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An abundance of limiting soil nutrients such as nitrogen may encourage Canada thistle infesta- <br />tions. Therefore, local fertilizer use is discouraged if control of the weed is desired.5 <br />4.3 CHEMICAL CONTROL <br />Herbicides targeting Canada thistle (see Table 2) are most effective when used in conjunction <br />with mechanical, cultural, and/or biological controls. In general, effective chemical control re- <br />quires multiple applications. Timing and application rate are critical, whereas more herbicide <br />does not necessarily mean greater control. <br />For more information on chemical control see: <br />httq://www.coogext.colostate. edu/bou Ider/AG/aoweedssgecificweeds.shtm I <br />4.4 BIOLOGICAL CONTROL <br />The stem mining/boring beetle (Ceutorhyncus litura), available from the Colorado Department of <br />Agriculture (CDA), is a weevil currently used as a biocontrol agent in Colorado. Females lay <br />eggs under Canada thistle leaves in early spring. Larvae bore into the main leaf vein, then into <br />the plant's crown area and feed on ("mine") the plant's leaves and stems. Adults feed on Can- <br />ada thistle externally. In addition to feeding damage, the entrance and exit holes made by the <br />larvae provide infection sites for diseases that stress the plant even further. Unless Ceutorhyn- <br />cus populations are high, the beetle alone will not effectively control Canada thistle, but can in- <br />troduce stress that works well in conjunction with other control techniques. <br />The Canada thistle stem gall fly (Urophora cardw) is another biocontrol insect also available <br />through the CDA. Females lay eggs on the apical meristems of developing shoots, and larvae <br />burrow into the shoots, where their feeding triggers the formation of substantial galls. At the <br />minimum, these galls stress the plant and in some cases will kill it. Galls near the plant's termi- <br />nal meristems (e.g., where flowers develop) keep the weed from flowering and reduce seed set. <br />There is a fungus (Puccinia) that, if used in conjunction with 2,4-D, has shown some promise in <br />controlling Canada thistle. <br />s Walters (1991) states that the ideal soil nutrient levels for controlling Canada thistle are (in pounds per acre) cal- <br />cium, 2,000; phosphate, 400; potassium, 200; sulfate, 200; nitrate-nitrogen, 40; "ammonicaP nitrogen, 40; iron, 40. <br />11 <br />