Biological control of Musk Thistles

After years of digging, mowing and occasionally using spray I wrote of our learning to  manage for and allowing musk thistle weevils to keep our thistles under control. Well after last years record setting drought the weevils have quite a challenge. The drought was severe enough to kill all of our ladino clover and I didn’t realize what a large percentage of our pastures ladino had become. Large bare spots appeared last fall from the decimated Ladino clover and it looks like thistle seeds have been laying in wait for just such a moment. We know that the weevils, in combination with a good healthy sod, control musk thistles very well–what I don’t know is how they will do when the sod is not as strong as in the past because of the drought. As I check the pastures I’ve been looking for signs of the eggs laid by our flower head weevil [Rhinocyllus conicus forelich]. The weevil lays her eggs and adds partially chewed plant material which makes the eggs look like warts on the plant. I found them and they are in abundance!

underside of thistle flower-with weevil eggs attached

We also have the Rosette Weevil [Trichosirocalus horridus panzar] working on the plants. This weevil starts earlier and the larvae feed on the center stem, stunting, or even killing the plant. But what really surprised me was finding our yearling heifer group starting in on immature flowers. While we’ve had a few of our brahma cross cows nibble and bite off flowers I’ve never seen the large bunches of flowers eaten like our young girls are doing this year. And no they aren’t starving, in fact the last weighing showed they were gaining just about 3lbs. a day. Here’s a photo of the chewed off flower heads.

Where have all the flowers gone?

While I know what biological control can do, it is tempting to bushhog the patches this year. But I have come to think of the weevils as tiny livestock and I don’t want to take their food source away! It will be interesting to see the results this fall as the grass and sod are returning to their previous vigor and it looks like the weevils may have some help with thistle control.



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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