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

Abstract

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.

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

“…Overall, feeding damage from larvae of Cassida rubiginosa did not affect the growth (height, or biomass) or reproductive performance (number of seedheads and seeds) of Carduus pycnocephalus.This result was expected based on a previous study where the same larval densities were applied to Cirsium palustre and had no significant impact on the weed (Hettiarachchi et al 2018). However, we expected that the effect of the beetle would be expressed under the constrained growth conditions imposed by small pot size.…”

“…The maximum feeding damage reached was approximately 50% defoliation in the highest larval density treatment. This level of feeding damage is lower than the 60 to 70% defoliation reported for the same larval density (200 per plant) in the experiment with Cirsium palustre (Hettiarachchi et al 2018) and, therefore, was understandable that it did not have an impact. A similar experiment using Carduus nutans (nodding thistle) reported that 23% defoliation by larvae of Cassida rubiginosa had no effect on plant growth or seed production (Cartwright & Kok 1990).…”

“…Based on this finding, the impact of Cassida rubiginosa was tested on the closely related biennial thistle, Cirsium palustre (L.) Scop. (marsh thistle) at a range of larval densities (0, 50, 100, and 200 per plant) (Hettiarachchi et al 2018). Despite high levels of feeding damage (60 to 70% defoliation) the beetle had no effect on plant size or reproductive output (number of seedheads and seeds produced).…”

“…The authors noted the weed was grown in large pots (12 litre) with plenty of space, nutrients, and water, which may have enabled the plants to compensate for the damage caused by larvae feeding. The beetle might be more effective if the plants were grown under more natural conditions where they must compete for limited space and resources (Hettiarachchi et al 2018).…”

Effects of Temperature and Photoperiod on the Immature Development in Cassida rubiginosa Müll. and C. stigmatica Sffr. (Coleoptera: Chrysomelidae)

“…rubiginosa was also released in New Zealand for the same purpose 33 . In both new locations, the beetle alone only slightly reduced thistle performance, although its effect may be stronger in conjunction with other natural enemies and competitors 34,35 . Throughout its distribution range, C .…”