Deb Babcock: Using insects to control weeds

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In their native habitat, most plants have natural enemies that hold their spread in check. This includes insects, foraging animals, pathogens in the soil and the ecological environment in which the plant originated.

Unfortunately, many of the plants which are considered noxious weeds here in the Steam-boat area were introduced from Europe and other overseas habitats without bringing along the predators and conditions that keep down their uncontrolled spread.

One such noxious weed, Can-ada thistle (Cirsium arvense (L.) Scop.), was introduced from Europe and can now be found throughout the county scratching our legs along forest trails, ruining cultivated farmland, and crowding out natural vegetation along stream banks, roadsides and in pastures, lawns and gardens.

Because it has no natural enemies here, it grows vigorously and is very hard to control. The underground root system is extensive, going as deep as 10 feet and spreading as far as 15 feet wide. Each flower head will produce 40 to 80 seeds. Pulling it out by hand is difficult as it will re-establish itself from even the smallest bit of root left in the ground (as small as a half-inch of root).

In areas where the population of Canada thistle is most prolific in Routt County, the weed department has had some pretty good luck with the introduction of stem-mining weevils. This weevil only attacks the Canada thistle and its close relatives such as Musk thistle in the early spring before the plant flowers and starts producing seeds.

The county receives these insects from the state agricultural department insectary in Palisade after the insects have undergone extensive testing to ensure that they won't attack desirable native vegetation or cause other problems when the Canada thistle population has been reduced. It takes about 10 years for a population of weevils to reach significant numbers to have a satisfactory effect on the thistle. "We've been involved with controlling Canada thistle since the 1930s," said C.J. Mucklow, Routt County extension agent.

For homeowners needing to eradicate just a few thistle plants that crop up in the yard or garden, the best control method is to spot spray with herbicides, continue mowing or digging up the plant so flower heads cannot disperse seeds. Each time the plant is dug or mowed, you weaken the root system and cause the plant to direct its energy to the plant body rather than to reproduction. Canada thistle is very tenacious and just pulling it once a summer won't eliminate the plant and will likely make it worse.

Other successful releases of weed-eating bugs include stem gallflies released along Yampa River State Park to attack Spotted Knapweed, leaf feeding Chyrsolina beetles to attack St. John's Wort on Rabbit Ears Pass, toadflax moth, Calophasia lunula on Dalmation toadflax along Spring Creek and flea beetles to attack leafy spurge in Hayden and West Routt.

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