I used to spend a large part of May and June digging thistles, mowing thistles and worrying about the thistles I wasn’t digging or mowing. Now that a musk thistle weevil population has been established on our farm, that worry is long behind me.
Musk thistles were mistakenly imported to the US in the late 1800s from Europe, but the insects that controlled their spread were left behind. In the late 1970s, government officials released musk thistle weevils in Webster County, Missouri – the county immediately west of us – and it soon worked well in those locations that had a large population of weevils. But it took quite a few years to reach this threshold on our farm, and I learned that a lot of my management practices during the 1990s actually inhibited the growth of the weevil population for us.
Now we do not dig, mow or spray thistles in the spring. The thistles are almost always full of weevils and the last thing I want to do is eliminate their food source. It’s tough to do, but I usually leave thistles standing when I clip pastures, especially those with lots of weevils living in them. There are also rosette weevils and a fungus that can do some damage to the musk thistle, so almost every thistle is left alone to ensure these natural controls have some plants to live on.
We have gone from fear that thistles were going to win the war and take over our pastures to little concern about seeing a patch of thistles spring up, knowing that in a year or so they will be gone. Good grazing and a strong sod ensure most don’t get a foothold, but even when we open a patch of bare ground by digging waterlines or putting in water tanks and the thistles spring up, we know the weevils will take care of them. One less thing to worry about..
Check out this article from the University of Missouri to learn more about the history and additional control methods of musk thistles.