Controlling Musk Thistles with Weevils

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.

The white section in the center of the flower (above) is evidence of weevils at work.

This is the same flower after it was split in half. The brown/black sections at the base of the flower are additional signs of musk thistle weevils.

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.

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About Steve Freeman

Steve Freeman joined Forester Industries as a partner in 2005 after being one of the first customers to use the PasturePro post. He installed his first electric fence in the early 1980’s and implemented management intensive grazing in 1987. Presently, the operation is exclusively beef cattle, but in the past it has also included both goats and sheep. Steve is always happy to talk grazing practices, livestock raising and fence building.
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6 Responses to Controlling Musk Thistles with Weevils

  1. avatar Rob Thayer says:

    I’m interested in the thistle weevils. Where would I purchase them?

  2. avatar Steve Freeman says:

    Hi Rob,

    The weevils were released just west of our farm back in the 70′s and have spread all through southern Missouri. I don’t know nor heard of the weevils being available for purchase.

    I would offer to send you a thistle flower full of weevils but I don’t know enough about their life cycle to know if they would survive.

    Sure open to suggestions from readers.

    Thanks,
    Steve

  3. avatar Bob says:

    While these weevils do good job at eliminating thistles, they are also known for eating up the seeds of native plants, driving some to extinction.

    It is important to research the possible consequences thoroughly before importing such biological agents.

    Source: http://www.msnbc.msn.com/id/32031719/ns/us_news-environment/t/when-supposedly-good-bugs-go-bad/

  4. avatar Steve Freeman says:

    Hi Bob,

    Interesting reading, and it does show the importance of researching consequences before importing bugs and plants.

    There seems to be two very different schools of thought about natives. One feels all should be done to protect every native plant, animal and insect from pressure brought by non-native invasives.

    The other seems to feel that all of nature is evolving and pressure from non-natives that thrive in the natives environment is just following the natural course.

    I imagine biologists would still have a tough decision to make knowing that the imported musk thistle will rampage unchecked and probably dominate many natives grass fields as well as farm pastures without their natural control, the musk thistle weevil. The only other control is heavy herbicide spraying, which can also cause much environmental damage.

    I will say I feel better letting musk thistle weevils do their work than spraying herbicide to control the heavy spread of thistles

    • avatar Brian Wagner says:

      Steve,
      Oklahoma used to have these that they collected in Missouri and gave to land owners here. They worked great and the results lasted a couple of years. The problem is once they eat the food source then they move on. So I missed the date for Oklahoma this year but would love to get a couple of flowers full of weevils. The flowers with the weevils could be put in a cardboard box and would survive several days in the mail. I got them from the extension office in a small plastic container like you put left overs in. There is also another weevil that eats the plant itself and would be found on the leaves and stalks vs the flower. If you would be willing to collect a couple of flowers (and leaves if there is a weevil on it) I would be glad to pay the postage.

      • avatar Steve Freeman says:

        Brian,

        One thing we’ve never had to worry about here in Southern Missouri is the weevils running out of their food source! We’ll need to send some pretty quick, most are becoming beetles by now. After receiving the weevil filled flower head what do you do with it? How do you ensure the larve doesn’t just die? I’ll e-mail you and get an address.

        Thanks,
        Steve

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