Milfoil Management Technique Information Sheet

These information sheets are intended to be objective summaries of what we know (and don’t know) about each management technique.

Management Technique: Milfoil ( lecontei)

Description: The milfoil weevil is a small, herbivorous aquatic , belonging to the family . It is a milfoil specialist, meaning that it feeds and develops only on plants in this genus. The weevil completes all life stages fully submersed and the larvae are stem miners. These characteristics make it very rare among aquatic . These characteristics are precisely why the milfoil weevil has shown promise as a potential biocontrol agent for Eurasian water milfoil. The milfoil weevil is native to the northeastern U.S. (Connecticut, Massachusetts, New York, and Vermont as well as in the upper Midwestern U.S. This weevil thrives on the native water milfoil and tends to prefer Eurasian milfoil. The are put on a piece of milfoil and then tied to plants in the lake. Divers are required to do this.

Effectiveness: The milfoil weevil has been effective in some lakes and ponds and not in others. The milfoil weevil needs a water body with a shoreline conducive to over wintering. Water bodies which have rip rap, walls, and which are cleaned of leaves and debris make it difficult for weevils to survive the winter. In addition, water drawdowns make it difficult for overwinter survival by changing the habitat. Perch and sunfish are natural predators of the weevil and there are concerns that an abundant perch population leads to a decline in these weevils. It takes at least two years to see if the weevils are successful; crashes may occur after two years, with oscillations in milfoil and weevil populations thereafter. It has been reported that weevils enter the lake in the late spring and leave in the late summer although milfoil grows for a more extended period, which limits the weevils’ effectiveness.

Costs: Weevils are expensive. They cost over $1 per beetle. Treatment for Woodridge Lake is estimated at $60,000 for the first year. Every year the success of the weevil program will have to be evaluated

Risks: There are no known risks to the use of milfoil weevils. Public perception is usually positive.

History of Use at Woodridge Lake: Weevils were put into Woodridge Lake in 2000 and 2001, with 10,000 added each year. A survey conducted in July of 2002 indicated the weevils were impacting milfoil as they were intended but the community was not happy with the rate of change and decided to take a more aggressive path. No follow up surveys were conducted of the weevil population’s impact to date.

Use at Other Lakes in Connecticut/New England: Weevils have been used at many lakes and ponds in Connecticut. The largest is Candlewood Lake and there is not much success reported. In Vermont, New York, and Massachusetts there are several lakes in which weevils have been introduced. Success has been limited and appears to be in bodies of water with shorelines compatible to successful weevil overwinter. Low populations of sunfish and perch also appear to be important factors. Limitation to success is related to basic ecology: if the weevils greatly depress milfoil, they will then die off, and milfoil will resurge. Weevils may recover, setting up an oscillating pattern of milfoil abundance, but it is unlikely that weevils can maintain a low density of milfoil indefinitely, considering food and predation influences.

Permitting: Permits are not required to introduce weevils in Connecticut.

Considerations/Comments:

Public health concerns: None identified

Lake Structure: Rip rap, walls, and a clean shoreline make it difficult for weevils to over winter. Benthic barriers are also a problem for weevil over wintering.

Aquatic plant and impact: Weevils do not have a negative impact on animal life. They may provide food for fish and predator fish need to be part of the management process.

Siltation impact: None identified

Public fears: None identified

Water clarity: None identified

Algae: None identified

Biomass: By crashing milfoil early in the season it reduces that amount of biomass added to the lake. The larvae consume the plant starting and the top and continue down through the stems.

Phosphorus: No impact

Downstream: None identified

Weed grow back after treatment: Annual monitoring is recommended and restock as required.

Compatibility with other treatment methods: Drawdowns may result in negatively impacting weevils’ ability to over winter