Multiple stresses by insect and plant competition on growth and productivity of Canada thistl

Canada thistle is an aggressive perennial weed throughout temperate areas for both the northern and southern hemisphere. As various single tactics have shown limited success in controlling this weed, I believe that a combination of different stress factors is necessary for effective sustainable control of Canada thistle. This study evaluates the competitive abilities of tall fescue grass and crownvetch in conjunction with a thistle feeding insect for Canada thistle control. The goal of the study was to determine what levels of the herbivore and plant competition are needed to suppress Canada thistle.;Field studies were conducted to determine the effects of multiple stresses caused by defoliation and plant competition on growth and survival of Canada thistle (Cirsium arvense Scop.). Plant competition was from a combination of tall fescue (Festuca arundinaceae Schreb.) and crownvetch (Coronilla varia L). Artificial defoliation was used to determine the influence of level and frequency of defoliation in combination with plant competition on Canada thistle. The effects of various densities of a defoliator, Cassida rubiginosa (Coleop:Chrysomelidae), in the presence and absence of plant competition were also determined. A three-season study determined the combined effects of C. rubiginosa and plant competitors at various densities on the growth and productivity of Canada thistle plantings of various ages.;Defoliation, plant competition and their interaction can significantly reduce productivity of Canada thistle. In a wet year, plant competition would have a greater effect than defoliation, but in a dry year, it was the reverse. In a wet year, in the presence of plant competition thistle productivity at all levels of defoliation was lower than undefoliated thistles. Biomass of the thistle decreased with increase in density of C. rubiginosa. At 20 beetles/thistle (introduced at rosette stage) in the presence of plant competition, less than 1 out of 3 newly planted thistles survived the growth season. The optimum density of plant competitors, for suppressing Canada thistle is close to the recommended density of 20 kg/ha and 50 kg/ha of crownvetch and tall fescue, respectively. When thistles were planted concurrently with plant competitors in the presence of C. rubiginosa, an increase in density was observed in the second season. By the third season, thistle density was lower than the initial density. The use of the defoliator, C. rubiginosa, and plant competition offers a sustainable method for controlling Canada thistle.