STRUCTURE OF THE PHYTOPHAGOUS INSECT FAUNA OF ONOPORDUM THISTLES IN THE NORTHERN MEDITERRANEAN BASIN

Thistles of the genus Onopordum (Asteraceae: Cardueae) have become ser ious weed problems in parts of Australia and North America following i ntroduction from their native Eurasian ranges, and are the target of a biological control programme in Australia. This paper analyses the re sults of three separate surveys of insects Feeding and breeding on thi stles of the genus Onopordum in the Mediterranean region. Of the 129 s pecies found feeding, 74 species also bred on these thistles. Endophag es comprised 54% of the breeding insects, with species that fed in the capitula and within the rosette crown/root area predominating. The Co leoptera and Hemiptera were the dominant endophage and ectophage taxa, respectively. Differences in the Onopordum fauna were noted between h ost species and between geographic regions, but this was largely due t o the generalist component of the fauna. Onopordum specialists showed little variability across regions or between different species within the host genus. On comparing the fauna of Onopordum with that of other thistle genera, it was suggested that plant-herbivore interactions be tween Onopordum and its fauna are less highly evolved, in that there i s less host specialization and a lesser degree of niche partitioning. This appears to be related to the low rate of speciation and habitat s pecialization within the host genus itself. The potential of certain g enus-specific insects as biological control agents for Onopordum spp. is discussed. It is argued that the absence of a high degree of niche specialization could favour successful biological control, as the agen t would be capable of inflicting damage over a wider range of resource gradients.