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Eurasian watermilfoil control options

Eurasian watermilfoil (Myriophyllum spicatum L.) is an invasive exotic aquatic plant that was introduced to North America in the New York state in 1880. It grows rapidly and tends to form a dense canopy on the water surface, which often interferes with recreation, inhibits water flow, and impedes navigation. As with most weeds, there are three general control strategies that can be employed: mechanical/manual, chemical, and biological.

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, but generally are not permitted in Minnesota.

Remember that simply cutting plants is analogous to cutting your lawn. Depending on growing conditions, several cuts may be needed each season. More disruptive approaches such as dredging or rotavation can eliminate all plants, reducing habitat for fish and food for waterfowl and potentially destabilizing sediments, resulting in murky water. Hand pulling can be effective in localized areas or for scattered plants, but is labor intensive. Contact your local authorities before taking action.

Large mechanical harvesters can be effective at reducing vegetation. The degree of selectivity is dependent on the plant community and skill of the operator – all plants beneath the harvester are cut and some fish and other vertebrates will be incidentally removed. Mechanical harvesting will also reduce the abundance of milfoil herbivores, especially when large areas are harvested. These harvesters are effective at providing access paths and clearing areas around beaches or docks. Commercial harvesters are expensive: capital outlays can range from $30,000-100,000. Annual costs per hectare can range from $350 to $4000 for regular control and contractors may charge from $300 to $600 per acre per cut. Harvesting generally needs to be repeated each year and often more than once annually. The logistics of transport and milfoil disposal often present greater challenges than the actual harvesting.

Chemical control can be effective, however, long-term eradication of larger infestations is unlikely and chemical controls can be expensive. Chemical controls also often need to be repeated every year to every three years. Systemic herbicides such as 2-4-D, fluridone (Sonar) or triclopyr are most effective for Eurasian watermilfoil and can, under appropriate circumstances, give selective control. Generally, the aim is for selective control, to reduce Eurasian watermilfoil, but retain a native plant community. Thus, systemic herbicides, which are taken up by the plant and will kill the entire plant, are preferable to contact herbicides which will knock down the plant, but do not affect the roots and prevent regrowth. The most commonly used herbicide for milfoil control in Minnesota is 2-4-D (often Aqua-Kleen) which is selective for dicots. Control is most effective with spring or fall applications and some damage to other dicots (e.g., coontail, water lilies) can be expected. 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 Minnesota DNR Aquatic Plant Management Program.

More detailed information of the use of chemicals and some state specific regulations can be found at:

Biological control (biocontrol) is the use of parasitoid, predator, pathogen, antagonist or competitor populations to suppress a pest population. The aim in biological control of weeds is not to eliminate the pest (and thus the control agent), but to suppress the pest population to levels that are no longer a nuisance.

Biocontrol offers several potential advantages over conventional methods, including reduced cost, long-term effectiveness, and little or no negative impacts on other aspects of aquatic systems. However, targeted physical, chemical or mechanical controls may be needed to ensure clear channels or swimming areas. Current efforts in Minnesota and elsewhere are focused on the native milfoil weevil, Euhrychiopsis lecontei, which has been associated with natural declines of Eurasian watermilfoil and has shown potential in controlled experiments in the field and experimental tanks.

Once exposed to the exotic Eurasian watermilfoil, the milfoil weevil prefers Eurasian over its native host northern watermilfoil. Adult weevils live submersed and lay eggs on milfoil meristems. The larvae eat the meristem and bore down through the stem, consuming the cortex, and then pupate (metamorphose) lower on the stem. Development from egg to adult occurs in 18-30 days at summer temperatures. The consumption of the meristem and stem mining by larvae are the two main effects of weevils on the plant and this damage can suppress plant growth, reduce root biomass and carbohydrate stores, and cause the plant to sink from the water column. Although the weevil has been quite effective at some sites, it has not been effective at other sites, mainly due to failure to maintain adequate population density throughout the summer. Currently, we cannot predict when, where and how the weevils will or will not be effective, but predation by sunfish appears to be a primary limiting factor. The aim of our current work is to improve our understanding so we can predict effects and appropriate circumstances for use of biocontrol.

Eurasian watermilfoil control options Eurasian watermilfoil ( Myriophyllum spicatum L. ) is an invasive exotic aquatic plant that was introduced to North America in the New York state in 1880. It

How to Kill Eurasian Watermilfoil

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Eurasian watermilfoil (Myriophyllum spicatum) is a pesky aquatic weed that rarely germinates by seed but can re-establish itself from fragments or pieces. It is adapted to U.S. Department of Agriculture plant hardiness zones 2 to 10 and is considered invasive from Florida to Canada. It’s a fast-growing weed that can become established in all kinds of water bodies – from small, backyard ponds to slow-flowing mountain rivers to massive, deep, high-altitude lakes. It foils propellers, ruins swimming holes and is thoroughly difficult to eradicate.

Assess the extent of the Eurasian watermilfoil (Myriophyllum spicatum) infestation. Eurasian watermilfoil is a submerged, rooted perennial that can grow up to 9 feet long. The invasive weed can be distinguished from native plants by its feathery leaves, which are 1 1/2 inches long and arranged around the stem in groups of four. Each leaf has 12 pairs of leaflets.

Remove large areas of Eurasian watermilfoil with large, barge-mounted harvesting equipment, which can be purchased or rented. The harvesting equipment works like underwater lawnmowers, cutting off the weeds, scooping up the vegetation and hauling it aboard the barge. This technique helps to control the weed’s spread in some areas but rarely eliminates it because pieces can break off and re-establish, sometimes in areas the plant hadn’t infested previously.

Remove small clumps of the weed with a hand-held rake. Scoop up small mats of the weed with the rake, and haul what you collect to dry land. This technique is an option when the water body is too small for a barge and the use of herbicide would kill all the other aquatic plants in the pond or lake and disrupt the food chain.

Place aquatic mats or other barriers over isolated clumps of Eurasian watermilfoil. These barriers can kill the weed by blocking sunlight, which cuts off the weed’s photosynthesis. These barriers often are used in swimming areas and around docks. An option is to add a non-toxic aquatic dye to the infested water. The dye kills the weed by preventing sunlight penetration, interfering with photosynthesis. Add about 0.6 ounces of the dye for every 10,000 gallons of water, adding the chemical from the shoreline or a boat and allowing the chemical to disperse on its own.

Apply an aquatic herbicide containing fluridone, triclopyr, endothall, diquat, copper, imazamox, penoxsulam, bispyribac or flumioxazin formulations to your infestation of Eurasian watermilfoil. Some herbicides are systemic herbicides, which are absorbed by the plant and kill all of it. Contact herbicides usually work quickly and kill the plant when they touch it. Herbicides, however, often kill all aquatic plants they contact, not just Eurasian watermilfoil. A National Pollutant Discharge Elimination System Permit is often needed before anyone can treat a natural body of water with an herbicide. In some states, only state-licensed individuals can apply an aquatic herbicide.

  • University of Nevada Cooperative Extension: Eurasian Watermilfoil
  • State of Washington Department of Ecology: Using Fluridone to Eradicate Eurasian Watermilfoil
  • Texas A&M AgriLife Extension: AquaPlant, a Pond Manager Diagnostic Tool — Eurasian Watermilfoil
  • Penn State Extension: Pocket Guide to Mid-Atlantic Water Garden Species
  • An aquatic herbicide typically is applied in spring or summer, depending on the climate and when the Eurasian watermilfoil begins a period of rapid growth.
  • When using an aquatic herbicide, treat just a section of the water body every two weeks until the problem is eliminated. Applying it that way can help to prevent oxygen depletion that will kill other aquatic life. Aerating at night after a treatment helps to offset oxygen lost to plant decomposition.
  • Check your boat for pieces of Eurasian watermilfoil before launching it in a non-infested waterway.
  • Killing large mats of Eurasian watermilfoil can cause oxygen depletion from dead plant decomposition, and that in turn can kill the fish in a pond or lake.
  • Don’t irrigate with water that is being treated with the herbicide fluridone, a slow-working herbicide. Swimming and fishing is typically allowed, however.
  • Read the label of an herbicide carefully before applying it, even if you’ve used the chemical previously.

Jim Sloan is a writer and editor in Reno, Nevada. He has been a journalist for more than 25 years and is the author of two books, “Staying Fit After Fifty,” and “Nevada: True Tales from the Neon Wilderness.”

How to Kill Eurasian Watermilfoil. Eurasian watermilfoil (Myriophyllum spicatum) is a pesky aquatic weed that rarely germinates by seed but can re-establish itself from fragments or pieces. It is adapted to U.S. Department of Agriculture plant hardiness zones 2 to 10 and is considered invasive from Florida to Canada. …