Eurasian watermilfoil is a submersed vascular plant in the family Haloragaceae. It is native to northern Europe and Asia and arrived in North America sometime between the late 1800s (Reed 1977) and the early 1940s (Couch and Nelson 1985; the later date has verified vouchers). Many other members of the genus Myriophyllum are native to North America. In Minnesota, several related species are relatively common and abundant (northern watermilfoil, Myriophyllum sibiricum Komarov = M. exalbescens Fernald; whorled watermilfoil, M. verticillatum L.). Parrot feather (M. aquaticum (Vell.) Verd. = M. brasiliense Cambess.), is another exotic watermilfoil species, native to South America. It is commonly sold as an ornamental and aquarium plant and is problematic in southern states and some coastal states (e.g., Washington), but does not occur in northcentral North America or Minnesota. More Info At These Links
Eurasian watermilfoil is a particularly problematic exotic aquatic weed in North America, due to its ability to reproduce from fragments and spread rapidly, its high growth rate in a range of temperatures and environmental conditions, and its tendency to reach the surface and form extensive mats of plant at the surface, which can allow it to shade and outcompete native vegetation (Madsen et al. 1991; Boylen et al. 1999) but see Valley and Newman 1998). Grace and Wetzel (1978), Aiken et al. (1979) and Smith and Barko (1990) provide good overviews of Eurasian watermilfoil biology and ecology and Madsen (1998) identifies some factors that influence invasion success. Eurasian watermilfoil is found in > 40 states and it is estimated that millions dollars are spent annually on Eurasian watermilfoil control (Sheldon and Creed 1995, Sales 1997; click here for report). Although Eurasian watermilfoil has been in Wisconsin since the 1960's, it was not reported in Minnesota until 1987, when it was found in Lake Minnetonka (Exotic Species Program 1993). Within three years, Eurasian watermilfoil had spread to 22 waterbodies and by 1992 was found in 60 waterbodies. In 1996, Eurasian watermilfoil was in nearly 80 waterbodies and 18 counties (Exotic Species Program 1996) and by 1999 it had colonized over 100 waterbodies (Exotics Species Program 2000, Crowell 2000). The rapid spread was likely due to inadvertent transport by boaters (e.g., Newroth 1993), however, some lakes appear to have been infested by natural means (downstream transport or waterfowl). More Info on Eurasian watermilfoil
As with most exotics, the best approach is to prevent invasion. Once an exotic is established it is highly unlikely to be erradicated (e.g., Crowell 1999). New tools are being developed to predict new exotics and to develop strategies for preventing their introduction (e.g., Madsen 1999). Minnesota's program of education, laws that prohibit transport and boater inspections appear to have reduced the spread of Eurasian watermilfoil (Crowell 2000), however, prevention will likely not be 100% effective and control measures will likely be required for established infestations.
Madsen (2000) provides an excellent review of aquatic macrophyte control strategies. His report is available online.
As with most weeds, there are three general control strategies that can be employed:
mechanical or manual, chemical, and biological. Biological controls, with pathogens and insects, are currently being evaluated; operational biological controls are available (EnvrioScience), but are still under evaluation. More predictable controls may be available in the next several years. Grass carp are not specific and will consume many native plants, but are used in some states. Mechanical and manual control, either by hand pulling, raking, or harvesting, is effective at reducing current abundance of plants and is useful to clear channels or maintain access. It will not result in long term control and depending upon growing conditions, several removals may be needed each year and regrowth may be fast unless roots are removed or plants are harvested close to the sediment. Professionally contracted mechanical harvesting can cost from $300 to 600 per acre. Water level management and bottom barriers may also be used in some locations. Chemical control can be effective, however, chemical control often needs to be repeated every year to every three years. Systemic herbicides such as 2-4-D, fluridone (Sonar) or Trichlopyr are most effective for Eurasian watermilfoil and can, under appropriate circumstances, give selective control. Selective control is difficult to achieve, however, and professionally-applied chemical control can cost from $200-2,000 per acre. Regulations may vary by state and municipality; check with local authorities before conducting control. Minnesota regulations are available from the MN DNR Aquatic Plant Management Program. A summary of benefits of aquatic macrophytes and control methods applicable to Minnesota is available in Krischik et al. (1997; click here for bulletin) and also from the Minnesota DNR. Cooke et al. (1993) and more recently, Madsen (2000) provide good general overviews on control methods for aquatic plants. Click below for more details on these control methods.