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flowering will reduce seed production, and since the plant is a biennial, it <br />will eventually reduce stands. <br />b. Chemical control is effective when applied to the rosette form of the plant. <br />This occurs in the fall or early spring. Herbicides lose their effectiveness <br />once the plant bolts. As before, local consultants should be contacted for <br />product and rate recommendations. <br />Biological control - As previously mentioned, musk thistle has provided one <br />of the great success stories associated with biological control on noxious <br />weeds. The musk thistle seed weevil, Rhinocyllus conicus, was introduced <br />into the United States in 1969. These weevils overwinter as adults. In May <br />to July the adults congregate on bolting musk thistle plants, feed, mate, and <br />deposit eggs. The larvae feed at the base of the flower and interfere with <br />seed production and viability. It may take several years for noticeable <br />decline to occur. If this control is utilized other control methods should not <br />be employed on the musk thistle in the insect release area. <br />4. Russian Knapweed -Centaurea repens <br />a. Mechanical control is possible but may take two to three years of intensive <br />cultivation. Best results occur with cultivation after 5-10 days of green <br />regrowth. <br />b, Chemical control provides the best option for controlling <br />Russian Knapweed. Several products are available with some of the newer <br />herbicides providing excellent control. As before, local consultants should <br />be consulted for product and rate recommendations. <br />5. Spotted Knapweed -Centaurea maculosa <br />a. Cultivation will be effective with spotted knapweed in an intensive <br />management program. The seeds of this plant remain viable in the soil for <br />up to 12 years after seed production is stopped. Therefore, although <br />cultivation may remove the plant, it also creates ideal seed beds for viable <br />seeds in the soil. <br />b. Chemical control is presently the best treatment for spotted knapweed. <br />Excellent control can be achieved with several different herbicides. Follow <br />up measures are required for successful eradication. <br />6. Diffuse Knapweed - Centaurea diffusa <br />a. Presently the best control for this knapweed is chemical treatment. Once <br />again a variety of herbicides are available and the appropriate authorities <br />should be contacted for recommendations. <br />-12- <br />