Competitive Growth of Canada Thistle, Tall Fescue, and Crownvetch in the Presence of a Thistle Defoliator, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae)

Abstract

Canada thistle (Cirsium arvense Scop.) was grown, for one to three seasons, together with tall fescue, Festuca arundinaceae Schreb., and crownvetch, Coronilla varia L., the latter two seeded at four densities, in the presence of a thistle defoliator, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae), before the resultant vegetation was evaluated. Cassida rubiginosa was monitored and maintained at densities of at least 50 beetles per square meter. This population level of C. rubiginosa enhanced the impact of the plant competitors as insect defoliation inhibited the ability of the thistle to compete against other vegetation. Biomass of Canada thistle decreased with increase in the density of the plant competitors seeded. Reduction of biomass of Canada thistle was greatest when the induced competitors were seeded between 1.2 and 1.25 times the recommended rate of 20 kg/ha of crownvetch and 50 kg/ha of tall fescue. In terms of dry aerial thistle weight, there was a mean reduction of 69.2 ± 13.8% for the duration of competition. At lower or higher densities, induced competition was less efficient at suppressing the thistle. Overall, biomass of Canada thistle was highest after two seasons, but decreased subsequently. Fescue established more rapidly than crown vetch and was the main competitor of Canada thistle initially. Crownvetch had very little growth in the first season. When seeded up to 1.25 times the recommended rate, it increased in biomass by the third season. Biomass of other dicots showed no response to the density of induced competition within one or two seasons of growth, but by the end of the third season, log dry weight of all other dicots decreased proportionally with increased density of induced competition. Results of this study indicate that plant competition and biological control can be combined to suppress Canada thistle, and C. rubiginosa as well as induced competition can be an integral part of a comprehensive approach for area-wide Canada thistle control.