The nation's top 15 nonindigenous pests, including the gypsy moth, zebra mussel, Medfly and even the common iceplant, cost the United States $134 billion in 1992, according to the U.S. Office of Technology Assessment. In an attempt to build a stronger scientific base to guide future research and public policy regarding invading species, two dozen leading U.S. and international scientists will gather May 16-19 at UC Davis for a workshop on the "Population Biology of Plant and Animal Invasions." "Although biologists have described many of these species and characteristics of their life history, we remain woefully ill-informed about the biological behavior that makes these organisms important invaders," says James Carey, a UC Davis professor of entomology and workshop coordinator. Scientists need to identify the origin of the species, the process of invasion, and the final distribution and abundance of the invading species, he says. Since the workshop is a working meeting, there is room for only a limited number of reporters.