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Use of the weed risk assessment tool in Australia’s approach to pest risk analysis

Paul Pheloung

Office of the Chief Plant Protection Officer, Australian Government Department of Agriculture, Fisheries and Forestry, GPO Box 858, Canberra, ACT 2601, Australia; e-mail:


The weed risk assessment (WRA) system is a decision support tool that is employed by Australia in applying the principles of pest risk analysis to the review of sanitary and phytosanitary risks associated with the intentional importation of exotic plant species not yet established in the country. The WRA system is used specifically to assess the weed potential of such species. It generates a score for a species based on its history as a weed in other parts of the world, climatic suitability to Australia and biological attributes. The system has been adapted for related purposes in New Zealand and Hawaii and a modified version is under development for use in the Galapagos Islands.


Australia applies the principles of pest risk analysis to the identification and management of sanitary and phytosanitary risks associated with new trade proposals and, where the need is identified, to the review of sanitary and phytosanitary risks of existing trade. One such trade activity is the intentional importation of exotic plant species not yet established in Australia.

Of approximately 250 000 plant species in the world, 30 000 have been intentionally introduced to Australia since European colonization, along with an unknown number of unintentional introductions, of which about 2 700 have naturalized (Cunningham et al., 2003). Some are problem plants and about 300 have become serious weeds of agriculture and natural ecosystems. Clearly, the majority of plant species have yet to be introduced to Australia and some of these would become weeds if allowed into the country.

Introduced to help stem this flow, the weed risk assessment (WRA) system is one means by which the assessment component of pest risk analysis is addressed. The WRA system is a question-based scoring method that can be operated using a computer or manually using paper-based forms. A Microsoft® Excel spreadsheet version of the system, including documentation, has been made available through the International Phytosanitary Portal (at

The WRA system in operation

The score generated by the WRA system assists Australian policy makers to determine if a plant species can be introduced. The questions deal with biogeography and a range of biological and ecological attributes that are indicative of, or can contribute to, the weedy character of a plant species. The WRA system was calibrated and validated using about 350 exotic plant species that have been present in Australia for sufficient time to reveal their potential as weeds or non-weeds.

It should be emphasized that the WRA system addresses only part of the requirements of a pest risk analysis. In particular, pathway analysis is not included. However, in the context in which the WRA system is predominantly used (namely assessing the risk of plant species that an importer wishes to introduce and grow in Australia), the pathway has already been established. The WRA system is applied only to plant species that satisfy the IPPC definition of a potential quarantine pest. In other words, the candidate plant should not yet be present in the country or, if present, have a limited distribution and be the subject of official control.

Between 1997 (when the WRA system was formally adopted) and 2003, assessments have been done on close to 1 000 proposals to import plant species. The WRA score for 30 percent of these was considered too high; the risk of the species becoming a weed in Australia was too great, and the species were prohibited from entry into the country. Close to half of the species (46 percent) were allowed to be imported into Australia. The weed potential of the remaining 23 percent of species could not be determined with sufficient clarity. These species require closer examination, which could involve experimentation in order to determine if the risk is low enough to allow introduction.

The WRA system has been an effective decision support tool in Australia for managing the phytosanitary risk associated with numerous proposals to introduce living plant material while allowing the entry of plants with low weed risk reasonably expediently.

The system has been adapted for related purposes in New Zealand (Pheloung, Williams and Halloy, 1999) and Hawaii (Daehler et al., 2004). A modified version is under development for use in the Galapagos Islands.

For further information on the WRA system, including detail of the questions involved, see appendix 2, workshop exercises.


Cunningham, D.C., Woldendorp, G., Burgess, M.B. & Barry, S.C. 2003. Prioritising sleeper weeds for eradication: Selection of species based on potential impacts on agriculture and feasibility of eradication. Canberra, Australia, Bureau of Rural Sciences (available at

Daehler, C.C., Denslow, J.S., Ansari, S. & Kuo, H. 2004. A risk-assessment system for screening out invasive pest plants from Hawaii and other Pacific islands. Conservation Biology, 18: 360-368.

Pheloung, P.C., Williams, P.A. & Halloy, S.R. 1999. A weed risk assessment model for use as a biosecurity tool evaluating plant introductions. Journal of Environmental Management, 57: 239-251.

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