The Minnesota Department of Natural Resources says zebra mussels are still alive in two lakes treated with chemicals designed to kill the invasive species.
The DNR treated lakes in Otter Tail and Douglas counties with copper sulfate last fall. They took that action because the zebra mussel infestation of each lake was recent and contained to a small area.
DNR Research Scientist Gary Montz said there's no evidence zebra mussels reproduced in the lakes this summer. But researchers found a small number of mussels on docks pulled from the water this fall.
"We knew when we tried these treatments that this was not a case of a slam dunk, that we were just going to treat and be able to walk away and say, 'there, we got them,' " Montz said.
The results show the difficulty of using chemical treatment, Montz said.
"It comes back to the idea that it's far, far better to prevent them from ever getting into the lake through inspecting boats and cleaning off equipment than it is to try and do an emergency treatment like this," Montz said.
Montz said it's too soon to say if the treatment is a total failure. He won't rule out trying the chemical treatment on other lakes. The DNR will monitor the lakes next year to see if the zebra mussels are reproducing.