Potential impact of the stem-boring weevil Lixus cardui on the growth and reproductive capacity of Onopordum thistles

Using plants grown in large field cages, it was shown that heavy feeding by the weevil, Lixus cardui, reduced both the plant growth and reproductive capacity of Onopordum thistles. At high initial densities of adult weevils, the plant height and biomass were reflected by up to 50%. Fewer capitula were produced on Lixus-attacked plants, and these were smaller, suffered higher levels of abortion and produced ca. 80% fewer viable seeds. Attacked plants senesced 2-3 weeks earlier than unattacked plants and late-developing capitula aborted causing a shortening of the normal flowering period. At extreme density levels, where survival of the weevils themselves was affected, damage levels were even greater. It was not possible to separate the contributions of damage by adults, which destroyed 30-40% of leaf tissue, from that by larvae, which hatched from eggs laid in the stems and mined the pith and cambium tissue. This was, however; not essential for the purpose of demonstrating the weevil's potential to contribute to biological control of the target thistles. It was considered that even a lower impact in the field would enable L. cardui to complement agents that had already been released or were planned for release against Onopordum spp. in Australia and would reduce the competitiveness of these weeds in infested pastures.