Can an a priori strategy be developed for biological control? The case of Onopordum spp. thistles in Australia

Between 1992 and 2000, seven insect agents were released in Australia for the biological control of Onopordum spp. thistles. This paper describes the protocol used for the selection of these agents, starting with the development of a preliminary strategy, based on the ecology and population dynamics of the target weed. The strategy informed the surveys for natural enemies in the native range of Onopordum, targeting insects that attacked key transitional stages of the weed's life cycle. Ongoing studies of Onopordum populations in both Australia and Europe, plus experimental studies on the ecology, potential impact and preliminary host specificity of the agents, led to the refinement of the strategy and the selection and prioritisation of the agents. It is argued that development of an explicit strategy prior to release should be encouraged, as it forces researchers to revisit the ratio- nale for and aims of particular biological control projects, ensuring that the process of agent selec- tion remains focused. It also provides a tool to improve the process of agent selection, as subsequent results can be measured against the strategy and agent success or failure evaluated against the a priori expectations.