A Strategy for the Biological Control of Onopordum spp. Thistles in South-eastern Australia

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Abstract
A strategy is described for the biological control of exotic Onopordum spp. thistles in Australia. This strategy is based on identification of critical transitional stages in the life-cycle of the weed and targeting a complementary group of insect herbivores against these stages. The strategy has both long- and short-term aims and is focussed on the long-lived soil seed bank of these thistles. In the longer term it is considered essential to reduce soil seed reserves to levels at which emergent plant densities are not economically damaging, and in the shorter term it aims to minimize the impact of those plants that do emerge during the period of seed bank decline. From more than 120 insect species found to feed on Onopordum spp. in their native European range, six were selected to form a complementary herbivore guild, targeting the small rosette ¤ large rosette, stem elongation ¤ flowering and seed production ¤ soil seed reserves transitions of their host plant. Descriptions are given of these agents and the role that they are expected to play. The importance of determining the order of agent release from the host-plant interactions is also discussed.