Impact of the biocontrol beetle, Cassida rubiginosa, on the secondary weed target, marsh thistle (Cirsium palustre)

Background: The folivorous beetle Cassida rubiginosa was introduced to New Zealand to control the weed, Californian thistle (Cirsium arvense). Although Californian thistle is the primary host, many thistles are accepted hosts. The objective of this study was to test if the beetle can reduce the fitness of the weed, marsh thistle (Cirsium palustre). Methods: A potted plant experiment was established with four treatments (0, 50, 100, and 200 larvae/plant). Plant growth (width, height, and number of branches) and reproductive performance (number of flowers, seeds, seed weight and % germination) parameters were measured. Results: No significant differences were found for any of the measured parameters, except % germination. Higher larval densities (100 and 200) showed approximately 10% less germination compared with lower densities (0 and 50). Conclusions: Under these experimental conditions, C. rubiginosa had minimal impact on the measured performance parameters of marsh thistle, and it is likely that additional stressors would be necessary to control this weed.