MELBOURNE-- To stop millions of foliage-eating introduced rabbits from turning huge tracts into desert, Australia has become the only nation to successfully use a biocontrol agent on a vertebrate, namely the European rabbit calicivirus disease. The virus appears to be a stunning success--rabbit numbers are way down and once barren deserts are blooming. Yet for biocontrol officials, the calicivirus experience has been a major embarrassment, a sobering lesson in the unpredictability of biocontrol agents.