Eurasian and hybrid watermilfoils

Aquatic plant management: Eurasian and hybrid watermilfoils 

What are Eurasian and hybrid “milfoil”?

Milfoil is a common name for Eurasian Watermilfoil. Many people call it simply a lake weed. Like many submerged plants, it has roots and spreads when parts of the plant break off. The fragments grow roots and become a new aquatic plant. It grows in depths of 3 to 9 feet, but underwater plants can grow deeper if the water is clear.

There are good and bad kinds of water milfoil in the waters of the Great Lakes area. This includes native and non-native milfoils that are related and look similar. The native milfoil is also called northern watermilfoil. It can breed with the nuisance non-native to make hybrid milfoil. Hybrid milfoils have traits of both the species. They may require genetic screening to distinguish them from non-hybrids. Both Eurasian and hybrid water milfoils can have unwanted impacts on the environment and recreation. The aquatic plants form dense beds that can block out beneficial native species and impair swimming, boating, and fishing.

Lake Management: milfoil control

Effective control of Eurasian and hybrid water milfoils is difficult to achieve. Few aquatic weed killer products (herbicides) have been consistently effective for long-term control. The results of maintenance lake weed control can be variable and ineffective. This is especially true for partial-lake water weed treatments. A whole-lake water weed treatment using a product called fluridone has been the most effective. It has provided good watermilfoil control for up to three seasons. The herbicide florpyrauxifen-benzyl (procellacor) has shown promising results. Initial research results suggest fewer non-target impacts as compared to other lake weed killer products. However, more research is needed to be sure. Aquatic weed killers can have non-target effects on wildlife (see “Herbicide control” sheet for more information).

A Wisconsin study compared the effects of Eurasian watermilfoil versus the effects of aquatic herbicide treatments. They found that lake-wide 2,4-D treatments had larger effects on native aquatic plants than milfoil itself. As such, any Eurasian watermilfoil control effort using aquatic weed killers should be carefully considered. For example, if the invasive aquatic weeds do not impede recreation, treatment may not be worthwhile. The same goes if invasive aquatic plants are mixed with native aquatic plants.

Physical and mechanical aquatic weed control methods have been tried in areas where aquatic herbicide use is restricted. Their results have been with mixed. Benthic mats and lake weed removal are two examples of these approaches. Burlap benthic mats can provide immediate relief in small areas. That said, mats can be difficult to set. Watermilfoil may also settle over the mats in the long term if they are not maintained. Lake weed cutters and hand-pulling can remove small patches of milfoil. However, it can be labor intensive. See “Physical and mechanical methods” fact sheet for more information on these options.

Biological control has also had partial successes. However, it does not provide immediate reductions in lake weed populations. The milfoil weevil has been used for several decades. It is native to the Great Lakes, and efforts for milfoil often involve augmenting weevil populations. However, due to limited stock, lake groups will need to raise sufficient stock themselves.

Integrated lake management strategies are still in pilot testing. An example includes using a combination of methods like aquatic herbicide and benthic mats. Integrated lake management strategies for other species have been effective. People are encouraged to work with a lake management consultant to explore integrated strategies. They can often save money and reduce the amount of herbicide needed. 

Part of the aquatic weed control challenge is the hybridization of Eurasian milfoil and native milfoil. While these strains are all categorized as “hybrid” water milfoil, variation exists within them. There are thousands of hybrid strains, each with a different response to herbicide treatment. Some show some aquatic herbicide resistance. Many hybrid strains are more aggressive and resistant to chemical aquatic weed control than native strains. To effectively manage this lake weed, an understanding of the genetic makeup is important. For more information, or to find out which strain may be in your lake, contact the Thum lab: https://www.montana.edu/thumlab/

Where do I find out more?

Eurasian and hybrid watermilfoils are common in lakes in the Great Lakes region. Their populations vary with factors like nutrient levels and temperature. If your lake has nuisance levels of these aquatic plants, you should contact a lake management service provider. They will help determine whether aquatic weed control efforts are appropriate. They can help decide how to proceed. See the “Management Plan” fact sheet for information on finding a lake management service provider. Many factors may determine whether and how control is attempted. These include: how much of the lake is populated by Eurasian watermilfoil? Are the patches only water milfoil, or are there native aquatic plants? Has the amount of water milfoil been staying about the same or increasing? For those lakes without this aquatic plant, early detection allows a small chance for eradication. At the least, early detection will make subsequent control efforts less costly and labor-intensive.