Canada thistle (Cirsium arvense Scop.), a widely distributed weed in northern United States, according to Dewey (3), was introduced from Europe in 1777. In 1901 Blankenship (1) reported it to be rare in Montana having been identified in only five locations. However, a report issued in 1956 by Heikes (6), showed an estimated 625,000 infested acres in Montana. Another report of a survey of the four northwestern states including Montana, Idaho, Oregon, and Washington estimated their total of Canada thistle at more than 1.5 million acres infested (4). This was considerably more than any other weed of this region. Control measures for this weed are, therefore, of primary importance.