Biological control is achieved by introducing some of the biological agents which control their distribution and density in their native habitats elsewhere in the world. Before exotic agents can be released into Australia they are extensively tested to make sure they are host specific i.e. only attack the weed species. Once these agents are approved by the Australian Quarantine Inspection Service they are bred up in laboratory and greenhouse conditions where they can be nurtured and monitored.
Biological control will not totally eliminate a weed but it should reduce its vigour and abundance to a level that is either tolerable or that can be managed by more conventional means. In the long term, a more natural balance between the weed and its predators is achieved. Best results have occurred when more than one agent attacks a specific weed.
A successful example of biological control is the gorse spider mite, Tetranychus lintearius, which attacks the mature foliage. First released in December 1998 it is now present at 25 sites throughout the State.