Milfoil

New types of milfoil have been discovered that could be more difficult to treat than the Eurasian variety.

Zebra mussels may be getting most of the attention when it comes to aquatic invasive species, but milfoil is still causing problems in Lake Minnetonka.

Among the biggest milfoil concerns are new, hybrid varieties. Minnehaha Creek Watershed District AIS Program Manager Eric Fieldseth said the hybrids are a combination of native northern milfoil and the invasive Eurasian milfoil. Currently, it’s unknown exactly how many varieties of hybrid milfoil are in Lake Minnetonka, he said.

Hennepin County recently awarded the MCWD a $15,470 grant to study milfoil in Lake Minnetonka in collaboration with the University of Minnesota and Montana State University. The goal of the study is to discover as much as possible about the different kinds of milfoil.

Fieldseth said that the researchers will compare and contrast milfoil found in bays that have undergone treatments for the invasive plant and those that have not been treated. They will also study weevils, which are small native beetles that feed on milfoil. Fieldseth said the goal is to see if the weevils have a preference among the native, Eurasian or hybrid varieties.

The knowledge gleaned from the study will help inform future milfoil management decisions.

“Eradication really isn’t feasible,” Fieldseth said. “However, there can be ways to effectively manage milfoil.”

According to the Minnesota Department of Natural Resources, Eurasian milfoil was accidentally introduced to North America from Europe. It spread by boats and waterbirds, and in the 1950s it first started to reach into Midwestern states. The invasive was first discovered in Minnesota in Lake Minnetonka in 1987.

Eurasian milfoil and the native northern milfoil can be easily confused with each other, Fieldseth said. According to the DNR, Eurasian milfoil typically has 12 to 21 leaflet pairs per leaf, while the native milfoil usually has five to 10 leaflet pairs per leaf.

Eurasian milfoil can easily spread by stem fragmentation. According to the DNR, a single segment of stem and leaves can take root and start a new colony. The plant is harmful because it can crowd out other native plants in a lake, Fieldseth said. In summer, big mats of the invasive plant can form on the surface of a lake, limiting the light that gets through to the aquatic life below it. The mats can also interfere with recreational use of lakes.

Northern milfoil is considered a beneficial native plant. Fieldseth said that unlike the Eurasian milfoil, northern milfoil doesn’t come together in large mats on the surface of lakes.

The study on the hybrid milfoil varieties will take place throughout the summer. Fieldseth said he didn’t know yet which bays would be included in the study, but that lake users won’t be affected by the work.

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