What is Musk Thistle?
Musk thistle or nodding thistle (Carduus nuttans) is a biennial forb in the Aster family. It was introduced to North America from southern Europe and western Asia in the early 19th century. Since this time it has become invasive across North America and is classified as a noxious weed in many states including Colorado.
How to Identify Musk Thistle
Musk thistle spends its first year after germination as a rosette. Its leaves are deeply lobed and spiky with sharp spiny tips. They are dark green in color with whitish spines at the tips of the lobes. It can grow from 1 to 1.5 meters tall in its second year of growth when it bolts. After bolting it bears large showy purple flowers 3-5 cm. in diameter. Each flower contains many large spear-shaped bracts where they attach to the stem. When the flower heads complete maturation they droop or “nod” distinctively from the stem lending the frequently used common name of nodding thistle.
Effects of Musk Thistle
Musk thistle is weedy because it forms a dense monoculture by outcompeting native grasses and forbs. It tends to thrive in disturbed habitats where resources are limited and native plants cannot establish or grow as well. Once the plant establishes it can easily spread with each flower head producing up to 1200 seeds that may last in the soil for up to ten years.
Currently, there is one biological control agent released by the Insectary for control of musk thistle, the Musk Thistle Rosette Weevil or Crown Weevil, Trichosirocalus horridus. This weevil develops in the crown of the musk thistle rosette. Damage by the weevil happens in the developing tissue of the plant. The damage stunts its growth as well as reduces the number of stems it can produce. There is a distinctive yellowing of the stem and foliage observable as weevil damage increases.
Life Stages of the Rosette Weevil
Adults emerge from overwintering in March and April. These adults lay eggs on the thistle rosettes. As the eggs hatch, the larvae of the weevil begin feeding in the center of the rosette for about seven weeks. The larvae leave the rosette for pupation in the soil near the plant which takes 12 to 20 days. The first new adults of the summer emerge by late June through early July in CO. This generation may produce a second generation that survives over the winter as eggs, larvae, or pupae.
Like most biocontrol agents, musk thistle weevils will not wipe out an infestation of musk thistle. Instead, the weevils are capable of reducing the density of the target weed over a lengthy period of time, typically two to three years minimum. Reductions in your musk thistle infestation will take patience when these weevils are used as the only means of control. Other IPM (integrated pest management) tactics are recommended including mechanical removal of the plant by pulling or mowing before the development of mature seed heads. Herbicides may also be helpful but are not recommended at the same time as the release of weevils. The insects may be sensitive to certain chemicals and or plant damage caused by herbicide application.