Musk thistles are invasive, noxious weeds in Oklahoma and many other states. They can infest roadsides and ditches, range and pasture lands, and occasionally even no-till farms and hayfields. The seeds have fluff attached to them, which allows them to spread on the wind. They can be either annual or biennial, which allows them to germinate and establish almost all year long. Frequently, they go unnoticed until they bolt in the spring and their showy purple flowers become evident.
Flower (right): Inside of the musk thistle shows burrow holes from the weevil larvae. To the naked eye, the weevil looks gray and dusty.
While thistles are in the rosette stage (before they bolt and flower), they can generally be controlled rather easily with herbicides. Once they flower, they are no longer effectively controlled with herbicides. In the past, the only control option at this stage was to chop them down and physically remove and destroy the flowers to prevent them from making seed.
However, there is hope. If you find yourself with flowering musk thistles, check them for signs of musk thistle head weevils. The musk thistle head weevil was introduced specifically to provide biocontrol of musk thistles. In recent years, weevil populations have grown large enough in many areas to provide natural control. This weevil lays its eggs on the underside of the musk thistle flower. When the eggs hatch, the larvae burrow into the flower and begin consuming the developing seed. If there are enough larvae present, the flower will appear to mature normally and still have the fluff; but there will be no viable seeds to create the next generation of thistles. The weevil larvae then pupates and emerges as an adult that will fall to the ground and remain dormant until the following spring when it will find new thistle flowers to lay eggs on.
Close-up of egg masses on leaves.
If you find yourself with musk thistle flowers, I encourage you to inspect them for eggs, larvae and adults. Hopefully, they will be present in your thistle population and you can let them do the work of destroying the flowers for you.