COMPETITIVE GROWTH OF CANADA THISTLE, TALL FESCUE, AND CROWNVETCH IN THE PRESENCE OF A THISTLE DEFOLIATOR, CASSIDA-RUBIGINOSA MULLER (COLEOPTERA, CHRYSOMELIDAE)

Canada thistle (Cirsium arvense Scop.) was grown, for one to three sea sons, together with tall fescue, Festuca arundinaceae Schreb., and cro wnvetch, Coronilla varia L., the latter two seeded at four densities, in the presence of a thistle defoliator, Cassida rubiginosa Muller (Co leoptera: Chrysomelidae), before the resultant vegetation was evaluate d. Cassida rubiginosa was monitored and maintained at densities of at least 50 beetles per square meter. This population level of C. rubigin osa enhanced the impact of the plant competitors as insect defoliation inhibited the ability of the thistle to compete against other vegetat ion. Biomass of Canada thistle decreased with increase in the density of the plant competitors seeded. Reduction of biomass of Canada thistl e was greatest when the induced competitors were seeded between 1.2 an d 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 l ower or higher densities, induced competition was less efficient at su ppressing 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 thistl e initially. Crownvetch had very little growth in the first season. Wh en seeded up to 1.25 times the recommended rate, it increased in bioma ss 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 dico ts decreased proportionally with increased density of induced competit ion. Results of this study indicate that plant competition and biologi cal control can be combined to suppress Canada thistle, and C. rubigin osa as well as induced competition can be an integral part of a compre hensive approach for area-wide Canada thistle control. (C) 1994 Academ ic Press, Inc.