The aim of biological control is to restore the natural balance between the pest and its enviroment by the introduction of natural enemies which can help control the pest to a level where it is no longer considered a problem.
The process of introducing and releasing a natural enemy of a weed into Australia is a lengthy one. The first step is to gain approval from the Standing Committee on Agriculture and Resource Management to declare the plant a target for biological control. After an extensive consultative process, the Committee will declare the plant target if there is sufficient benefit to the community in having the weed controlled. Then a search is made for natural enemies which feed on it and control its population density. As most weeds are introduced into Australia, we need to look in the country of origin of the weed to locate its natural enemies. The natural enemies which appear to have the greatest impact on the weed are brought back to Australia and tested in quarantine to make sure they do not become pest themselves. This is done by determining the range of plant species the biological control agents will feed on that are economically important, such as vegetables, ornamentals, and native plants that are closely re lated to the weed species. If the agent feeds and successfully reproduces on plants of economic and ecological importance other than the target weed, then it is not "host specific" and is not considered safe for release into Australia. Agents that are specific to the target plant will then be given approval for release by the Australian Quarantine and Inspection Service (AQIS) and Environment Australia, if a review which includes expertise from State and Territory Departments of Agriculture and Conservation, unanimously agree.
Once approved for release, the agents are reared in large numbers for release in weed infestations. The release sites are closely monitored for establishment. If establishment is successful, further studies are carried out to assess the impact of the agent and to carry out redistribution programs if the agent is slow to spread.
Biological control programs are therefore lengthy due to the time it takes to locate, test and monitor the agents. It is therefore not uncommon for individual programs to run 10 years or much longer. In addition, it is not possible to predict the likely level of control that might be achieved by the agents before they are released. This is because it is uncertain:
- whether any agents that are released will actually establish.
- what the population repsonses of the agents will be in their new environment without their natural enemies.
- the effect of climate on control agent performance.
- what the impact of native parasites and predators will be on the agents.
Furthermore, once the agent or agents are established in a given locality, the degree of control achieved can vary from site to site depending on how site conditions or different land management techniques suit either the pest or the control agents. In some places total control may be achieved while in others the agents may not affect the pest. Control can also vary from year to year according to weather patterns.
Biological control does not involve the eradication of a pest species, but reduces the vigour of the pest to a stage where it can be controlled by traditional methods less often and at lower level.
Even if only partially successful, the long term economic benefits of biological control can be enormous. The benefits are permanent and, for weeds in agricultural areas, accrue through reduced farmer inputs, reduced herbicide usage and improved management options for the weed infested land.
There have been a number of successful biological control programs on weeds carried out in Australia. One of the best was the release of the salvinia weevil to control a huge infestation of salvinia on Lake Moondarra - the Mt Isa water supply dam. Three thousand insects were released and multiplied to an estimated population of 100 million destroying 40,000 tonnes of salvinia over several months. Perhaps the most often quoted success was the introduction of a moth called Cactoblastis cactorum to control prickly pear. Over a period of about 9 years in the 1920's and 1930's the insect cleared about 20 million hectares of land in northern Australia.
In Tasmania, research into the biological control of weeds is being conducted by the Tasmanian Institute of Agricultural Research , based at New Town Laboratories near Hobart. The major work programs at present are focussing on ragwort, gorse and horehound. They are among the most important agricultural weeds in Tasmania. Ragwort and gorse alone cause multi-million dollar losses in lost production annually.
A biological control program for ragwort which began in Tasmania in 1979 is now achieving some major successes. The aim now is to facilitate the biological control process by getting community groups and schools involved in the project.
 
|
