Taming the milfoil monster

08/12/2005

Mary Lynn Smith,
Star Tribune
August 12, 2005

After taking root in Minnesota more than 20 years ago, the invasive Eurasian water milfoil plant is extending its reach into more state lakes and spreading frustration among some who love them.

“We’ve tried to work with the DNR (Minnesota Department of Natural Resources) on this issue, but there have been too many roadblocks,” said Dick Osgood, executive director for the Lake Minnetonka Association. “It’s being mishandled and mismanaged. ... I think they need to be more aggressive. If we wait, what happens is that milfoil grabs a foothold in more and more lakes. ... There will be a snowball effect.”

Osgood’s group, along with the Minnesota Lakes Association, opened a public discussion this week on measures to more forcefully fight Eurasian milfoil.

“Given the fact that milfoil continues to spread, I understand the idea they want us to be more aggressive,” said Dave Wright, who oversees management of the DNR’s invasive species program. “But I don’t know what the answer is.”

Osgood and others think they might have one possible solution: a herbicide known as fluridone, which is being used successfully in other states but is not approved for use on a lakewide basis in Minnesota.

“No one is saying it’s a silver bullet,” Osgood said. “We know that it’s not a cure-all. It’s just one more tool in the toolbox.”

DNR officials, who have tested the herbicide on a limited basis for more than a decade, haven’t been convinced.

They remain concerned about the damage it can do to native plants and other parts of the lake’s ecosystem. But Wright said that doesn’t mean they won’t listen to Osgood and others as the science continues to evolve.

Meanwhile, so does the problem.

A commercial herbicide applicator first discovered milfoil in Minnesota in 1987 on Lake Minnetonka. “It had been in the U.S. since at least back in the mid-’40s if not before that,” said Chip Welling, coordinator of the DNR’s Eurasian water milfoil program. “And we knew it was causing problems in the Madison (Wis.) lakes in the ‘60s and had been concerned since then that it would be transported here from some other place.”

About 170 Minnesota lakes and streams are now infested, with about nine lakes a year added to the list, Welling said. The good news is that there are at least 10,000 lakes that aren’t on that list, he said.

“There’s steady (increase),” Welling said. “Clearly, we wish it wasn’t spreading, but at least we’re not getting 50 or 60 new lakes a year.”

But some of the infested lakes, such as Lake Minnetonka, are among the largest and most used lakes in the state. The weed takes root in the soft, mucky bottom and creates thick underwater stands of tangled stems that often crowd out native plants. It can quickly reach the surface and form mats of vegetation that can stop boats dead and tangle swimmers. At the very least, it’s not very attractive to most people.

In the Minneapolis Chain of Lakes, the clearer lakes such as Calhoun, Cedar, Harriet and Lake of the Isles have become ideal breeding grounds for the pesky plant.

“It catches fish that die, and trash, and it gets kind of nasty,” said Marcia Holmberg, Minneapolis natural resources coordinator.

The Conservation District for Lake Minnetonka spends about $100,000 each year to harvest milfoil on about 500 to 600 acres of the 14,000-acre lake.

Better methods

Meanwhile, lake property owners, with the DNR’s permission, can also yank, cut and chemically treat milfoil along their shorelines.

Over the years the DNR has looked for newer and better ways to attack milfoil. The weevil, an insect that loves to munch on milfoil, can do some serious damage to the plant. The problem is that sunfish eat most of the weevils.

An experiment with a fungal pathogen worked well in a controlled experiment but not very well in a real lake. Some southern states have used grass carp to control milfoil, but Minnesota doesn’t want to introduce yet another non-native species to its waters.

That brings Osgood and others back to fluridone as a way to attack the problem on a lakewide basis.

They agree with DNR officials that the milfoil problem can be managed but not eradicated.

“There’s just a lot of frustration among lake association members that the rules and guidelines that the DNR has used are three and four years old,” Osgood said.

“The landscape has changed and the nature of understanding the science (of fluridone) has increased. ... The state has to be more aggressive in fighting invasive plants and more protective of native ones.”