Eurasian Watermilfoil Control Methods

Managing Eurasian Watermilfoil

Eurasian watermilfoil management has been practiced in North American water bodies for more than 50 years, generating the most extensive body of aquatic plant management experience of any invasive submerged species. This experience demonstrates that sustained, adaptive, integrated management programs — combining targeted herbicide treatment, biological control with the milfoil weevil, and prevention of new introductions — provide the best long-term outcomes. Single treatments or unsustained programs consistently result in temporary suppression followed by regrowth and reinfestation. All aquatic weed control activities require state permits.

Herbicide Treatment

Triclopyr (Renovate OTF, Navigate Granular, others)

Triclopyr is the herbicide most specifically recommended for Eurasian watermilfoil. It is a synthetic auxin that disrupts growth regulation, causing abnormal growth followed by death. It is highly effective against Eurasian watermilfoil and most native broadleaf aquatic plants but is not effective against grasses and most graminoids. Application typically uses granular formulations broadcast from a boat over the treatment area, or liquid formulations applied by spray. Treatment timing is critical: triclopyr is most effective on actively growing milfoil with well-developed stems (late spring to early summer in most of the U.S.). Water temperature should be above 15°C (60°F) for optimal uptake. Results are visible within 2–4 weeks.

Hybrid caution: Some Eurasian × Northern watermilfoil hybrid populations show reduced sensitivity to triclopyr. If treatment results are unexpectedly poor despite correct application, consider hybrid presence and potentially seek species confirmation before retreating.

ProcellaCOR (Florpyrauxifen-benzyl)

ProcellaCOR is a newer herbicide that has demonstrated exceptional efficacy against Eurasian watermilfoil (including hybrid forms) at very low use rates. It works by disrupting auxin signaling, causing rapid abnormal growth and tissue death. Its low application rate and favorable aquatic environmental profile make it increasingly preferred in sensitive ecosystems. State registration is expanding. This herbicide is particularly useful in situations where triclopyr has shown reduced efficacy against hybrid populations.

Fluridone (Sonar)

Fluridone is effective against Eurasian watermilfoil at appropriate whole-pond concentrations maintained for 60–90 days. It is most used in enclosed water bodies where concentrations can be maintained. Unlike triclopyr, fluridone has broader spectrum activity and will affect other aquatic plants (both invasive and native) in the treatment area. It is most appropriate when multiple invasive species need to be managed simultaneously.

Endothall (Aquathol, Hydrothol)

Endothall is a contact herbicide used against Eurasian watermilfoil and several other aquatic weeds. It acts quickly and degrades relatively rapidly, making it useful in flowing water or situations where residual herbicide effects are undesirable. It is typically applied as a spot treatment or in combination with other herbicides.

Biological Control: The Milfoil Weevil

The native North American milfoil weevil (Euhrychiopsis lecontei) is a small (~3 mm) insect that is highly host-specific to Eurasian watermilfoil and its hybrids. Adults feed on the growing stem tip and deposit eggs in the stem. Larvae bore down through the stem, mining the vascular tissue and causing stem dieback. Populations of milfoil weevils that develop naturally or are augmented through stocking can significantly suppress milfoil populations over time — several lake case studies document 70–90% reduction in milfoil biomass over 3–7 years following weevil augmentation programs.

Milfoil weevil augmentation is available through several commercial suppliers and state programs. Best results are achieved in lakes where: water clarity allows dense native plant communities to develop after milfoil is suppressed (providing refuge habitat for weevils); the lake is nutrient-limited enough that milfoil regrowth after herbicide treatment is not immediate; and weevil populations are given time to establish before herbicide retreatment is applied. Herbicide and weevil programs should be carefully integrated — applying herbicide that reduces milfoil to very low densities before weevil populations are established can reduce the food supply that sustains the weevil population. Timing and integration of these tools is an active area of management research.

Mechanical Harvesting

Mechanical harvesting (cutting and removing plant biomass) provides rapid navigational access and aesthetic improvement. Properly equipped harvesters with containment systems can harvest milfoil without generating significant fragment dispersal. However, harvesting does not reduce the root system and plants regrow rapidly — typically requiring re-harvest every 4–8 weeks during peak growing season. Harvesting costs $300–800 per surface acre per operation. It is best used as a short-term management tool for access and recreation while longer-term herbicide or biological control programs take effect.

Prevention: The Most Cost-Effective Strategy

New Eurasian watermilfoil introductions to uninfested lakes are preventable. Boat inspection and decontamination programs at lake access points — particularly in lake districts that receive boats from distant infested water bodies — are the most effective prevention tool. Many states have implemented mandatory Clean-Drain-Dry requirements for boats moving between water bodies, with inspection stations at heavily used launch sites. Early detection monitoring programs (regular diver or hydroacoustic surveys of the littoral zone) enable rapid response to newly detected plants before infestations become established. A new milfoil infestation of a few plants can sometimes be spot-treated and eradicated; a full-scale infestation cannot. See prevention and early detection strategies for complete guidance.

References

• Creed, R.P. (1998). Euhrychiopsis lecontei as a biological control agent. Journal of Aquatic Plant Management 36:12–19.
• Madsen, J.D. (1997). Eurasian watermilfoil biomass. Journal of Aquatic Plant Management 35:15–21.
• Getsinger, K.D. (1998). Herbicide treatment of Eurasian watermilfoil. Journal of Aquatic Plant Management 36:72–81.