Hettiarachchi, Dilani KasundaraCripps, MichaelJackman, S. D.Van Koten, C.Sullivan, Jon J.Rostas, Michael2018-10-292018-06-092018-07-26Hettiarachchi et al. (2018). Impact of the biocontrol beetle, Cassida rubiginosa, on the secondary weed target, marsh thistle (Cirsium palustre). New Zealand Plant Protection, 71, 66-71. doi:10.30843/nzpp.2018.71.1451175-9003https://hdl.handle.net/10182/10319The 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 other thistles are accepted hosts. The objective of this study was to test if the beetle can reduce the fitness of marsh thistle (Cirsium palustre). 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 percent germination) parameters were measured. No significant differences were found for any of the measured parameters, except percent germination. Higher larval densities (100 and 200) resulted in approximately 10% less germination compared with lower densities (0 and 50). Under these experimental conditions, C. rubiginosa had minimal impact on the performance of marsh thistle. For the beetle to have an impact, it would likely need to attack smaller, non-bolting rosettes, or be combined with additional stressors that might be encountered in a natural field population.66-71en© 2018 New Zealand Plant Protection Society (Inc.)green thistle beetleseed viabilityalternative hostCirsiumJournal Article10.30843/nzpp.2018.71.145ANZSRC::060808 Invertebrate BiologyANZSRC::0602 EcologyANZSRC::0501 Ecological ApplicationsANZSRC::070603 Horticultural Crop Protection (Pests, Diseases and Weeds)ANZSRC::070308 Crop and Pasture Protection (Pests, Diseases and Weeds)1179-352X