A sign, required by law, warns of a pesticide application of methyl bromide on a field near Watsonville, Calif., Aug. 12, 2005. The pesticide is used to fumigate the soil as preparation for strawberry planting. The U.S. continues to permit the methyl bromide to be used despite signing an international treaty that banned its use by 2005. Its survival demonstrates the difficulty of banishing a chemical that is a powerful toxin but that also helps deliver abundant, pest-free and affordable produce for farmers and consumers.