The effect of a rosette-crown fly, Botanophila turcica, on growth, biomassallocation and reproduction of the thistle Carthamus lanatus

Plant growth and reproductive output of the winter annual invasive thistle, Carthamus lanatus was characterised in relation to plant size in three nat ive populations in southern France. The effects of the rosette-crown feedin g fly Botanophila turcica on these plant characteristics were assessed by c omparing unattacked with naturally attacked plants at each site and by a fi eld experiment. Indirect effects of B. turcica on plant seed production wer e also compared with direct seed loss caused by a guild of capitulum-feedin g insects that incidentally attacked the marked plants at these sites. C. l anatus showed no size or weight requirement for flowering, but larger flowe ring plants produced less total receptacle surface and less seed production (female reproductive potential) in proportion to plant weight than smaller flowering plants. B. turcica did not select hosts on the basis of size or density. B. turcica reduced plant relative growth rate (RGR) in all situati ons, but attacked plants compensated fully at two of three sites as attack failed to hall rosette growth. Attacked plants suffered 12 % mortality, and 71 % lower seed production than unattacked plants at the site with the low est RGR. This corresponded to 9 % lower seed production for the whole thist le population compared to 8.6-19.5 % direct seed loss to capitulum insects across all sites. (C) 2000 Editions scientifiques et medicales Elsevier SAS .