Los festivales de teatro

For both pathogens (Thrall and Burdon, 2004) and arthropods (Hopper et al., 1993), releasing genetically diverse founder populations is considered the best strategy for biological control agent efficacy to avoid evolution of resistance in the target. Recent releases of Phragmidium violaceum rust against several blackberry (Rubus spp.) clones in Australia, by example, have adopted a strategy using multiple-characterized rust strains (Morin et al., in press). Release permits, however, are limited to single agent populations or groups of populations for arthropods and usually genetically-pure isolates for pathogens with defined specificity profiles. Whether pathogen introductions should be more acutely constrained for genetic diversity than arthropods in this way deserves more scientific debate (Morin et al., in press), but despite this, pathogens have proved highly effective biological control agents. Counter-balancing a need for genetic variability is a need to know the characteristics and genetic identity of the material released, regardless of whether or not host-mediated selection of agent strains occurs following release. There is a need to better define and regulate release strategies with respect to genetic variability that fairly encompass both arthropods and pathogens to allow the release of multiple, but characterized genetically-pure strains of agents. Streamlined protocols for assessing non-target risks to allow releases of multiple characterized strains or biotypes of agent species are also needed to ensure the best strategy can be adopted without unnecessary delays in the process.

CONCLUSIONS

The survival and longevity of biological control as a discipline depends on empirical, science-based agent selection. Grab-and-run approaches to agent acquisition may be expeditious, but ignore the science needed to ensure high agent efficacy. Biological control needs to be accountable and the number and nature of releases increasingly requires justification. A selection process based on chance alone appears hard to justify, not least because it provides no opportunity to improve or refine the process. Waage (1991) argues that it is better to make

some form of judgment based on accepted ecological principles than to flip a coin. At least the value of the judgment can be assessed in the future. While universal criteria for agent selection appear unlikely, documenting a process of assessment tailored to each project may, over many projects, provide more reliable strategies. To achieve this, strategies need to be declared from the outset, including, not only which agents will be released and in what order, but also why agents are to be released in that order. This is rarely done (e.g. Blossey et al., 1996; Briese and Zapater, 2002; Briese et al., 2002), but is vital to allow strategies across targets of similar and contrasting life history strategies to be compared and improved.

Where there is acceptance that pre-release efficacy testing can assist agent selection, the level to which such studies can be carried out will be largely determined by available resources. Project characteristics will influence the ecological investment made (Sheppard, 2003). Certainly the greater the number of potential agents per weed the greater the investment should be in the decision process, unless the proportion of potentially effective agents is constant. Whether each activity is worth the resource cost will depend on the importance of the weed and the number of agents to choose from. Biological control projects, however, are notoriously under-funded on a per annum basis to carry out all the science necessary to advance the discipline. Operational and time constraints in the program itself always force compromises. Underpinning the ecological and evolutionary science opportunities behind the discipline has traditionally been the role of university-based scientists and their research students (particularly in insect biological control), who are unconstrained by the urgency for applied outcomes. Improving agent efficacy evaluation may also require the help of such scientists to adopt particular weed biological control programs as model systems and test the theory, while the practitioners focus on delivery. As can be seen from many of the references in this paper, in Australia the Cooperative Research Centre for Australian Weed Management has tried to take this initiative.

ACKNOWLEDGEMENTS

I would like to thank André Gassmann for pointing out that nearly all temperate weed targets for weed classical biological control have been controlled by beetles and for some stimulating debate along with his colleagues, Hariet Hinz, Urs Schaffner, and others at CABI and USDA concerning where efficacy efforts should be focused in the native range. I also thank those participants of the CRC for Australian Weed Management workshop on agent selection in 2005 for access to their manuscripts prior to publication. The proceedings soon to be published in the Australian Journal of Entomology should help set the scene for agent selection for us all.

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BIOLOGICAL CONTROL OF SQUARROSE KNAPWEED IN NORTHERN CALIFORNIA: A