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.

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