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6 What’s New In Biological Control of Weeds Working under a tight time frame, we were not able to track down enough information to support or discount a number of hypotheses, such as the importance of host plant quality, genetic variability and susceptibility to secondary infection. Further research into the signifi cance of these factors is needed. However, our analysis did reveal that biocontrol impacts have, on average, been greater against biennial and perennial versus annual weeds, plants capable of vegetative reproduction versus those that reproduce solely by seed or spores, aquatic and wetland weeds versus terrestrial weeds, and plants that are not reported to be weedy in the native range versus those which are known to be weedy in the native range. Having sorted out the key factors, we developed a prototype scoringframework, and ran examples of South African and US biocontrol targets through it. “To validate our framework we compared our likely impact of biocontrol scores with the impact index scores we had calculated earlier and found that there was a good correlation between the two, so we were comfortable that our framework was on the right track,” explained Quent. We presented our preliminary framework at a workshop in Canberra to obtain feedback on the proposed scheme and seek further information about Australian weeds. Following the workshop we further refi ned the scoring system by incorporating data from Australian weed biocontrol programmes and undertaking a sensitivity analysis to ensure questions were weighted appropriately (see Table 2). We then ran all 112 Australian weeds nominated as targets for biocontrol through the framework. Next we excluded species for which biocontrol programmes Table 1: Plant traits that might aff ect biocontrol success Hypothesis Our analysis Plants that are weeds in their native range are harder to control. Weeds with closely related non-target plants are harder to control. Annuals are more diffi cult to control than biennial and perennial weeds. Clonal weeds that reproduce solely vegetatively are easier to control. Terrestrial weeds are harder to control than aquatic/wetland weeds. Success is more likely for weeds that belong to a genus with many species (i.e. more agents available). Host plant quality aff ects success. Biocontrol is more diffi cult if there is diff erential susceptibility due to genetic variability (e.g. lantana). Plants that are susceptible to secondary infections are more susceptible to biocontrol. Success is greater against environmental weeds than agricultural weeds. were considered completed and prepared a prioritised list of the remaining 75 species (see Table 3). While the framework appears to be robust and useful, as always there is room for improvement: “While we can now identify programmes that are likely to be ‘winners’ or the most diffi cult targets, with a fair degree of confi dence, there are still many intermediate scoring weeds, where predicting success or failure is still a bit of a lottery,” said Quent. Our analysis only explains about half the variation in the Issue 48 May 2009 There was some evidence from the USA to support this. No, but non-target attack on native plants was a minor consideration in the past. We consider this to be an important factor because many past programmes would not be able to proceed under today’s regulations. Yes for the USA, data not available for South Africa where only one annual weed has been targeted for biocontrol. There was evidence from South Africa to support this There was some evidence from South Africa and the USA to support this. No evidence was found to support this. Not enough information was available to assess this. This appears to be important, but there was not enough information available to assess this statistically. There was not enough information available to assess this. We did not attempt to assess this because many weeds do not fall neatly into one or other category. success of past biocontrol programmes. If we can identify further factors that aff ect biocontrol success and modify the scoring system accordingly, we should be able to make signifi cant improvements to the predictive power of the framework. For example, the relevance of criteria that authors have suggested are important, such as food quality, but which we were unable to include due to a lack of data should be investigated. For this reason, we have recommended that the framework is expanded and reviewed regularly and revised as more weed biocontrol impact
What’s New In Biological Control of Weeds Issue 48 May 2009 Table 2: Excerpt from framework 9. Habitat Score Weight Aquatic/wetland Higher probability of success. 10 3.5 Terrestrial Lower probability of success. 4 3.5 Table 3: Top 20 Australian weed targets for biocontrol Rank Weed Likely Biocontrol Impact Weed Importance Eff ort Total Impact × Importance × 1/Eff ort 1 Spartina anglica 90.00 62.34 21.00 267.16 2 Alternanthera philoxeroides 100.00 67.53 26.00 259.74 3 Cabomba caroliniana 100.00 79.65 38.00 209.61 4 5 6 Chrysanthemoides monilifera Solanum elaeagnifolium Macfadyena unguis-cati 64.00 84.85 26.00 208.86 100.00 51.08 25.00 204.33 77.00 64.94 26.00 192.31 7 Prosopis spp. 50.00 99.13 26.00 190.64 8 Ulex europaeus 56.00 70.13 21.00 187.01 9 Tamarix aphylla 59.00 77.92 26.00 176.82 10 Schinus terebinthifolia 76.00 59.74 26.00 174.63 11 Parkinsonia aculeata 62.00 100.00 38.00 163.16 12 Nassella neesiana 52.00 78.35 26.00 156.71 13 Nassella trichotoma 52.00 77.06 26.00 154.11 14 15 Genista monspessulana Hymenachne amplexicaulis 59.00 52.38 21.00 147.17 85.00 82.68 48.00 146.42 16 Rubus fruticosus agg. 48.00 93.07 31.00 144.11 17 Anredera cordifolia 76.00 50.65 28.00 137.48 18 Salix spp. 75.00 77.49 43.00 135.16 19 Argemone ochroleuca 59.00 35.50 17.00 123.20 20 Thunbergia grandifl ora 76.00 64.50 40.00 122.55 data become available. Methods for ranking weed importance also need further debate. Should new weeds of currently limited importance, that are known to be serious weeds in other countries, be attibuted a higher weighting compared with widespread medium-inportance weeds that have been around for ages? Also, how do we factor in the likely infl uences of climate change on weed distribution and abundance? While it is helpful to have tools like this framework to guide us in our decision making, we believe that some pragmatic decision making should also always be used when deciding on a portfolio of targets for biocontrol. While it is tempting and sensible to pick off some of the easier targets fi rst, there may be good reasons for embarking on some higher risk projects, such as when other eff ective control methods are lacking for a very serious invader. “After all, a 5% reduction in a really important weed doesn’t sound like much, but can result in economic or environmental benefi ts that outweigh the complete control of a minor weed,” added Quent. While developed initially for Australia this framework could be used to rank weed biocontrol targets for New Zealand or any other country. However, before this could happen a lot of data would need to be collated, especially on the importance of each weed and the likely ease of undertaking a biocontrol project. This project was funded by Land and Water Australia as part of the Australian Government’s Defeating the Weed Menace Programme. A full report on this project is available from www.lwa. gov.au/weeds 7
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