Non-native species like zebra mussels make national news, but the dangerous plant variable milfoil is rarely discussed outside of lake communities.

Myriophyllum heterophyllum, commonly referred to as variable milfoil, is an invasive aquatic plant that has been contaminating lakes throughout the Northeastern United States since the 1960s. It looks harmless enough, resembling a green squirrel tail with the occasional small, reddish flower. Yet, variable milfoil can grow up to 15 feet long, forming dense mats of vegetation that choke out native species. These mats block sunlight from reaching other submerged plants, killing them, and can deplete oxygen levels in the water while decaying, which hurts fish and other aquatic animals. The plant not only destroys ecosystems but also inhibits recreational water activities, as the dense mats of milfoil make boating or swimming impossible. Furthermore, these large clusters of plant matter are the perfect breeding grounds for mosquitoes, more bad news for those who visit lakes.

Maine and New Hampshire Hard Hit

Variable milfoil most seriously affects Maine and New Hampshire due to an absence of natural predators and ideal water conditions for plant growth. The plant is found in over 90 water bodies in these two states alone, including Lake Winnipesaukee, the largest lake in New Hampshire. Variable milfoil was likely brought to the Northeast from the Southern United States, its native habitat, attached to the bottoms of boats as a kind of “aquatic hitchhiker.” Small fragments of milfoil were chopped up by boat propellers and then floated to different parts of the lake, quickly growing to form masses of plant matter. Milfoil spreads most easily by fragmentation, but loose milfoil seeds can also grow into full plants in a short period of time.

Prevention the Most Effective Control

Leslie J. Mehrhoff/Wikimedia Commons/CC BY 3.0Despite the dangers of variable milfoil, it is rarely discussed, except by members of already affected lake communities. Today, the plant is even sold in pet stores for use as a decoration in fish tanks with little or no indication of its invasive nature. Minimal research has been conducted concerning variable milfoil, but there do exist some studies that focus on another member of the milfoil genus, Myriophyllum spicatum or Eurasian watermilfoil. Native to Europe, North Africa, and Asia, Eurasian watermilfoil is now found in every continent except Antarctica and Australia. Its global impact has caused variable milfoil to take a backseat in the discussion of invasive plants, often being written off as a less severe relative of Eurasian watermilfoil. However, Eurasian watermilfoil and variable milfoil are not always found in the same lakes due to differing pH preferences, and control methods for the two species can vary. For example, Euhrychiopsis lecontei, colloquially referred to as the milfoil weevil, is a small beetle that eats certain milfoil species. Its ability to consume and damage M. spicatum stems allows the weevil to be a successful biocontrol agent for Eurasian watermilfoil, curbing populations when introduced into infested lakes. Yet, it won’t touch variable milfoil.

With no practical biocontrol available and heavy regulations on herbicides, lake communities affected by variable milfoil mainly resort to pulling the weeds from the lakebed by hand. These hand-pulling programs have proven to be effective in the long run, but manual removal is a slow and expensive process. With a lack of milfoil awareness, lake communities face insufficient funds for these projects, and improperly supervised harvesting can allow fragments of plant matter to break off of the milfoil plants. This can potentially cause new infestations as these fragments float to other parts of the lake.

The best way to prevent milfoil infestations is to stop the spread of the plant in the first place. For information on how to prevent aquatic hitchhikers like variable milfoil from spreading, check out this video.