Research on the use of a copper-based pesticide to fight zebra mussels in Lake Minnetonka continued to show the treatment's effectiveness in the study's second year, University of Minnesota researchers have reported.

The copper solution, first applied in July 2019, dramatically decreased the population of zebra mussels — especially on veligers, their larval form — while leaving native fish mostly unharmed, said Angelique Dahlberg, a Ph.D. student and researcher at the U's Minnesota Aquatic Invasive Species Research Center. Dahlberg spoke Tuesday at the Upper Midwest Invasive Species Conference.

Zebra mussels are among the most problematic invasive species in Minnesota, harming wildlife and altering natural habitat. They have been found in more than 200 lakes, including Lake Minnetonka.

Copper has been used to try to eradicate zebra mussels since about 2011, Dahlberg said. Large amounts may be toxic to other species of aquatic life. But researchers have seen evidence that low concentrations of copper can be effective against veligers, while potentially less harmful to other species.

In the study, researchers from the U and the U.S. Geological Survey applied a copper sulfate solution to the water in St. Alban's Bay near Excelsior. Another bay with a similar zebra mussel population and water chemistry, Robinson's Bay in Deephaven, did not get the copper treatment and was used for comparison.

The solution was applied in July. When the researchers conducted follow-up tests in August and October, zebra mussels had practically vanished from the treated site. Using submerged testing equipment to which the zebra mussels attached themselves, they found 125,000 zebra mussels per square meter at Robinson's Bay, the untreated site, while at St. Alban's Bay, the treated site, the number was just 69 per square meter. Algae production also fell at the treated site.

Gabriel Jabbour of Orono, a longtime Lake Minnetonka advocate who helped with the project, said he saw the study's effects at marinas he owns — one in St. Alban's Bay and another in nearby Excelsior Bay.

"When we pulled the boats out of there [at the end of the season], there was a profound difference between the boats on the St. Alban's Bay side vs. the Excelsior side," Jabbour said.

Most native species at the St. Alban's Bay site — including bluegills, largemouth bass and yellow perch — did not appear seriously affected, the researchers found. The notable exception was fathead minnows, whose numbers declined at the treated site.

Zebra mussels were found to have higher concentrations of copper in their tissues than other species, Dahlberg said.

Follow-up tests this year showed the zebra mussel population had climbed a little, but not to previous levels. There were 5,500 per square meter at the treated site, Dahlberg said. That might sound like a lot, but at the untreated site the number was 200,000.

"We weren't necessarily trying to wipe out everything — this is more of a long-term tactic," Dahlberg said.

Next year, researchers plan to add another lake to the study.

"We know we did have effects," Dahlberg said. "This is probably not going to be a one-and-done scenario."

Jabbour compared the zebra mussel battle to the search for treatments for COVID-19.

"This is just like the virus; we're all looking for the silver bullet," he said. "But just like the virus or other viruses, you're probably going to have two or three things that will make the difference."