BELLEVUE, Wash. -- A quarter-century ago, American rocket scientists proposed the "Star Wars" defense system to knock Soviet missiles from the skies with laser beams. Some of the same scientists are now aiming their lasers at another airborne threat: the mosquito.

In a lab in this Seattle suburb, researchers in long white coats recently stood watching a small glass box of bugs. Every few seconds, a contraption 100 feet away shot a beam that hit the buzzing mosquitoes, one by one, with a spot of red light.

The...