POTENTIAL IMPACT OF THE STEM-BORING WEEVIL LIXUS-CARDUI ON THE GROWTHAND REPRODUCTIVE CAPACITY OF ONOPORDUM THISTLES

Using plants grown in large field cages, it was shown that heavy feedi ng by the weevil, Lixus cardui, reduced both the plant growth and repr oductive 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 we re smaller, suffered higher levels of abortion and produced ca. 80% fe wer viable seeds. Attacked plants senesced 2-3 weeks earlier than unat tacked plants and late-developing capitula aborted causing a shortenin g of the normal flowering period. At extreme density levels, where sur vival of the weevils themselves was affected, damage levels were even greater. It was not possible to separate the contributions of damage b y 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 cambi um tissue. This was, however; not essential for the purpose of demonst rating the weevil's potential to contribute to biological control of t he target thistles. It was considered that even a lower impact in the field would enable L. cardui to complement agents that had already bee n released or were planned for release against Onopordum spp. in Austr alia and would reduce the competitiveness of these weeds in infested p astures.