The Minnesota Department of Natural Resources says a new treatment failed to kill zebra mussels in two lakes.
The DNR used copper sulfate, a chemical commonly used to treat lakes for swimmers' itch, to treat Lake Irene in Douglas County and Rose Lake in Otter Tail County.
DNR invasive species specialist Nathan Olson says the agency did not find any zebra mussel larvae or large zebra mussels during scuba searches this summer. But Olson says during fall searches the DNR did find adult zebra mussels in both lakes.
Olson says the DNR cannot confirm that zebra mussels have reproduced in either lake. But the recent find means efforts to kill the zebra mussels were unsuccessful.
In both cases, boat lifts used in infested lakes were moved to non-infested waters.