Milfoil Weevil Study

Project Status: 
Current Status: 

(Updated July 16, 2013) -- The milfoil weevil study has been put on hold. State law requires that any weevils stocked in lakes must come from Minnesota lakes, and researchers have been unable to collect enough of the small beetles to propagate the number needed for the study. Now that Eurasian watermilfoil has flowered, it is also too late to use the weevils to their full effectiveness. 

A variety of factors, including the late spring, may have reduced the number of available milfoil weevils. District staff is working with the University of Minnesota to monitor weevil populations in lakes across the District. 
About this project: 

Eurasian water milfoil (Myriophyllum spicatum) is an invasive species from Europe that can grow in dense underwater stands and form thick mats at the surface of the water. It outcompetes native plants and interferes with recreational activities like boating, fishing, and swimming.  Eurasian water milfoil has the ability to reproduce through stem fragmentation and runners meaning a single segment can start a new colony.

Milfoil weevils (Euhrychiopsis lecontei) are native beetles that feed and develop on milfoil plants. Because they are small and  This makes the weevil very unique, as specialist herbivores are very rare among aquatic insects.  This is why the milfoil weevil has shown the most promise as a potential bio-control agent for Eurasian water milfoil and why it continues to be the subject of research.


In 2012 the District contracted with the biological services firrm EnviroScience to implement their Milfoil Solution in the watershed. Due to a very early spring flowering season, the study was scaled back to one location in 2012, Christmas Lake, but will expand to Christmas Lake and Lake Minnetonka's Veteran's Cove in 2013. EnviroSciene’s Milfoil Solution® includes surveying and stocking milfoil weevils into multiple sites around each lake over the course of multiple years.

The project aims to:

  • Establish significant populations of the milfoil weevil E. lecontei  the test sites
  • Reduce Eurasian watermilfoil density in thes areas, to the point that the canopy of topped out milfoil is largely eliminated and milfoil is kept below the lake surface at non-nuisance levels
  • Increase native plant diversity in the stocking areas
  • Be monitored by independent, third parties for the eventual publication of the study results in a peer-reviewed journal. 


2012 – 2015: EnviroScience’s Milfoil Solution®
Every year for three years:

  • An early-summer  survey of the milfoil beds to be stocked along with a number of unstocked control areas.
  •  Stocking of milfoil weevils in June.
  • A follow-up survey in the fall to track the progress of the programs. 


  • Conduct a fish survey to monitor bluegill sunfish and other omnivores that may prey on milfoil weevils.
  • Monitoring of milfoil and native plants by an independent third-party
  • Bi-weekly weevil population assessments by the University of Minnesota

2014 and 2015:

  • Monitoring of milfoil and native plants by an independent third-party
  • Bi-weekly weevil population assessments by the University of Minnesota

2016: Follow-up Survey

  • Conduct final surveys to assess the overall effectiveness of the milfoil weevils at controlling Eurasian water milfoil. 
Informational Materials: