"There is lots of evidence that these weevils are doing a massive job of consuming many of the seed in the flower heads scattered across our county. There is no way that they can keep up with all the seed produced, but if they are getting perhaps 30% to 40% of the billions of seeds that are produced each year, they are having an impact," said Schnakenberg.
Schnakenberg says to watch for dried up seed heads and cut them open for evidence of weevil damage. Many times, a person will find two to four flower head weevils in the heads.
"At this time of year, the weevil is probably the best control," said Schnakenberg. "Biological control does not take us, as landowners, off the hook for keeping thistles from going to seed. Obviously the weevils need our help."
Using chemical control
Other control options include spraying at appropriate times of the year with products such as 2,4-D, dicamba, Grazon P+D, GrazonNext, Chaparral or other registered products.
Schnakenberg says the best times to spray are when the plants are still in the rosette stage which is the stage these plants are in for 70% to 80% of their lifespan. This corresponds with an ideal time of the year to spray being in the fall (October) or early spring (March-April). Sometimes widespread broadcast spraying is necessary for control over spot spraying.
Mowing multiple times is also an option in the spring or early summer. A Kansas study found that only 11% of the musk thistles mowed at the early bud stage were killed. When mowed a second time four weeks later, 79% of the thistles were controlled.
The best time to start mowing is within two days after the terminal flower head blooms in order to inhibit seed production and prevent rebolting. Remember however, that viable seed can start to develop within seven days of the first pink coloring in the heads.
Source: University of Missouri Extension