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Tiny weevils new weapon in war on noxious weeds

Becker County Ag Inspector Marsha Watland demonstrates how she collects the tiny weevils that are used to control the noxious weed known as spotted knapweed. The weevils are placed on the weeds, where they lay their eggs and eventually destroy the plants. Vicki Gerdes/DL Newspapers

DETROIT LAKES, Minn. -- July is the height of the growing season for many grains, fruits and vegetables in northwest Minnesota.

Unfortunately, it is also prime time for the spread of a particularly noxious weed, known as the spotted knapweed.

Once it has infested a meadow, pasture, roadside ditch or similar area, the weed can take between 6 and 8 years to eradicate, said Marsha Watland, agricultural inspector for the Becker County Soil and Water Conservation District (SWCD).

Minnesota law requires all landowners who find this -- and other plants on the state's noxious weed list -- on their land to make every effort to control its spread into surrounding areas, she added.

"There are several different methods you can use (to get rid of them)," Watland said. "There's mechanical (cutting and tearing them out by hand), or herbicide ... or there's an alternative, biological control."

Spotted knapweed and its noxious weed counterpart, leafy spurge, can both be controlled biologically, without the use of herbicides or mechanical methods.

Instead, a host-specific insect -- one that feeds and reproduces on the plant itself -- is introduced into the weed's environment, and eventually kills it.

With leafy spurge, a beetle is introduced near the base of the plant, where it lays its eggs. Once the larvae has hatched, it develops in the crown of the leafy spurge plant, cutting off the plant's food source. Eventually, the leafy spurge will die off, Watland said.

"We can usually get control within three years using this method," she added.

"As of this week, I've collected about 60,000 beetles and placed them at different (leafy spurge infested) sites in Becker County," Watland continued.

The prime time for the spread of leafy spurge ends in early July, while the spotted knapweed's prime season starts around the same time, she added.

Two different types of weevils can be used to prevent the spread of spotted knapweed.

One type lays its eggs in the flower, or seed head, of the plant, while the other lays them at the base of the plant.

The seed head weevil larva develop inside the plant's flower during the winter months, eating the seeds as food and thus preventing them from reproducing.

The root weevil larva, meanwhile, feed on the roots of the spotted knapweed.

Most of the insectiaries for the development of the weevils are located in northeast Becker County -- Two Inlets, Carsonville and Pine Point -- except for one, which is located right next to the SWCD offices in Detroit Lakes.

"Spotted knapweed has been up there (in northeast Becker County) for over 100 years, and it covers really large areas -- some of them 80 to 100 acres," Watland said.

So why is spotted knapweed such a problem? Basically, Watland said, once an area is infested, the knapweed will overtake all other types of vegetation, because it produces a toxin that inhibits the growth of other plants in the same area.

Through cooperative weed management grants from the Minnesota Board of Soil and Water Resources (BWSR) and the National Fish & Wildlife Service, the Becker County SWCD has managed to obtain roughly $200,000 in funding for invasive plant control since 2006, Watland said.

Those landowners who are interested in implementing biological controls for the spread of spotted knapweed may be able to obtain some grant funding for introducing the weevils in infested areas of their property, she added.

Funding may also be available for leafy spurge control, though the primary season for biological control pf this weed usually ends around July 4, Watland said.

For more information on how to request biological controls for leafy spurge and spotted knapweed, please contact Marsha Watland at the Becker County Soil & Water Conservation District offices, 218-846-7360.