J. Vitou

Prevalence and impact of the crown fly, Botanophila spinosa, on its host thistle, Onopordum acanthium , in southern France

Feeding by larvae of the crown fly, Botanophila spinosa, was found to directly reduce the growth of Onopordum acanthium rosettes during spring and to indirectly reduce the subsequent growth of flowering stems and seed production. The impact of feeding damage by larvae was greatest when plants were attacked in early spring and declined to be non-significant for plants attacked later in April and May. No pattern in attack levels was observed between patches. Within patches, there was a trend for increased attack on larger, more apparent rosettes and the distribution of attack per plant was aggregated due to variability in the egg-laying behaviour of B. spinosa, resulting in more concentrated attack on some individual plants than expected if attack were random. B. spinosa was considered capable of complementing the already released crown weevil, Trichosirocalus briesei, to extend the period of attack by biological control agents on Onopordum rosettes in Australia, particularly in the case of O. acanthium. Following host-specificity testing, it was released in 1999, though establishment has not yet been confirmed.