Current status of biological control of Cirsium arvense in New Zealand

Cirsium arvense (L.) Scop. (Californian, Canada or creeping thistle) is an exotic perennial herb that successfully established in New Zealand (NZ) approximately 130 years ago, and is now considered one of the worst invasive weeds in NZ arable and pastoral systems. Two insects, Cassida rubiginosa and Ceratapion onopordi, were recently released for classical biological control. Studies carried out from 2006 to 2009 in both the native (Europe) and introduced (NZ) ranges of the plant aimed to quantify C. arvense growth characteristics and assess incidence of the specialised rust pathogen, Puccinia punctiformis, in regions with and without the supposed pathogen vector, C. onopordi. In permanent field plots natural enemies were excluded with insecticides and fungicides, and compared with controls. The impact of C. rubiginosa was also assessed under different pasture competition scenarios. The survey data indicate that C. arvense expresses similar growth characteristics in both ranges, and that incidence of the rust pathogen is similar in both ranges, regardless of the presence of C. onopordi. The data suggest that the overall suite of natural enemies is capable of exerting some regulating influence on the plant in its native range, but that the released biocontrol agents will not likely have a significant impact on this weed in NZ.