Biological control is the use of natural predators and diseases as agents to attack weeds. Biological control agents are usually insects, but fungi, bacteria and nematodes (eelworms) are sometimes used.
Potential biological control agents are found by examining the weed in its overseas area of origin and looking for damage caused by natural enemies of the weed. These are usually organisms that have evolved along with the weed, but for some reason did not spread with it to new habitats in Australia.
Once possible biological control agents have been identified, they are tested overseas to ensure that they are host-specific, that is, they will not attack any plant other than the weed to be controlled and therefore will not become a pest if introduced to Australia. Native and cultivated plants related to the weed are exposed to the biological control agent to see if they too are attacked. Other plants not related to the weed, especially those that it would not have encountered in its home range, are also tested.
Many insects will starve rather than attack another plant. Some agents are so specific that they will attack only one variety of a plant species. For example, the rust fungus Puccinia chondrillina will only attack the narrow leaf form of skeleton weed, leaving the intermediate and broad-leaved forms untouched. Others have a limited range of possible hosts. If the agent appears to be specific to its host, it is usually brought to Australia for further testing in quarantine. Final approval for release of the agent depends on further host specificity tests confirming that the agent is indeed host specific. Before release, the agent is bred under laboratory conditions to make sure that it is free of disease organisms, parasites or predators that might attack it.
Advantages of biological control
- Once the cost of testing and introducing control agents has been met, the on-going costs are small.
- In most cases, there is no need to find and identify every individual weed to be treated. An effective agent will search out all suitable plants of the weed.
- Biological control has no adverse effect on human health or the environment.
- Biological control is self-sustaining. Once the initial research work has been completed and the biological control agents released, little or no further money is required.
- Unlike certain herbicides, development of host resistance is not normally a problem.
- Biological control is compatible with most other control techniques (except sometimes the use of insecticides and herbicides).
- Biological control reduces the vigour, competitiveness and reproductive capacity of the weed, making it more manageable.
Disadvantages of biological control
- Biological control agents are expensive to find. The greatest expense is during the field survey and early testing stage which must be conducted overseas.
- Suitable agents may not even exist.
- Potential agents are also expensive to test for specificity. Host specificity testing may take several years to complete because of the need for thoroughness (however, herbicides often take as long and cost even more to develop).
- Biological control can never eradicate a pest organism completely, because if the control agent reduces the pest population too far, it destroys its own food source.
- Biological control operates over large areas. Therefore it cannot be limited to individual properties or paddock.
Further information on controlling declared plants can be found through the Declared plant control handbook link.