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Canada Thistle <br />Pag <br />quarts/A) or Telar (1 ounce/A) to re-growth. Use a surfactant (0.25 percent to 0.5 <br />percent v/v) with Telar fnr adequate control. Banvel/Vanquish/Clarity also may be <br />applied in early spring at 2 quarts/A when Canada thistle is in the rosette stage. Tordon <br />(1 quart/A) or Tordon plus 2,4-D (1 quart + 1 quart/A) is effective whenever Canada <br />thistle is actively growing. Fall applications are especially effective. <br />Curtail and Transline are effective when applied in spring after all Canada thistle plants <br />have emerged. Apply Curtail (2 to 3 quarts/A) when the oldest Canada thistle plants are <br />entering the bud growth stage and the youngest are in the rosette to bolting growth <br />stages. Apply Transline (2/3 to 1 pt/A) when Canada thistle is in the rosette to bud <br />growth stages. Transline at 1 pt/A also is effective when applied in fall. <br />Recent research at Colorado State University shows that the performance of Curtail to <br />control Canada thistle can be improved when preceded by two or three mowings. When <br />Canada thistle infestations occur in situations where root growth would be restricted, <br />such as habitats with high water tables, begin mowing when it is 12 to 15 inches tall. <br />Repeat mowings at about one month intervals. Apply Curtail at 2 to 3 quarts/A in <br />October or about one month after the third mowing. Follow this regimen for two <br />consecutive years. <br />Mechanical control. Mowing hay meadows can be an effective tool if combined with <br />herbicide treatments. Mowing alone is not effective unless conducted at one-month <br />intervals over several growing seasons. Always combine mowing with cultural and <br />chemical control. Mowing at hay cutting stimulates new Canada thistle shoots to develop <br />from its root system. <br />In irrigated grass hay meadows, fall herbicide treatments that follow mowing can be an <br />effective management system because more Canada thistle foliage is present after <br />cutting to intercept herbicide. Additionally, root nutrient stores decrease after mowing <br />because the plant draws on them to develop new shoots. <br />If a Canada thistle infestation exists in a field that will be rotated to alfalfa, control the <br />weed before seeding alfalfa. Alfalfa is an effective competitor only after it is established. <br />It will not adequately establish in a well-developed Canada thistle infestation. A Canada <br />thistle management system can start with crop or grass competition combined with <br />herbicides, with the field rotated to alfalfa when the management plan ends. <br />Biological control. Ceutorhyncus litura is a weevil currently used as a biocontrol agent <br />in Colorado, The female lays eggs underneath the Canada thistle leaves in early spring. <br />Larvae bore into the main leaf vein, then down into the plant's crown area. If the <br />population is high enough, plant death can occur, otherwise Canada thistle is stressed <br />and less vigorous. <br />Ceutorhyncus alone will not effectively control Canada thistle. It must be rnmbined with <br />other methods to be successful. Combine the weevil with cultural techniques that allow <br />for maximum desirable plant competition. Research to combine Ceutorhyncus with <br />herbicides or mowing has not been conducted. Research has shown that biological and <br />chemical controls are compatible for musk thistle. This is most likely true for Canada <br />thistle as well. Ceutorhyncus litura is available through the Colorado Department of <br />Agriculture. <br />Urophora cardui is another biocontrol insect available from the Colorado Department of <br />Agriculture, Females lay eggs on apical meristems of developing shoots. Larvae burrow <br />into shoots. Their feeding triggers huge galls to form that stress the plant, perhaps killing <br />it. Galls that form near the terminal meristems (e.g., where Flowers develop) keep the <br />weed from Flowering and reduce seed set. <br />tCOlorado State University Cooperative EMension weed science specialist and professor, bioagricuRural sciences <br />and pest management. Revised 3/03. <br />Go to top of this oaae• <br />Updated Monday, August 23, 2004. <br />http://www.ext.colostate.edu/pubs/natres/03108.htm1 6/20/2005 <br />