Impact of the rosette crown weevil Trichosirocalus briesei on the growth and reproduction of Onopordum thistles.
Several European thistle species of the genus Onopordum have become naturalized in Australia and are considered serious pasture weeds, leading to a project aimed at their biological control. The population structure of a potential control agent, the crown weevil T. briesei, a newly described member of the T. horridus species group, was studied under natural conditions on its Onopordum spp. host plants in northern Spain, during 1994-96. T. briesei was found to be the most abundant of three species of crown-feeding insect utilizing these plants. Adults of this univoltine weevil began to lay eggs in mid-autumn and larval population sizes increased over winter until pupation in early spring. The spatial distribution of the immobile larvae was not random, indicating that females were selective in their oviposition behaviour. Field data suggested that plants in dense patches were attacked less frequently, and larger rosettes were preferred for egg-laying. A preliminary cage experiment indicated that feeding by T. briesei could damage Onopordum spp. thistles. A second experiment using plants in individual cages showed a clear relationship between the density of the weevils and the reduction in several growth parameters and seed production. At the highest densities, larval feeding could kill Onopordum rosettes before they produced flowering stems. These results indicated that T. briesei has the potential to contribute to the biological control of Onopordum spp. thistles in Australia. Following host-specificity testing, the weevil was released in 1997.