Musk Thistle: Natural History and Management

Musk Thistle or Nodding Thistle (Carduus nutans) belongs to the sunflower family (Asteraceae). This species is a biennial generally requiring two years to complete its life cycle. These flowers are native to Europe and Asia, but are now a common agricultural pest throughout the United States and much of southern Canada. Musk Thistle was introduced to North America in 1853. Throughout most of its range C. nutans is considered a noxious weed, but is sometimes cultivated in gardens for its showy flowers.


Musk thistle management consists of mechanical, chemical, and biological control methods. Mowing and hoeing tend to be expensive. Herbicide control has been effective and is probably the most common. Several biological control methods have been successfully implemented using insects including: the Thistle Head Weevil (Rhinocyllus conicus), Musk Thistle Rosette Weevil (Trichosirocalus horridus), Musk Thistle Flower Fly (Cheilosia corydon), and the Musk Thistle Leaf Beetle (Phylliodes chalcomera).


References:

Boldt, P. E., and J. A. Jackman. 1993. Establishment of Rhinocyllus conicus Froelich on Carduus macrocephalus in Texas. Southwestern Entomologist 18(3): 173-181.

Cory, V. L. 1940. Six thistles recently introduced into Texas. Madrono 5: 200-201.

Department of Entomology. 2003. Biological control of weeds in Texas. http://bc4weeds.tamu.edu/weeds/rangeland/muskthistle.html.

Jackman, J. A., P. Boldt, J. W. Stewart and T. W. Fuchs. 1992. Biological Control of Musk Thistle in Texas. L-5067. Biological Pest Control. Texas Agricultural Extension Service. Texas A&M University System, College Station, TX.

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) and Agent Orange

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) is a broad leaf plant defoliant developed in the 1940s and widely used in agriculture through the early 1980s when the Environmental Protection Agency (EPA) ended its use on food crops in the United States. Agent Orange is a defoliant that is a mixture of 2,4,5-T and 2,4-D that was used during the Vietnam War. Agent Orange has subsequently been suspected of causing serious illness including cancer in Vietnam veterans that were exposed to the herbicide during the war.
Both 2,4-Dichlorophenoxyacetic acid (2,4-D) and 2,4,5-T are synthetic auxins. Auxins occur naturally as plant growth substances (growth hormones) that regulate plant growth behavior. Both of these chemicals are members of the phenoxy family of herbicides. The half-life of 2,4-D is estimated to be 7-10 days and soil microbes are primarily responsible for its degradation. Although microbes also break down these chemicals in aquatic systems tests have shown trace amounts in surface and groundwater across the United States and Canada.

For more details about the manufacture, chemistry, and toxicity of the phenoxy family of herbicides, see http://en.wikipedia.org/wiki/2,4-D

Herbicide Uses in Agriculture: 2,4-Dichlorophenoxyacetic acid (2,4-D)

Herbicides like 2,4-Dichlorophenoxyacetic acid (2,4-D) are designed to kill target species while leaving desirable species such as certain crops unharmed. 2,4-D is used to control broad-leaf weeds. It is the most widely used herbicide in the world, and the 3rd most common in North America. More than 1,500 herbicide products contain 2,4-D as an active ingredient sold under a wide variety of brand names. 2,4-D is generally applied as a salt solution or a more potent ester formulation. These chemicals interfere with the growth hormones of target species or so called “weeds” and are classified as plant growth regulators. The chemical is absorbed through a plant’s leaves and effects growth at the apical meristems. The effected plant literally grows itself to death. In a short time plants show leaf curl and withering. Herbicides like 2,4-D are commonly used on industrial sites, railways, and roadsides. Lesser quantities are applied to forest stands, pastures and wildlife management areas. 2,4-D was a component in "Agent Orange" widely used as a defoliant in Vietnam that has now been linked with cancer.