Eurasian watermilfoil control options

Eurasian watermilfoil (Myriophyllum spicatum L.) is an invasive exotic aquatic plant that was introduced to North America in the early 1940s. 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 eurasian watermfoilmay 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.