Hybrid Milfoil: Coming to a Lake Near You?

Post by Sara Fox, Aquatic Invasive Species Outreach Assistant, University of Wisconsin-Extension. This post was originally featured on the UW-Sea Grant Blog. 

This summer, while water skiing in Waukesha County, Wisconsin, I recall taking a massive wipeout. I remember expecting to feel the painful sting after hitting the water, similar to that of a belly flop. Instead, I landed on a cushion of plants. I looked around and found myself floating on top of a giant mat of milfoil!

If you’re a Wisconsin lake user, patches of milfoil probably aren’t new to you. However, a new milfoil hybrid combines genes of native and invasive species and may be a cause for concern on some Wisconsin lakes.

Wisconsin has many native milfoil species that provide cover and sometimes food for fish, invertebrates and aquatic insects. Milfoil species such as northern or whorled watermilfoil are native to Wisconsin and have contributed to lake ecosystems for a long time. More recently, in the early 1960s an invasive milfoil species called Eurasian watermilfoil was discovered in Dane County and since has spread to over 800 lakes in Wisconsin.

A diver surfaces through a heavy infestation of Eurasian watermilfoil.

As its name suggests, Eurasian watermilfoil is native to Europe, Asia and North Africa. Like many other plants, Eurasian watermilfoil grows up to the surface to access sunlight. Yet, unlike many other plants, it doesn’t stop at the surface; it grows across the surface, causing dense mats of vegetation that can get tangled in boat propellers and block sunlight from native plants below.

Eurasian watermilfoil can spread quickly and efficiently, primarily because of its ability to produce adventitious roots after fragmenting. This means that if you cut a strand of Eurasian watermilfoil into ten pieces, each piece could sprout new roots and become its own plant.

Treatment options for lakes that contain Eurasian watermilfoil differ by lake and vary by levels of efficacy, cost, benefits, and drawbacks. Attempted treatments have included: chemical application, manual hand pulling and use of biological control weevils.

Even more recently, a hybrid milfoil has evolved on some lakes in Wisconsin and it might be a big deal. The hybrid (sometimes referred to as a “super milfoil”) is a genetic cross between native northern watermilfoil and invasive Eurasian watermilfoil. It exhibits characteristics similar to both northern and Eurasian watermilfoil and genetic testing is often needed to verify its presence.

A possible hybrid species collected and pressed in Jefferson County, Wisconsin.

The hybrid species hasn’t caused any verified negative effects to lake ecosystems that differ from Eurasian watermilfoil, but it’s potential to cause trouble for lake users in coming years makes it worth keeping an eye on. For example, state biologists have observed that the hybrid may be harder to control using the chemical application methods we use to treat Eurasian watermilfoil. Specifically, in some cases, it has shown a reduced sensitivity to  2, 4-D and fluridone. (It’s important to note that if your lake hasn’t seen success treating for Eurasian watermilfoil it doesn’t necessarily mean you have the hybrid.)

Additionally, the hybrid’s growing habits seem to have a competitive advantage over both its parents. Expect more information on the hybrid’s behavior soon from organizations conducting research such as the University of Minnesota Aquatic Invasive Species Research Center and the Wisconsin DNR Aquatic Plant Management Program.

If you’re worried about hybrid milfoil making its way into a lake you care about, don’t panic! There are preventative measures individual lake users can take to make sure hybrid milfoil–and every other aquatic invasive species–doesn’t enter a new waterbody:

  • INSPECT your boat, trailer and equipment.
  • REMOVE any attached aquatic plants or animals (before launching, after loading and before transporting on a public highway)
  • DRAIN all water from boats, motors and all equipment
  • NEVER MOVE live fish or plants away from a waterbody.
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7 Responses to Hybrid Milfoil: Coming to a Lake Near You?

  1. Bill Scott says:

    Or, we do the responsible thing as trustees of the navigable waters, and close all the boat landings to transient boats, allowing use of only boats designated for use at that water, and made avaialble to the public by rental at the landings. Management of an unnecessary nuisance that devalues lakeshoe property and makes water more difficult to navigate and disrupts the native flora and fauna is not a solution and should not be an option. Let’s stop the invasives where they stand now and not facilitate continued spread by enabilng the transport to uninfected waters. Lastly, a hybrid that naturally develops is a product of evolution and if it thrives it is natural selection, bluring the lines of justification for regulation, which is yet another reason to stop enabling the transport of invasive aquatics.

    • sarafox says:

      Thank you for the response. We share your concern that aquatic invasive species hold potential to devalue lake property and disrupt navigation.

      I’m not sure if you’re familiar with the watercraft inspection program called Clean Boats Clean Waters that many Wisconsin lake associations have recently began using. This program seeks to inspect watercraft vehicles on their way into and out of waterbodies to make sure AIS are not being spread from lake to lake. I think this is along the same line of thinking your comment expresses.

      Here is some more information on the program: https://dnr.wi.gov/lakes/cbcw/

  2. Jim Gehrke says:

    Ms Fox makes li sounds as though the DNR is making use of 24-d to combat EWM on a regular basis. I wish that were true. I, and a group of dedicated members of Crescent Lake Assn. in Oneida County, have been watching EWM go from a single plant in July of 2015 to an estimated 24 – 26 acres currently. We have repeatedly met with DNR staff and asked to take an aggressive approach to managing the invasive species. But the DNR essentially says we do not “have enough” yet to get serious about controlling it. We have communicated with other lake associations that have had similar response when trying to use the most effective means of treatment for EWM. Even more frightening it’s potential to hybrid into an even greater problem.

    • sarafox says:

      Thank you for the comment. Sorry to hear about the EWM spread on your lake, because of its invasive tendencies we’ve heard similar stories from other lake residents around the state.

      Here is more information about applying for permits to chemically treat aquatic invasive species:

      Otherwise, you can reach out to your local AIS coordinator for more information regarding feasible treatment options. Find your coordinator here:

  3. bre says:

    This is a complex problem and not all boaters or other movers between lakes take it seriously. It does destroy environments. Maybe a test program to limit boats to certain lakes would help or a certification process before being able to move between. You can see boats being hauled up and the managing that would be a real challenge.

    I personally beleive people should not be able to posses non native speices in their care and control.

    • sarafox says:

      Thank you for the comment! You might be pleased to know there is currently a Wisconsin law that prevents possession of regulated species: https://dnr.wi.gov/topic/invasives/classification.html

      If you have suggestions for the Wisconsin DNR to include species on this list, you can make suggestions here until January 4th:

  4. Dawn says:

    Your article discusses chemical application, manual hand pulling and use of biological control weevils; how come there is no mention of mechanical harvesting.
    I realize there is concern on harvesting fragments, but boaters and swimmers are causing fragments also. At least a harvester will collect the majority of milfoil harvested and then perhaps following up with the chemical treatment.
    I am just curious why the Wisconsin Department of Natural Resources seems to exclude mechanical harvesting.

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