Folivory impact of the biocontrol beetle, Cassida rubiginosa, on population growth of Cirsium arvense

The folivorous beetle, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae), was released in New Zealand in 2007 as a biocontrol agent against the pasture weed, Cirsium arvense (L.) Scop. The impact of the beetle on shoot population density and spread was assessed over two years on isolated experimental plots within an established population of the weed. Four folivory treatments were imposed by applying 0, 5, 10 or 20 larvae per shoot. Folivory in the 10 and 20 larvae per shoot treatments caused C. arvense population declines of 29% and 75%, respectively, although this effect was not consistent between the two years. Shoot spread was reduced in both years where 10 or 20 larvae per shoot were applied. This study represents the first post-release assessment of this biocontrol agent in New Zealand, and indicates that average densities of ≥ ten larvae per shoot can reduce population density and spread of C. arvense.