Montana Outdoors

July 15, 2017

A weed and its nemesis.

Filed under: Montana — Tags: , , , — montucky @ 3:05 pm

Spotted Knapweed

Spotted knapweed ~ Centaurea stoebe

While its blossom might look pretty, this plant is one of the worst of the invasive plants that has long infested this area. It develops into huge masses and is found nearly everywhere, including the forests where it has been introduced from seeds carried by logging equipment and in some cases from seed in hay brought in by horse packers and riders.

It has a large tap root that sucks up available water faster than the root systems of its neighbors and it releases a toxin from its roots that stunts the growth of nearby plants of other species. Once started in an area, knapweed is very difficult to eradicate. Mowing or cutting only helps it spread faster. Spraying is expensive and introduces all of the negative impact involved with toxic chemicals. Often sheep are used to control it with some degree of success. Knapweed does however have a nemesis: the Knapweed Root Weevil.

Knapweed Root weevil

Knapweed Root weevil ~ Cyphocleonus achates

This half-inch long weevil is a specialist that feeds exclusively on spotted knapweed and does not attack any of the native flora. The female lays her eggs on the top of the knapweed’s root crown. After the eggs hatch, the larvae burrow into the root. This larval feeding destroys knapweed’s vascular root tissue and prevents the plant from transporting water and nutrients. Roots become swollen and deformed as a result of this larval feeding and the plants eventually die.

The hillside on the east side of my driveway is quite steep and exists in its historical natural condition, but somehow became severely infested with spotted knapweed. In August of 2010 we learned of a field seminar conducted by an extension agent from MSU on biological control of knapweed by using these weevils and I attended. We met in one of the pastures of a very large ranch whose owner supported the program and proceeded to collect some weevils. Upon returning home with several hundred of them I distributed them according to the instructions and promptly forgot about the whole thing. One day a couple of years later I was walking down the drive and suddenly noticed that there wasn’t a single knapweed plant around. The weevils had done a remarkable job! None have returned since.

Yesterday I asked the new extension agent about the program because a friend has a huge problem with knapweed and found that they may possibly conduct that seminar again either this summer or next. I hope they do because I saw an ad just today for a weed control business that is selling the weevils for $1.40 each.


  1. Ouch! That sure sounds like an inflated price. Maybe you should ask the extension agent if there’s another source. Congratulations on finding an eco-friendly solution for that invasive.

    Liked by 1 person

    Comment by Pat — July 15, 2017 @ 3:15 pm

    • There has been a high price on the weevils for years. They were about a buck apiece back in 2010. The extension course includes collecting weevils and there is no cost in that.


      Comment by montucky — July 15, 2017 @ 4:42 pm

  2. Very interesting. I know that packers have to use “certified weed free” hay when they go into the back country. Maybe you should start growing them wholesale!

    Liked by 1 person

    Comment by Ron Mangels — July 15, 2017 @ 3:28 pm

    • The signs are fairly recent, but the problem goes much further back and the problem has been there for a long time. I don’t know if there is an active eradication program in the National Forests.


      Comment by montucky — July 15, 2017 @ 4:45 pm

  3. Once again nature solves its own problems, with a little help in this case. This is interesting stuff that I hadn’t heard of.
    I doubt too many people will pay that price for a single weevil. That’s a great shot of it!

    Liked by 1 person

    Comment by New Hampshire Garden Solutions — July 15, 2017 @ 3:48 pm

    • As you mentioned, any program with the weevils helps the natural process, Or actually starts it because the plants were imported without the insect control. It doesn’t take all that many weevils to get a good start in clearing up an area. They don’t fly so you have to place a dozen or so in each place so there will be a breeding population and the weevils do the rest. If they kill a lot of weeds, because that’s the only food for the weevils, then their numbers naturally decrease. The knapweed is never completely eliminated; there’s always some left and the same with the weevils until a natural balance is established and the population of both reaches the natural normal.

      Liked by 1 person

      Comment by montucky — July 15, 2017 @ 4:49 pm

      • I usually worry that the imported insect will turn to native plants when it runs out of those it feeds on naturally, but this sounds like a win-win situation for everyone.

        Liked by 1 person

        Comment by New Hampshire Garden Solutions — July 15, 2017 @ 4:52 pm

        • That’s the good news. This insect is a true specialist, so it has been a safe program with a good record.

          Liked by 1 person

          Comment by montucky — July 15, 2017 @ 4:55 pm

  4. Thank goodness for that weevil!

    Liked by 1 person

    Comment by wordsfromanneli — July 15, 2017 @ 3:55 pm

    • I was initially concerned about bringing in the weevils too, but the programs using them have been established for about two decades and monitored closely and there have not been any problems caused by the weevils.

      Liked by 1 person

      Comment by montucky — July 15, 2017 @ 4:52 pm

      • There’s always the potential for making things worse (as in the cane toads and rabbits in Australia), but sometimes it turns out to be a good thing.

        Liked by 1 person

        Comment by wordsfromanneli — July 16, 2017 @ 1:34 pm

  5. Yes, nemesis falls on all.

    Liked by 1 person

    Comment by nvsubbaraman — July 15, 2017 @ 5:23 pm

  6. What a fantastic result.
    Now if a similar kind of weevil could be introduced to other non-indigenous plants (or weeds), it could be the start of clearing out other invasive plants.

    Liked by 1 person

    Comment by Vicki — July 15, 2017 @ 6:21 pm

    • I’m not tuned into that very much, but I would think there is a lot of effort going into looking for similar situations. This is a unique insect though because it is so specific in its diet. It seems to be bringing back the natural balance that the plant had in its native home.

      Liked by 1 person

      Comment by montucky — July 15, 2017 @ 8:22 pm

  7. Great closeups!

    Liked by 1 person

    Comment by Michael Andrew Just — July 15, 2017 @ 7:40 pm

  8. Nasty plant.

    Liked by 1 person

    Comment by Malcolm R. Campbell — July 15, 2017 @ 11:17 pm

    • It is one of the worst around here. I’ve even read that pulling it with your bare hands can lead to skin problems, and pulling it is not very successful.


      Comment by montucky — July 16, 2017 @ 7:40 am

  9. This really is interesting. First, it brought to mind the relationship between the monarch butterfly and milkweed. I suspect there are many, many more such relationships that I don’t know about. One I recently discovered is the fire ant/tick relationship. Fire ants apparently adore munching on ticks, and that’s part of the reason we have so few. I found one on my shirt this week after ploughing around in some grass, but it’s the first I’ve ever come across.

    It’s great that the weevil’s beneficial work has been nurtured, and that it works without any downside. I once gave some ladybugs to a friend’s father for his birthday. He was ecstatic, and so were the ladybugs, since he had entirely too many aphids for one man. The trick with the ladybugs was to put out only a few at a time, and leave the rest in “hibernation” in the refrigerator. His wife wasn’t so keen on it, but they worked it out, and he had aphid-free plants for a season.

    Liked by 1 person

    Comment by shoreacres — July 16, 2017 @ 5:05 am

    • There are plenty of ticks here, but we get used to dealing with them. During tick season when I come home from a hike I usually come inside and put my clothes right in the washer and head for the shower. I usually bring home over a dozen every spring, but haven’t been bitten in years. I would rather put up with them than mosquitoes of those darned biting black flies that live on the peaks.

      I’m glad that we can use some of the natural controls over invasive species as long as it is done under strict control so we don’t inadvertently bring in something that causes problems by itself. That is something I liked about the weevil program. It was closely controlled and monitored by the folks at MSU and turned out to have an outstanding record.


      Comment by montucky — July 16, 2017 @ 7:39 am

  10. It’s a nature-horror story, Montucky. But it is brilliant when invasive species can be combated biologically!!

    Liked by 1 person

    Comment by Hanna — July 16, 2017 @ 8:05 am

    • Yes it is. I do not use chemicals to control unwanted plants because of all of the other organisms that would be adversely affected and because no amount of chemicals will ever attain the natural balance that is the real answer.

      Liked by 1 person

      Comment by montucky — July 16, 2017 @ 8:34 am

  11. Pretty remarkable how nature has a cure. An unattractive cure but still a cure.

    Liked by 1 person

    Comment by Candace — July 16, 2017 @ 2:11 pm

    • I’m assuming that the weevil came from the same place as the plant and that it is part of Nature’s way of maintaining balance.


      Comment by montucky — July 16, 2017 @ 3:08 pm

  12. I’ve heard about knapweed, mainly through efforts to control it, but I doubt I could identify it. Your story sent me on a hunt. “Weeds of Canada and the Northern United States” (Royer and Dickinson, 1999) says it was introduced from Europe into Victoria, BC in 1893. Careless Canucks. Sorry ’bout that. It and Russian knapweed are both on our county’s hit list of Prohibited Noxious in Alberta so I best learn to spot it. In my search for more info I discovered the Crown of the Continent Research Learning Center (CCRLC). I’d never heard of this group — I tracked down an online copy of their “Invasive Plants of the Crown of the Continent.” A great resource covering BC, Alberta and Montana with detailed info and photos of a wide selection of invasives. No mention of weevils here, it’s more a guide to mechanical control (digging it up.)

    Liked by 1 person

    Comment by Sally — July 16, 2017 @ 4:13 pm

    • Thanks for the info. I downloaded the PDF with the invasive weeds info. I have seen nearly all of those weeds in this general area to at least some extent. The most prevalent here is the spotted knapweed.

      Liked by 1 person

      Comment by montucky — July 16, 2017 @ 7:33 pm

  13. Life is learning and today I learnt new things from You. Thank You.

    Liked by 1 person

    Comment by Sartenada — July 18, 2017 @ 4:34 am

    • I was happy to learn of this insect and plant relationship too. Many people have never heard of it.


      Comment by montucky — July 18, 2017 @ 7:16 am

  14. Interesting that it releases a toxin … thumbs up for the weevil!

    Liked by 1 person

    Comment by Julie@frogpondfarm — July 21, 2017 @ 1:11 pm

  15. Very nice! I learn a lot from your blog!

    Liked by 1 person

    Comment by Reed Andariese — July 21, 2017 @ 6:58 pm

    • I stumbled across the weevils years ago and have been fascinated with them since. They are very effective!


      Comment by montucky — July 21, 2017 @ 7:30 pm

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