Impact of the Onopordum capitulum weevil Larinus latus on seed production by its host-plant

1. The impact of the oligophagous capitulum weevil Larinus latus on seed production by its host Onopordum species was studied in its native range in order to assess its potential as a biological control agent of these thistles in Australia. 2. Field surveys in Greece during 1988-89 showed a strong positive relationship between attack levels of L. latus and seed destruction in Onopordum species Differences in attack levels were found between populations of different species, with heavier attack on those having larger capitula. 3. In the case of O. bracteatum, attack levels attained 100% with total seed destruction. More detailed studies at one location over a full season in 1991 confirmed that L. latus is the dominant capitulum insect, being responsible for the loss of 37% of seed, compared with 8% for the lepidopteran Homeosoma nebulella and 2% for the tephritid fly Tephritis postica. 4. A regression surface was produced that estimated seed loss in terms of capitulum size and number of developing L. latus larvae. This provides a tool for measuring overall population seed loss from easily measurable parameters. 5. While able to reduce seed production by Onopordum thistles severely in a single season, the longer-term impact of these weevils on their host-plant population dynamics may not be as great. Onopordum species have long-lived seed banks and thus escape in time will dampen the effects of high seed loss in any one year. Moreover, populations of L. latus are themselves subject to density-dependent regulatory processes that would further buffer the system. 6. This weevil was released in Australia in 1992 for the control of Onopordum thistles, and these findings suggest that L. latus would be most effective as part of a complementary guild of biological control agents.