CANADA THISTLE [CIRSIUM-ARVENSE (L) SCOP] RESPONSE TO DENSITY OF CASSIDA-RUBIGINOSA MULLER (COLEOPTERA, CHRYSOMELIDAE) AND PLANT COMPETITION

Response of Canada thistle, Cirsium arvense (L.) Scop., to density of Cassida rubiginosa Muller (Coleoptera: Chrysomelidae), a defoliator, a nd plant competition from a combination of tall fescue, Festuca arundi nacea Schreb., a grass, and crownvetch, Coronilla varia L., a legume, was determined in a a-year field study. Both C. rubiginosa and plant c ompetition significantly reduced biomass and survival of Canada thistl e. In both years, C. rubiginosa had a greater detrimental effect on th e thistles than plant competition. When there were significant plant c ompetition effects, thistle biomass and survival were consistently hig her in treatments without plant competition at all densities of C. rub iginosa 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, t all fescue provided Canada thistle with the primary source of plant co mpetition. Treatments without plant competition produced more dicots ( excluding Canada thistle) than treatments with plant competition, indi cating the added advantage of tall fescue and crownvetch in keeping ou t other nondesirable vegetation as Canada thistle is suppressed. This study demonstrates that long-term control of Canada thistle is achieva ble with C. rubiginosa and plant competition. (C) 1995 Academic Press, Inc.