The leaf-feeding beetle, Cassida rubiginosa, has no impact on Carduus pycnocephalus (slender winged thistle) regardless of physical constraints on plant growth

The leaf-feeding beetle, Cassida rubiginosa, is an oligophagous biocontrol agent capable of feeding on most species in the tribe Cardueae (thistles and knapweeds). The beetle was released in New Zealand in 2007, primarily to control Cirsium arvense (Californian thistle), with the recognition that it had potential to control multiple thistle weeds. The objective of this study was to test the impact of different densities of Cassida rubiginosa larvae (0, 50, 100, or 200 per plant) on the growth and reproductive performance of the annual thistle weed, Carduus pycnocephalus (slender winged thistle). Since the effectiveness of biocontrol agents is often enhanced when plants are stressed, different levels of growth constraint were imposed by growing the weed in different pot sizes (0.5, 1, 5, and 12 litres). We hypothesised that feeding damage by Cassida rubiginosa larvae would have a greater impact on the weed when grown in smaller pots, since root growth would be constrained, and the weed’s ability to compensate for feeding damage would be restricted. Contrary to our hypothesis, pot size had no effect on feeding damage by Cassida rubiginosa on Carduus pycnocephalus. As expected, most measures of plant performance increased with larger pot sizes, including plant height, biomass, and the number of seedheads per plant. The results of this study indicate that Cassida rubiginosa is unlikely to contribute to the control of Carduus pycnocephalus. Additional oligophagous biocontrol agents targeting the rosette stage and seed production should be considered for release in New Zealand.

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

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 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.

Introduced leaf beetles of the Maritime Provinces, 7: Cassida rubiginosa Muller and Cassida flaveola Thunberg (Coleoptera: Chrysomelidae)

Cassida rubiginosa Müller is newly recorded in Nova Scotia and on Prince Edward Island. Cassida flaveola Thunberg is newly recorded in Nova Scotia, on Prince Edward Island, and in the Maritime Provinces as a whole. The systematics, identification, introduction history, distribution, zoogeography, biology, parasitism, predation, host plant preferences, and biocontrol potential of both species are surveyed in detail. Early records of C. flaveola are also recounted and questions are posed with respect to its presence and origins in the region and whether it may have been introduced via human agency. Although previously considered an introduced Palearctic species, recent evidence indicates that C. flaveola is, at least in part, a native species with a Holarctic distribution.