 days, some of the new compounds lasted for up to 73 days, scientists at the University of Florida and the U.S. Department of Agriculture found. Buzz about the findings in the May 27 Proceedings of the National Academy of Sciences USA.