Maligned as a bee-killer and possibly cancer-causing, a common herbicide has turned out to be a boon for tadpoles making them more toxic to predators, researchers said Wednesday. Common toad larvae exposed to glyphosate, used in the well-known herbicide Roundup, contained higher doses of bufadienolides -- life-saving natural chemicals found in some plants and animals. Bufadienolides taste bad to put off potential tadpole diners, and can kill in large quantities.