Farms are battlefields, pitting growers against rapacious pests and aggressive weeds in never-ending, costly campaigns that often involve chemical weapons. Those weapons, alas, also harm innocent bystanders such as bees, fish, and crustaceans. Now, a large study charts epic shifts that have occurred in recent decades as U.S. farmers have changed their arsenal of pesticides. Birds and mammals have fared much better, whereas pollinators and aquatic invertebrates are suffering. The toxic impact to land plants has also skyrocketed, likely because farmers are using increasing kinds of chemicals to fight weeds that have become resistant to common herbicides.