Evaluation of Pst Interactions with Canada Thistle in Non-disturbed Sites

Canada thistle is the most invasive and challenging weed in grazing systems in the Upper Midwest (Doll, personal observation) and its membership on the noxious weed lists of many states demonstrates its tenacity against conventional control tactics. The resilience of this weed can be credited to its ability to effectively reproduce through both seed production/dispersal and its extensive root system. This is especially true in non-disturbed sites where few management strategies exist for its control. In pastures and non-crop sites, control is often not economically justifiable and only makes marginal gains against Canada thistle in the long run (Tichich and Doll 2001). However, non-disturbed sites appear to be a better fit for biological control than annual cropping systems as complete control is not required in the short run. Rather, one can afford to wait three or more years for a control tactic to reduce the infestation to acceptable levels.