A recently approved herbicide called Imprelis, widely used by landscapers because it was thought to be environmentally friendly, has emerged as the leading suspect in the deaths of thousands of Norway spruces, eastern white pines and other trees on lawns and golf courses across the country. Manufactured by DuPont and conditionally approved for sale last October by the federal Environmental Protection Agency, Imprelis is used for killing broadleaf weeds like dandelion and clover and is sold to lawn care professionals only.