Musk Thistle Control
By Alix Carpenter, Agronomy Specialist
University of Missouri Extension, Marion County
April 19, 2004
Musk thistles exist throughout the Missouri (as well as several other midwestern states), and have been the subject of a lot of calls I’ve received recently. Introduced to the country in the 1880s, musk thistle spread, and was declared a noxious weed by Missouri state statute in 1979. Not a serious pest in spring planted row crops, musk thistle is a significant weed of fall-planted grain, alfalfa, and clover. In their first year of growth, thistles are somewhat inconspicuous, and may not be noticed until they flower the following year. In pastures, a moderate musk thistle infestation will reduce yield by an average of 23 percent. Livestock, which will not graze the rest of the plant, may feed on the flower head.
Musk thistle is a biennial plant: after a seed germinates, the plant grows in a rosette form. This rosette increases in diameter, and begins to produce a fleshy taproot. It is in this form that the plant overwinters. The following spring, new leaves emerge and the plant bolts. Flowering typically lasts from mid-May through August. A few may, however, develop as an annual, or even a winter annual. Proper identification of thistle species prior to treatment is critical, as there are very effective biological control agents for musk thistle in Missouri.
Each musk thistle plant produces an average of 10,000 seed. Seventy percent of musk thistle seed geminate the year after their production, and 20 percent germinate the following year. The remaining 10 percent either never germinate, or do so more than two years after production. This has led to a buildup of a viable seed bank in areas where musk thistle is present. Musk thistle spreads by wind-blown seed, and seed in contaminated hay.
There are three control categories for musk thistle: chemical, mechanical, and biological. There are several herbicides available for musk thistle control; applications should be made prior to bolting. Mechanical control of the thistle involves mowing, optimally within two days of the terminal flower head blooming. In 1985, USDA Agricultural Research Service entomologists released flower head weevils in Missouri; in 1979, rosette weevils were introduced. Both of these weevil species, native to Europe, feed on the developing plant, thereby preventing it from producing seed. Both weevil species were studied extensively prior to their release, to ensure that they would not complete with one another, and that they would not become economic pests of plants in this country.
Since their introduction, the weevils have been responsible for a 70 to 90 percent reduction in the musk thistle population. Through collection and reintroduction, as well as natural dispersal, the weevils have spread and increased in number throughout Missouri.
Prior to taking chemical or mechanical action against musk thistle, it should be determined that weevils are not present. It is easiest to determine the presence of flower head weevils in June, when the thistle is blooming. The flower should be cut or broken in half lengthwise. If the weevils are present, small, white, grub-like weevil larvae will be visible in the region of seed development. Rosette weevils feed, mate, and lay eggs on musk thistle when it is in the rosette form. They are best scouted for in March and April, by cutting through the rosette of the plant. If rosette weevil larvae are present, black streaks will be visible in the normally white tissue of the plant. It may be necessary to sample several plants in order to find visible signs of weevils.
If weevils are present, chemical and mechanical methods of musk thistle control should be carefully timed, so as not to disrupt the weevils’ life cycle. If the flower head weevil is present, herbicides may be used in mid-March through late April, and September through October. Mowing may be done in mid-July. If both weevils are present, mowing may still be done in mid-July, but herbicide use should be limited to September through mid-October.
Timing alternate methods of control ensures that the weevils will continue to work at controlling this weedy pest. If you have any questions about weed control, please contact me at the University of Missouri Extension Center, Marion County, (573) 769-2177.