August 1997 Biological control of weeds LC0163 Anne-Marie Tenni (Melbourne), El Bruzzese and Melinda Newnham (Frankston) ISSN 1329-833X Biological control cannot eradicate a pest species. Weeds What is biological control? occur as part of complex ecosystems which vary according Biological control involves using one living species, the to climate and land use. This makes it difficult to predict agent, to control (usually suppress) an unwanted species, the degree of control that can be achieved in a given the target. Many weed species introduced to Australia situation. The effect of biological control may vary from often do not have any natural enemies here to restrict their year to year and place to place. When successful, spread. The introduction of the weed’s natural enemies can biological control reduces the weed population and slows be used to control it to a level where it is no longer down its invasion of new areas. considered a problem. Usually biological control must be used with other control methods such as cultivation and herbicide application. Biological control means that these more traditional methods of control may be needed less often and at lower levels. Biological control should be considered as part of an Integrated Weed Management Program aimed at efficiently using introduced biological control agents, physical and chemical control methods and changing management techniques so that the weed is disadvantaged. In general, biological control of weeds is best suited to infestations which have a low priority for control using chemical and cultural methods because of financial, practical or environmental constraints. In addition, sites which have a dense, chronic weed infestation which have a long time-frame for control may also be suitable. Biological control is particularly suitable for use in weed It takes time management programs. It is also useful in controlling Biological control programs are usually lengthy because of unwanted insects and animal pests, for example the the time it takes to select those natural enemies of the weed European wasp and the rabbit. which will do the most damage. This is followed by complicated studies to determine if these enemies are How it works specific to the weed and will not attack other non-target In Victoria the Department of Sustainability and species. Finally, the best methods for mass rearing the Environment (DSE) is responsible for research into the control agent, its release and establishment into the weed biological control of pest plants. All biological control population has to be determined. research is based at the Keith Turnbull Research Institute It is not uncommon for programs on one pest to span over (KTRI) in Frankston where a world-class quarantine ten years while several of its natural enemies are studied facility exists. and released in Australia. At KTRI a team of researchers study the natural enemies The classic example of successful biological control is the of target weeds. This work establishes an agent’s safety for prickly pear in Queensland earlier this century. release into the Australian environment and its This involved the introduction of 18 species of insects over effectiveness as a control agent. Only natural enemies 15 years. The outstanding success with the moth which are proven to be specific to their host and therefore Cactoblastis cactorum took a further nine years to achieve. safe, will receive approval for release from the Australian Quarantine and Inspection Service and Environment Australia. © State of Victoria, Department of Sustainability and Environment Page 1 Biological control of weeds LC0163 the growth of other plants. This means there is minimum competition for nutrients and water from other plants. For example, cereal rye secretes chemicals which inhibit other plants growing around it! The biological control process Finding suitable biological control agents for a target weed species is usually a long, involved and expensive process. 1. The cost of controlling a weed is compared with the benefits to be gained from its control. This will assess whether a biological control program is worth commencing. A weed is then proposed as a candidate for biological control research to the Australian Weeds Committee and the Agriculture and Resource While it may take many years to effectively control a weed Management Council of Australia and New Zealand. using biological control, benefits accumulate continually This system allows for various groups to comment on due to savings in herbicide and application costs and/or the proposed target so that any possible objections can increased agricultural production. Biological control is an be determined. For example, Paterson’s curse is environmentally sound method of pest control. It poses considered a weed to graziers and a useful plant to bee- negligible danger to non-target species and allows the keepers. weed to be gradually suppressed and replaced by more desirable species. Biological control types Increasing public concern over the use of pesticides in general has resulted in more interest being placed on biological and other non-chemical methods of control. Another type of biological control is the use of fungal spores or bacteria in the production of mycoherbicides. To create a mycoherbicide, the spores of the agent fungi (chosen because it will only attack the weed), are mixed in a commercial formulation and applied to the weed using the same techniques as herbicide application. There are few examples of these for weed control, but formulations 2. The taxonomy (naming), biology and ecology of the of the bacteria Bacillus thuringiensis are commonly used target weed is studied both in Australia and in its native as bio-insecticides for a number of insect pests. country of origin. 3. The country of origin of a weed is surveyed for potential biological control agents. The most damaging candidates are selected for further studies. The biology, distribution and level of damage caused by the most promising agents are studied. 4. When promising candidates are determined, permission is sought from the Australian Quarantine and Inspection Service (AQIS) and Environment Australia (EA) for the importation of proposed agents into an AQIS approved quarantine facility. Here each agent is tested against an extensive list of commercial, ornamental and native plants to determine the agent’s host range. Scientific experts from each state in Australia review both the test list and results of host In a wider context biological control also includes the use specificity testing. of grazing animals. For example, goats have been 5. Once the proposed agent is proven host specific, results successfully used to control blackberry and thistles and of tests are submitted to AQIS and EA for permission fish have been used to eat aquatic weeds which were to release. If permission is granted the agent must be choking waterways. bred through a minimum of one generation under A natural phenomenon exhibited by some plants which can quarantine conditions to ensure that it is free of its own be considered as biological control is allelopathy. In this parasites and diseases. The progeny of this initial process, a plant secretes a chemical or toxin which inhibits generation can then be released into the Australian environment. © State of Victoria Department of Sustainability and Environment Page 2 Biological control of weeds LC0163 6. The best methods for breeding large numbers of each process is slow and expensive because of the need to agent is then determined. Widespread releases are ensure the safety of the agents that are released. However, made at the appropriate time of year to ensure if successful, the savings gained from having to use less of establishment and survival of the agent. the traditional control methods will continue indefinitely. 7. The agent is evaluated over a number of years. Its Biological control cannot eradicate pests because the establishment, spread and the impact it exerts on the control agent eventually reaches a natural balance between target weed population are assessed and the program is itself and the target weed. Hopefully, this is at a level reviewed. where the target is no longer considered a pest. Usually a The process of finding suitable agents, testing, breeding combination of control methods in conjunction with and finally releasing them can take ten or more years. biological control, must be used to achieve the desired Several more years are then needed for the agents to level of control. spread throughout the area affected by the weed. Further information Several species of agents and several strains of the same Refer to Agriculture Notes and other Landcare Notes, agent may have to be introduced to ensure that including the Pest Plant and Biological Control series. the weed is controlled throughout its distribution, Contact the local office of the Department of Sustainability especially if this spans over several different climatic and Environment for more detailed advice on weed zones. management. Conclusion Increasing resources have been allocated to implement biological control of pests in Australia. The research This publication may be of assistance to you but the State of Victoria and its officers do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication. © State