Crews began using a copper-based pesticide Tuesday in Lake Minnewashta to halt the spread of zebra mussels, following the discovery of 14 mussels last month at a boat launch on the suburban Twin Cities lake.

Minnewashta is the third west-metro lake to use the copper product, EarthTec QZ, to kill mussels. This is also the largest open-water treatment in Minnesota; the Department of Natural Resources has approved the product’s use on 29 acres.

The fingernail-sized pest clings to boat hulls and docks, alters ecosystems, and poses a hazard for swimmers.

The Minnehaha Creek Watershed District, the Lake Minnewashta Preservation Association and Carver County are partners on the $40,000 project and will keep applying the copper this week.

The Watershed District has gone on the offensive with the spread of the invasive species.

“This is our best opportunity yet to test the feasibility of in-lake treatments for zebra mussels,” said Eric Fieldseth, the district’s aquatic invasive species program manager.

In the first study of its kind, researchers are testing low doses of the copper-based pesticide on zebra mussel larvae, called “veligers,” on Lake Minnetonka. The copper product, along with two other chemicals, successfully killed mussels on nearby Christmas Lake, but more were later found outside the treated area.

Minnewashta’s treatment area is about three times bigger than Christmas Lake’s.