Invasive Exotic Plant Tutorial - Eurasian water-milfoil MC

Species Management and Control Information

Eurasian water-milfoil
Miriophyllum spicatum L.

Source: The following information is taken from a number of sources and those sources are identified at the beginning of the referenced information. The intent of this resource is to provide the user with as much of the information that exists for management and control of this species as is practical. It is important to note that new and improved methods are added regularly which will require you to visit the websites provided for updates on this information. Bibliographies and resources referenced by these sources are not included here, but are included at the websites from which this information was extracted.

READ THIS FIRST!

Before administering any of the following management and control options, it is imperative that you are familiar with the background information provided under the General Management and Control Section.

For additional, and potentially more current, information on management and control of this species, use the Mid-Atlantic Exotic Pest Plant Council (MA-EPPC) listserve or any of the other listserves identified in the Resource - Listserve section of this tutorial. You will find directions for subscribing to the list serve there. The MA-EPPC listserve has an archives feature that saves past discussions (beginning in 1999) about specific species control. These messages are at: http://groups.yahoo.com/group/ma-eppc/messages.

GENERAL MANAGEMENT CONSIDERATIONS:

Plant Conservation Alliance, Alien Plant Working Group - Weeds Gone Wild - Fact sheet - Eurasian watermilfoil - http://www.nps.gov/plants/alien/fact/mysp1.htm - Authors: Tom Remaley, Editors: Jil M. Swearingen and Alison A. Dalsimer - April 1, 2004.

Large harvesting equipment can be used to mechanically remove milfoil in larger areas; a sturdy hand-rake can be used for smaller areas. Other available options include manipulation of water level, light penetration and chemical control. Potential impacts to existing native aquatic plant species should be evaluated carefully before implementing any of these techniques. For the single harvest, removal should take place just before peak biomass is obtained (early summer). Substantial regrowth may occur if this is done too early. Better results appear with multiple harvests in the same growing season. If multiple harvests are not possible, then sustaining annual harvests is an option. All fragments of milfoil plants must be removed to achieve adequate control.

Wisconsin Department of Natural Resources - Invasive Plant Fact Sheets - Eurasian Water-milfoil - http://www.dnr.state.wi.us/org/land/er/invasive/factsheets/milfoil.htm - September 3, 2004.

Preventing a milfoil invasion involves various efforts. Public awareness of the necessity to remove weed fragments at boat landings, a commitment to protect native plant beds from speed boaters and indiscriminate plant control that disturbs these beds, and a watershed management program to keep nutrients from reaching lakes and stimulating milfoil colonies--all are necessary to prevent the spread of milfoil.

Monitoring and prevention are the most important steps for keeping Eurasian water milfoil under control. A sound precautionary measure is to check all equipment used in infested waters and remove all aquatic vegetation upon leaving the lake or river. All equipment, including boats, motors, trailers, and fishing/diving equipment, should be free of aquatic plants. Lake managers and lakeshore owners should check for new colonies and control them before they spread. The plants can be hand pulled or raked. It is imperative that all fragments be removed from the water and the shore. Plant fragments can be used in upland areas as a garden mulch.

Delaware River Invasive Plant Partnership - Invasive Plant Fact Sheets - http://www.paflora.org/Invasive%20species%20fact%20sheets.htm - prepared by: Ann F. Rhoads and Timothy A. Block, Morris Arboretum of the University of Pennsylvania - April 2000.

Prevention should be a major goal of any control program since once milfoil becomes well-established within a water body, it is difficult or impossible to remove. Programs should target recreational lake users and educate them on how to avoid transporting Eurasian water-milfoil from site to site. In addition, every effort should be made to maintain the low nutrient status of naturally oligotrophic lakes as a way of reducing problems with this and other invasive species.

MANUAL AND MECHANICAL CONTROL:

Where water levels are under manual control, raising or lowering of the water can be an effective way to reduce the growth of milfoil. By raising the water level, plants can be "drowned" by not having access to enough light. By lowering the water level, plants can be dehydrated and, at the right time of the year, frozen to death. This type of control is best used in conjunction with herbicides and shade barriers.

Bankside plantings, floating native plant species, light limiting dyes, or shade barriers are effective ways of reducing the amount of light reaching the plants and may reduce overall growth rates. Barriers can be used to prevent the movement and spread of aquatic weeds in ponds and lakes. A barrier is usually a suspended blocking screen that hangs vertically from a cable to a depth of about 4 meters; the cable is suspended by drum floats.

Center for Aquatic and Invasive Plants, University of Florida and Sea Grant - Non-Native Invasive Aquatic Plants in the United States - http://plants.ifas.ufl.edu/seagrant/myrspi2.html#hpcontrol

The use of mechanical harvestors and chopping machines should be carefully considered because resulting plant fragments may easily regrow or be carried downstream to create new infestations. Harvesting machines are effective at reducing a large biomass in a short time, however harvesting may have to be done several times per year.

In Okanagan Lake, British Columbia, authorities have apparently successfully experimented with management by simultaneously rototilling plants and roots and underwater vacuuming (Newroth 1988).

Water level manipulation (drawdown) has been used effectively to control Eurasian water-milfoil in Tennessee reservoirs (Bates et al. 1985)

Ohio Department of Natural Resources - Invasive Plant Fact Sheets - http://www.dnr.state.oh.us/dnap/invasive/12milfoil.htm - January 8, 2003

Mechanical cutters and harvesters, as well as hand-pulling, are the most common methods of Eurasian water-milfoil control. To be effective, all fragments must be collected and removed from the site to eliminate new establishments. Manipulations of the water level, where feasible, may have an effect on the plant. Low water levels can desiccate populations and high levels will "drown" the plants by not giving them access to enough light.

Southeast Exotic Pest Plant Council - Invasive Plant Manual - Eurasian water-milfoil - http://www.se-eppc.org/manual/watermilfoil.html -2003

Harvesting: Large equipment exists to mechanically remove milfoil in larger areas. A sturdy handrake can be used for smaller areas, such as around docks, swimming areas and harbors. For the single harvest, harvesting should take place just before peak biomass is obtained. There may be substantial regrowth if done too early. Better results appear with multiple harvests in the same growing season. If multiple harvests are not possible, then sustaining annual harvests is an option. All fragments of milfoil plants must be removed to achieve adequate control. Water Levels: Where water levels are under manual control, raising or lowering of the water can have an effect on the milfoil. By raising the water level, plants can be "drowned" by not having access to enough light. By lowering the water level, plants can be dehydrated and, at the right time of the year, frozen to death. This type of control is usually used in conjunction with herbicides and shade barriers.

Heat: The viability of milfoil fragments is severely reduced after being subjected to temperatures between 45-50°C in the cooling systems of thermal electricity generating systems.

Light: The amount of light reaching the plant can be limited by changing water levels using bankside plantings or floating plant species, light limiting dyes, or shade barriers.

Booms: Barriers are used to prevent the movement and spread of aquatic weeds in ponds and lakes. Usually the barrier is a suspended blocking screen that hangs vertically from a cable to a depth of about 4 meters, and the cable is suspended by drum floats. This will not eradicate milfoil, but it can limit its spread

Wisconsin Department of Natural Resources - Invasive Plant Fact Sheets - Eurasian Water-milfoil - http://www.dnr.state.wi.us/org/land/er/invasive/factsheets/milfoil.htm - September 3, 2004.

Mechanical cutters and harvesters are the most common method for controlling Eurasian water milfoil in Wisconsin. While harvesting may clear out beaches and boat landing by breaking up the milfoil canopy, the method is not selective, removing beneficial aquatic vegetation as well. These machines also create shoot fragments, which contributes to milfoil dispersal. Harvesting should be used only after colonies have become widespread, and harvesters should be used offshore where they have room to turn around. Hand cutters work best inshore, where they complement hand pulling and bottom screening. Bottom screening can be used for severe infestations, but will kill native vegetation as well. A diver-operated suction dredge can be used to vacuum up weeds, but the technique can destroy nearby native plants and temporarily raise water turbidity.

Hand pulling is the preferred control method for colonies of under 0.75 acres or fewer than 100 plants. The process is both thorough and selective (not to mention time-consuming); special care must be taken to collect all roots and plant fragments during removal. Sites remote from boat traffic can be covered with bottom screens that are anchored firmly against the lake bed to kill grown shoots and prevent new sproutings, but screens must be removed each fall to clean off sediment that encourages rooting. Buoys can mark identified colonies and warn boaters to stay away.

Whenever possible, milfoil control sites should become customized management zones. For example, colony removal by harvesting can be followed by planting native plants to stabilize sediments against wave action, build nurseries for fry, attract waterfowl, and compete against new milfoil invasions.

Rotovation (underwater rototilling), installation of bottom barriers, hand pulling by divers, and dredging harvesters are the most common methods of Eurasian water-milfoil control. To be effective, all fragments must be collected and removed from the site. Manipulations of the water level, where feasible, may have an effect on the plant. Low water levels can desiccate populations and high levels will cause plants to decline by not giving them access to enough light.

BIOCONTROL:

Van Driesche, R., et al., 2002, Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04, 413 p. http://www.invasive.org/eastern/biocontrol/6EurasianMilfoil.html.

Grass carp have been widely used to suppress many different aquatic nuisance plants. Regulations as to whether individual states allow grass carp releases vary widely. By 1991, grass carp had been introduced into 35 states, following initial releases in Arkansas and Alabama in 1963. In 14 states, releases of both fertile diploid and sterile triploid fish have been allowed; 15 states have allowed only sterile triploids, and 19 states have prohibited all importations (Julien and Griffiths, 1998). Problems associated with the use of grass carp include difficulties in guaranteeing sterility, escape of individuals into areas with native fish species and, in particular, lack of specificity. In fact, in choice-tests with different plant species, M. spicatum was the least preferred food for grass carp (Pine and Anderson, 1991) The voracious appetite of this fish has potential to eliminate much of the aquatic plant biomass, greatly reducing the native plants that serve as important food and shelter for invertebrates, fish, and waterfowl (McKnight and Hepp, 1995).

The aquatic moth A. ephemerella now appears widespread from the east coast to Minnesota and Wisconsin (Johnson et al., 1998); however, the species is cryptic and probably often overlooked, particularly as early instars. Regional surveys in New York show that A. ephemerella is widespread and occasionally abundant in most lakes in New York State (Johnson et al., 1998).

The midge C. myriophylli has been collected at the Cornell Experimental Ponds (Ithaca, New York) and at many other places throughout North America. Indeed, it may be present in many lakes, but taxonomic difficulties in identifying larvae have prevented an easy assessment of the species distribution (Johnson et al., 1998).

The weevil E. lecontei occurred in 24 of 26 lakes surveyed in New York (Johnson et al., 1998) and appears widely distributed in North America. The species is now commercially available, but releases into lakes and ponds often only supplement existing (often much larger) populations (Hairston and Johnson, 2001) and therefore may not expand the range of the species. Several states require that the mass-reared individuals used for releases be offspring of locally collected weevils.

In laboratory and in lake-enclosure experiments, A. ephemerella successfully controls Eurasian watermilfoil growth by destroying the apical meristem, reducing biomass and plant height, and preventing canopy formation. Long-term monitoring in Cayuga Lake, New York, showed a dramatic decline of Eurasian watermilfoil associated with large populations of A. ephemerella (Johnson et al., 1998; Gross et al., 2001). Ten years after the initial decline in Cayuga Lake, Eurasian watermilfoil biomass remained at very low levels with no canopy formation. The reduction in Eurasian watermilfoil constituted a 90% decline of the species in Cayuga Lake. As the Eurasian watermilfoil population declined, native plant species returned and they now dominate the plant community (Johnson et al., 1998; Gross et al., 2001). Prevention of canopy formation in Eurasian watermilfoil due to A. ephemerella herbivory was the most likely mechanism explaining the shift in dominance from M. spicatum to E. canadensis in Cayuga Lake (Gross et al., 2001).

In the laboratory, in lake-enclosure experiments, and in the field, E. lecontei is capable of causing high levels of damage to the host plant (Creed and Sheldon, 1995; Sheldon and Creed, 1995; Newman et al., 1996; Hairston and Johnson, 2001; Newman and Biesboer, 2001). Certain declines of Eurasian watermilfoil have been associated with large populations of E. lecontei (Creed and Sheldon, 1995; Sheldon and Creed, 1995; Newman and Biesboer, 2001), and E. lecontei seems to be contributing to declines in Eurasian watermilfoil in some lakes in Vermont, Minnesota, and New York, (Creed and Sheldon, 1995; Sheldon and Creed, 1995; Newman et al., 1996; Newman and Biesboer, 2001) but not in others (Hairston and Johnson, 2001). High levels of seasonal weevil damage does not always translate into long-term declines in Eurasian watermilfoil populations (biomass or plant height) in lakes due to recovery of M. spicatum after adult weevils move to shore for overwintering (Hairston and Johnson, 2001).

Center for Aquatic and Invasive Plants, University of Florida and Sea Grant - Non-Native Invasive Aquatic Plants in the United States - http://plants.ifas.ufl.edu/seagrant/myrspi2.html#hpcontrol

Years of research to find insect biocontrols has resulted in the successful introduction of insects which are believed to be helping keep Eurasian water-milfoil under control; biocontrol fish also have been successfully used (Bonar et al. 1993.)

Ohio Department of Natural Resources - Invasive Plant Fact Sheets - http://www.dnr.state.oh.us/dnap/invasive/12milfoil.htm - January 8, 2003

A native weevil (Eurhychipsis lecontei) has been found to feed and reproduce on Eurasian water-milfoil. This insect may be an excellent control method in that it is selective to this species and does not appear to feed on native water-milfoils. Other biological control methods including a fungus (Mycoleptidiscus terrestris) are currently being researched.

Wisconsin Department of Natural Resources - Invasive Plant Fact Sheets - Eurasian Water-milfoil - http://www.dnr.state.wi.us/org/land/er/invasive/factsheets/milfoil.htm - September 3, 2004.

Eurhychiopsis lecontei, an herbivorous weevil native to North America, has been found to feed on Eurasian water milfoil. Adult weevils feed on the stems and leaves, and females lay their eggs on the apical meristem (top-growing tip); larvae bore into stems and cause extensive damage to plant tissue before pupating and emerging from the stem. Three generations of weevils hatch each summer, with females laying up to two eggs per day. It is believed that these insects are causing substantial decline in some milfoil populations. Because this weevil prefers Eurasian water milfoil, other native aquatic plant species, including northern watermilfoil, are not at risk from the weevil's introduction. Twelve Wisconsin lakes are currently part of a two-year DNR project studying the weevil's effectiveness in curbing Eurasian water milfoil populations. The fungi Mycoleptidiscus terrestris is also under extensive research.

A native North American weevil (Eurhychipsis lecontei) has been found to feed and reproduce on Eurasian water-milfoil. This insect may be a useful biocontrol agent; however, although it seems to prefer Eurasian water-milfoil, it also feeds on native water-milfoils including M. exalbescens and M. verticillatum, both of which are state endangered species. Other biological control methods, including a fungus (Mycoleptidiscus terrestris), are currently being studied. In some very limited situations triploid (sterile) grass carp may be useful. Stocking with grass carp is regulated by the Pennsylvania Fish and Boat Commission.

CHEMICAL CONTROL:

Fluridone (the active ingredient in Sonar AS) is a selective herbicide for milfoil and several other exotic aquatic weeds. There are no restrictions on swimming, fishing, or drinking after application, and season-long control can be achieved with one application. Fluridone is available in liquid or granular form, and can be used as a spot treatment or on an entire waterway. For best results, applications should be made before or during the early stages of active growth.

Center for Aquatic and Invasive Plants, University of Florida and Sea Grant - Non-Native Invasive Aquatic Plants in the United States - http://plants.ifas.ufl.edu/seagrant/myrspi2.html#hpcontrol

Registered aquatic herbicides such as endothall, 2,4-D and fluridone do provide temporary control of Eurasian water-milfoil, but efforts to eradicate the plant "are rarely, if ever, likely to succeed" (Smith & Barko 1990).

From the University of Florida Aquatic Weed Management Guide by V.V. Vandiver, 1999: According to this Guide, a number of aquatic herbicides may be used to manage "watermilfoil", including formulations of endothall, diquat, copper, 2,4-D, and fluridone. A concentration of 5 ppm 2,4-D for 1 h will kill all plants (Steward & Nelson 1972). As always, comply with federal law by following the herbicide label instructions, permissible sites and application rates.

Ohio Department of Natural Resources - Invasive Plant Fact Sheets - http://www.dnr.state.oh.us/dnap/invasive/12milfoil.htm - January 8, 2003

Fluridone® is a selective aquatic herbicide for Eurasian water-milfoil and other aquatic weeds that may be useful.

Southeast Exotic Pest Plant Council - Invasive Plant Manual - Eurasian water-milfoil - http://www.se-eppc.org/manual/watermilfoil.html -2003

Fluridone: Fluridone is a selective herbicide for milfoil and several other exotic aquatic weeds. There are no restrictions on swimming, fishing, or drinking after application and season-long control can be achieved with one application. Fluridone is available in liquid or granular form and can be used as a spot treatment or on an entire water-way. For best results, applications should be made before or during the early stages of active growth. Granular 2,4-D: This method is appropriate for large unmanageable areas of milfoil. This herbicide is formulated to release the active ingredient over several days. Apply granules at a rate of 100 lbs per acre of water. The herbicide granules will sink to the root zone and kill the plant. Liquid DMA 2,4-D: Application of liquid DMA 2,4-D may be used for milfoil control in areas with low dilution potentials such as ponds and lakes. Application rates should be less than 2.0 parts per million (ppm). Subsurface application rate has to be adjusted proportionately for varying water depths.

Wisconsin Department of Natural Resources - Invasive Plant Fact Sheets - Eurasian Water-milfoil - http://www.dnr.state.wi.us/org/land/er/invasive/factsheets/milfoil.htm - September 3, 2004.

Herbicide treatment is not recommended because it is typically disruptive to aquatic ecosystems and not selective in the vegetation it affects, thus threatening native plants.

In smaller water bodies (350 acres or less), there has been some success using fluridone to control Eurasian water-milfoil and other aquatic weeds.

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