Weed fighters unleash bug on thistle

Weed control experts have called out the weevils to help fight an infestation of Canada thistle which struck Carson Valley.

"Your county has the record for it," said Dawn Rafferty, Nevada Department of Agriculture invasive plant specialist. "There is a higher abundance of Canada thistle in Douglas County than anywhere else in the state."

Rafferty said the purple-flowered thistle is pernicious, spreading both by seed and through roots.

"These are highly aggressive invasive plants," she said. "They are bad in people's yards because they have aggressive root systems. If it gets into your yard you will have a devil of a time getting rid of it."

Spraying it isn't effective, and it is difficult to clear by digging.

"Digging it doesn't work, because every time you cut the root you make more plants," she said.

Canada thistle isn't really from Canada, but was imported by accident with some contaminated crop seed.

Once the flower matures, the seed is carried on the wind and in water to new sites.

"Once it is in the fields, you have to kill all the alfalfa to get it out," she said. "Bull thistle and Scotch thistle are biennials, so after two years, the plant dies. But Canada thistle is a perennial and is very long-lived."

That's why Rafferty and Douglas County weed control officer Larry Hughes introduced Canada thistle to their little friend, ceutorhynchus litura, or the Canada thistle stem weevil.

The weevil, which is about the same size as this capital M, lays eggs on the plants when they are sprouting. Newly hatched weevil larvae dig into the leaf toward the main vein while older larva head for the stem, root crown and root.

While the weevil itself doesn't do all that much damage, it leaves a door, or more appropriately a hole, in the stem on the way out, which other bugs can exploit in the winter.

"This is a weevil that helps to weaken it," Hughes said. "It is just one more tool out there to aid in the fight against this particular thistle."

This program is similar to one conducted on a Gardnerville development to control diffused knapweed.

"This is something we're using to try and combat the thistle that is a little different than the nuke and go kind of thing," Hughes said.

Rafferty said people should keep an eye out for Canada thistle in new landscaping particularly if they've brought in fill dirt or new trees.

Canada thistle appears on the state's list of noxious weeds and property owners are required to control the population under state law.

Rafferty credited Hughes with paving the way by contacting landowners when they have an infestation and helping by either eradicating the weed or showing property owners how to do it.

State law permits weed control officers to go onto property infested by weeds and clear them, then bill the owner for the work, Rafferty said.

She said the weevils are the first attack on the thistles and that by next summer, they will bring in a few other insects that prey on the plant to expand the fight.

"A lot of times a single biocontrol agent isn't going to knock this out," she said.

• Contact Kurt Hildebrand at khildebrand@recordcourier.com or 782-5121, ext. 215.