Identifying and managing invasive Eurasian and hybrid watermilfoils in Michigan lakes

Some hybrids of this invasive plant are resistant to herbicides, making genetic testing critical for management.

For decorative purposes.
A close-up of the invasive aquatic plant, Eurasian watermilfoil. Photo credit: J. Latimore

Eurasian watermilfoil (Myriophyllum spicatum) has been a problematic invasive weed in Michigan lakes for decades. Once introduced, it can rapidly grow and create dense stands that impede recreation and negatively impact lake life. Management approaches for Eurasian watermilfoil include herbicide application, biological control, and physical removal. Recently, lake managers and scientists observed that traditionally effective herbicides were failing to control invasive watermilfoil in some lakes. Genetic analysis of plants from these lakes revealed that Eurasian watermilfoil had begun crossing with native northern watermilfoil (Myriophyllum sibiricum), leading to concern that hybrid plants may be herbicide resistant.

Today, aquatic plant managers increasingly recognize that Eurasian watermilfoil (including hybrids with native northern watermilfoil) is genetically diverse, and that strains can differ in their growth, spread, impacts, and herbicide response. A practical challenge for Eurasian watermilfoil management is developing efficient and effective methods to predict how a specific watermilfoil population will respond to a proposed control tactic (e.g., a specific herbicide) before implementing management.

Because management of invasive Eurasian and hybrid watermilfoil is a widespread need in numerous Michigan lakes, we initiated a project to investigate the distribution of hybrid strains of watermilfoil across the state and to determine the sensitivity of these strains to the common herbicide, fluridone.

We genetically analyzed hybrid watermilfoil collected by Michigan State University Extension’s MiCorps Cooperative Lakes Monitoring Program volunteers and professionals from lakes across Michigan and tested the hybrid plants’ susceptibility to fluridone. We made two important determinations. First, there are many different genetic strains of hybrid watermilfoil in Michigan lakes. Second, these strains respond differently to fluridone, and some are highly resistant to it. Therefore, we strongly recommend genetic testing of populations being considered for herbicide control.



Monitor your lake regularly for possible aquatic invasive plants

Early detection of invasive species when invasions are small in scale allows for more choices in control options (below). Once invasions become dense or cover a large area, control may become cost-prohibitive – especially in the case of physical and biological control approaches. The MiCorps Cooperative Lakes Monitoring Program includes a training program called the Exotic Aquatic Plant Watch for volunteers interested in monitoring their lakes for invasive plants.

Identify aquatic plant species carefully

Michigan has many native watermilfoil species that typically do not cause a nuisance and are a beneficial part of the natural plant community in lakes. If you identify Eurasian watermilfoil and are considering herbicide treatment for control, we recommend genetic analysis to determine whether the population is purely Eurasian watermilfoil or a hybrid. Hybrids cannot be identified by physical characteristics alone. Because we know that herbicide resistance can occur in hybrid watermilfoils, you should determine what strain you are trying to control and gather the latest information on herbicide response for that strain before launching a control program. A small investment in genetic testing could save you thousands of dollars in wasted herbicide application. Our herbicide response data are public, and we encourage lake managers to share their experiences about various other strains with one another and their clients.

With this information in hand, develop a management plan

Your plan should include clear goals and objectives and a plan for monitoring and evaluating progress. Your plan should be developed in consultation with community members and stakeholder groups and may require involvement of permitting agencies and professional lake managers. Many lake management actions require permit(s). You will need to consider the resources available for investing in management and have a plan for adjusting your actions if the situation changes or your goals are not being met.

Management options for invasive Eurasian watermilfoil in lakes include:

  • Vigilance without active control:
    • The population is monitored annually, with the possibility of action if it grows to nuisance levels.
  • Physical control such as:
    • Hand pulling, including diver-assisted suction harvesting (DASH)
    • Smothering (benthic barriers)
    • Cutting/mechanical harvesting is not recommended because of the risk of spread by fragments.
  • Biological control such as:
    • Application of herbivorous insects like the milfoil weevil (Euhrychiopsis lecontei)
  • Chemical control:
    • Application of herbicide(s) approved for watermilfoil control
    • Selecting an herbicide known to be effective against your strain of watermilfoil and avoid an herbicide if there are strain-specific data indicating resistance to that herbicide. 

For more information on control options and permit requirements for treating aquatic nuisance species in Michigan waters, contact the Michigan Department of Environment, Great Lakes, and Energy (EGLE).

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