DAVID STEINER FIRST NOTICED THEM at his Suisun Marsh duck club, Joice Island Mallard Farms, a couple of years ago. "You'd see this animal swimming with just its head above the water," Steiner said.

Any mists spotted rising over the swamp may just be Washington wielding its newest weapon in its never-ending war on rats: dry ice. The District of Columbia's rodent control division's program manager ...