Life history of the cotton thistles capitulum weevil, Larinus latus (Col.: Curculionidae) and its impact on seed production in Urmia region, Iran

Cotton thistles, Onopordum spp. (Asteraceae), are plants native to Mediterranean basin, southwest and central Asia. Their reproduction is entirely by seed and the plants are biennial. The biology of cotton thistles capitulum weevil, Larinus latus (Herbst), was studied on Onopordum acanthium and O. leptolepis in fields and rangelands of Urmia. The weevil has a single generation annually and overwinters as an adult. Adults emerge from hibernation late May concurrently with the thistles starting to produce bolting stems. After emerging, they feed on the leaves, capitula and pollen of Onopordum species and mate from time to time throughout adult's life span. Then females lay eggs from the onset of the capitulum development until the completion of flowering. Eggs are laid singly by the female in between the bracts or at the base of the flower heads. Females cover their eggs by feces to protect them from natural enemies and dehydration. The mean number of eggs laid per female was 36.7 ± 3.6. Upon hatching, the young larvae bore into the capitulum where they feed on receptacle tissue and developing seeds. The larvae of the capitulum weevil are very destructive and one larva can destroy all the seeds in a flower head of 20-37 mm in diameter. In natural conditions, the development of L. latus from egg to emerging adult takes approximately six weeks. Mature larvae produce pupal cells in the margins of seed head and then pupate. A new generation of adults emerge from pupa from early August to the beginning of September and search for a protected site to pass the autumn and winter. The results showed that this weevil is responsible for the loss of 38-40% of seeds in O. acanthium and 49-57% in O. leptolepis. Two species of parasitic wasps, Baryscapus cf. crassicornis (Hym.: Eulophidae) and Bracon (Rostrobracon) urinator (Fabricius) (Hym.: Braconidae), were collected from the larvae of L. latus, of which the latter species, B. urinator, is newly recorded from Iran.