Bats have an image problem. Throughout much of the world, they "often are associated with the devil, the underworld and the supernatural," write Michael J. Harvey, J. Scott Altenbach and Troy L. Best, authors of the new field guide  Bats of the United States and Canada  (Johns Hopkins University Press, 2011).  In fact, these flying mammals are among the most agriculturally and ecologically beneficial of animals, consuming vast numbers of insects, dispersing seeds, and pollinating plants. A study published last year in Science showed that insect-eating bats save the U.S. agricultural industry at least $3 billion per year. Yet most North American bat species are in decline as a result of white nose syndrome , habitat loss and extirpation by humans. The book discusses threats to bat populations, efforts to save them and the latest research on how the nocturnal creatures live, breed and feed.