Biological control of weeds is the deliberate use of natural enemies to reduce the density of a particular weed to a tolerable level The objective of biological weed control is not eradication of the weed entirely, but simply the reduction of the weed population to an economically low level.
For biological control to be continuously successful, small numbers of the weed host must always be present to assured the survival of the natural enemy.
The two most frequently cited examples of successful biological weed control are the destruction of the prickly pear cacti in Australia by an imported moth and the control of St. Johnswort on range land in California, Oregon, Washington, and British Columbia, Canada.
These examples demonstrate that biological control can provide a permanent solution to serious weed problems. The first insects were introduced 50 years ago and the Australian prickly pear is still under control. St. Johnswort is presently controlled at two per cent of its former density in British Columbia.
Weeds are plants growing where they are not wanted and most of our serious weeds have been introduced from other parts of the world. One of the reasons why these plants are so noxious in their new habitat is that their natural enemies are often absent. Biological control has most frequently been applied against these alien weeds and attempts are made to restore the natural control of these weed pests by introducing one or more host-specific, damaging natural enemies from the native region of the weed.
Insects have been most frequently used as biological control agents of weeds and this will likely continue. The reasons are that there have been major successes using phytophagous insects and almost all of the scientists working in biocontrol of weeds are entomologists. However, recent research has demonstrated the potential of other organisms, including plant pathogens, nematodes, and fish for the control of weeds.
Plant pathogens offer two advantages over insects as biocontrol agents of weeds: they are often more host specific, and they can be applied with conventional spray equipment at a time when the weed is at its most susceptible stage.