Canada Thistle [Cirsium arvense (L) Scop.] Response to Density of Cassida rubiginosa Müller (Coleoptera: Chrysomelidae) and Plant Competition

Response of Canada thistle, Cirsium arvense (L.) Scop., to density of Cassida rubiginosa Müller (Coleoptera: Chrysomelidae), a defoliator, and plant competition from a combination of tall fescue, Festuca arundinacea Schreb., a grass, and crownvetch, Coronilla varia L., a legume, was determined in a 2-year field study. Both C. rubiginosa and plant competition significantly reduced biomass and survival of Canada thistle. In both years, C. rubiginosa had a greater detrimental effect on the thistles than plant competition. When there were significant plant competition effects, thistle biomass and survival were consistently higher in treatments without plant competition at all densities of C. rubiginosa tested. With 20 C. rubiginosa per plant (introduced to 12- to 15-cm-diameter rosettes), less than one-third of the planted thistles survived at the end of the season. As growth of crownvetch was slow, tall fescue provided Canada thistle with the primary source of plant competition. Treatments without plant competition produced more dicots (excluding Canada thistle) than treatments with plant competition, indicating the added advantage of tall fescue and crownvetch in keeping out other nondesirable vegetation as Canada thistle is suppressed. This study demonstrates that long-term control of Canada thistle is achievable with C. rubiginosa and plant competition.