N.D. using bugs to fight thistle

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AP photo This undated photo from the North Dakota Agriculture Department shows weed specialist Rachel Seifert-Spilde measuring a Morton County infestation of Canada thistle in Mandan, N.D. North Dakota is using weevils to try to fight the pesky plant.

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BISMARCK — Officials in North Dakota plan to release 100,000 bugs this fall to combat what has become the state’s peskiest plant.

Over time, entomologists believe the stem-mining weevils will help reduce stands of Canada thistle, a prickly elbow-high plant with a purple flower that infests more than a million acres in the state.

The weevils were brought from Europe to Canada in 1965, and to the United States in the early 1970s. They have been released in at least a half-dozen states where Canada thistle was a problem, from California to Montana.

The adult bugs spend the winter in the soil and emerge in the spring to mate and feed on the plants. Larvae tunnel into the stem and weaken the thistle, inhibiting its ability to survive diseases and winter weather.

Canada thistle is considered the worst on the state’s list of noxious weeds — those that are required by law to be controlled. It’s tough to tame because of a hardy, extensive root system.

“Canada thistle is definitely a priority,” said Blake Schaan, a state noxious weed specialist. “And out of all the biocontrol agents that have been studied extensively ... this (weevil) has proven the most effective.”

Dr. Edward Evans, a Utah State University entomologist who has studied the weevils, said the bugs work if given time. At long-term study sites in that state, “reduction (in Canada thistle) stands has been about 80 percent, but that’s over 20 years,” he said.

North Dakota’s Agriculture Department introduced the weevils to the state in 2004, releasing about 130,000 of the bugs in 34 of the state’s 53 counties. Last summer, agency officials went to 75 release sites that had not been disturbed by crops or cattle grazing, and found 44 still had weevils, Schaan said.

Rachel Seifert-Spilde, another state weed specialist, said many of the sites had a “noticeable reduction” in Canada thistle densities — some up to 80 percent.

“It is important to note that the releases were made five years ago and most of the time, it takes upwards of 10 years to see measurable success,” she said.

While research hasn’t identified any drawbacks to the weevils, some officials such as Seifert-Spilde say they work better in tandem with other control measures, such as herbicides.

The Agriculture Department is now planning another weevil release, using about $18,000 in federal grant money to free about 50,000 of the bugs at nearly 50 sites. A regional natural resource group in southwestern North Dakota is sponsoring a similar release in a 10-county area, with about the same number of bugs but double the sites, using about $25,000 in federal grant money.

“We want to try to establish areas where the weevils will multiply and colonize an area ... where in the future, other landowners can go out and collect these things,” said Jared Andrist, coordinator of the Dakota West Resource, Conservation and Development Council.

Schaan said the Agriculture Department has asked North Dakota State University researchers to study the issue and expects results in the fall.

The state will give its bugs to county weed boards or state agencies such as the Game and Fish Depatment, on a first-come first-served basis. The Dickinson-based resource council is asking landowners to nominate areas so it can develop a priority list, putting emphasis on areas that are poorly suited to herbicide control of Canada thistle.

Determining the effectiveness of the bug is not an exact science. In Utah, for example, Evans said efforts to compare Canada thistle sites with weevils and sites without the bugs failed when the bugs spread too quickly and overtook the control areas.

But Evans said ranchers in Montana have reported sharp declines in Canada thistle in some areas. Researchers in the Black Hills of South Dakota also have reported success in using the weevil to reduce thistle stands.

“It’s not a cure-all, but it seems to be helpful in the long term,” Evans said.

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