A pesticides ban in Europe could soon be overturned on the grounds that it was based on unreliable data. Meanwhile, revelations that one of the scientists behind the ban was also involved with a nongovernmental organization that campaigns against pesticides continue to undermine the ban’s integrity.

Two European chemical companies, Bayer and Syngenta, appeared before the European Court of Justice this week to argue that the European Union should revoke a ban on neonicotinoid pesticides. “Neonics,” as these sprays are known,...