Options to Control Eurasian Watermilfoil in Candlewood

Candlewood Lake is drawn down by nearly 10 feet every other winter.  The purpose is to expose Eurasian watermilfoil, an invasive aquatic plant that grows rapidly and chokes the shallow water of the lake, to the extremely cold temperatures needed to kill it.

However, Larry Marsicano, the executive director of the Candlewood Lake Authority, says "the best practice guidelines for the timing and duration of drawdowns, which assure maximum exposure of the plant to cold weather, have not always been followed in recent years."  

Additionally, "recent winter weather (except this past winter) has also been warmer that expected.  And there are concerns about the possible negative impacts of drawdowns on lake water quality."

"These factors have resulted in poorer lake-wide control of milfoil -- and have led many people to consider if different or even multiple strategies are needed to control the nuisance weed."

Thanks to Mr. Marsicano, the following is a brief overview of some accepted milfoil control strategies, other than drawdowns, and their financial and environmental pros and cons.

Milfoil Weevils -- Experiments with these tiny native insects, called the milfoil weevil, have been underway in Candlewood Lake for about five years.  The weevils feed exclusively on milfoil and they are a potential natural biological control. 

Tens of thousands of the sesame-seed-sized insects were placed on milfoil in several test sites in Candlewood Lake since the summer of 2008.  While the density of the weevil populations, and damage to the milfoil in the test sites has increased, the weevils have not yet increased to a level where they are producing a visible reduction in the milfoil.  CLA is continuing to monitor the test sites.

Pros: Native to the Northeast | Targets only milfoil | Ability to be self-sustaining.

Cons: Not yet proven effective in Candlewood during testing | Potentially costly to stock | Takes time to produce results.

Sterile Grass Carp -- While not native to North America, these fish feed on milfoil and other underwater vegetation.  They are bred sterile and unable to reproduce.  A permit from the CT-DEEP is required prior to their introduction and use.  Certain physical conditions must be met, such as barriers to assure the fish can't migrate out of their stocked body of water. 

Grass carp require an initial purchase and re-stocking in future years to assure their population remains at optimal numbers.  A population of about 7,500 is believed necessary to combat the milfoil problem in Candlewood.  At that volume, it is estimated the sterile grass carp will cost about $6 each, plus costs associated with preventing emigration out of Candlewood.

Pros: Should provide results in a couple of seasons | Record of success in many lakes.

Cons: Also eats beneficial native plants | Overstocking can eliminate all plant life | Requires continued funding | May cause algae blooms.

Chemical Herbicides -- Herbicides come in two forms: contact and systemic.  Contact treatments kill only the portion of the plant they come into contact with.  Systemic treatments are absorbed by the plants and transported to the roots, killing the entire plant.  Single herbicide treatments can range from around $300 per acre for contact treatments, to between $400 and $1,200 pre acre for systemic treatments.  Candlewood Lake has up to 500 acres of milfoil, depending on the year.

The application of a chemical herbicide to a body of water in CT is strictly controlled the CT-DEEP.  A permit is required.  Only licensed applicators are permitted to apply the herbicide.

Pros: Targeted to kill milfoil | Ability to provide results in one season.

Cons: Costly up front (due to size of application) and may require multiple/annual re-applications | Requires monitoring of private/community wells | May cause algae blooms | Possible swimming and lawn and garden irrigation restrictions after application.

Mechanical Harvesting -- Mechanical weed harvesters are similar to underwater lawn mowers.  They cut weeds (invasive and native alike) several feet below the surface of the water.  The cut weeds are then drawn up a conveyor belt onto the floating harvester, where they are stored until unloaded on land. 

Candlewood Lake has between 250 and 500 acres of milfoil distributed along 60+ miles of shoreline, depending on the year.  A mechanical harvester requires a significant initial capital investment, plus ongoing expenses to staff and operate and the equipment.  A single harvester is not believed capable of controlling the milfoil acreage on Candlewood Lake.  (A few private firms provide this service on a contract basis.)

Pros: No chemicals or introduction of live species into the Candlewood Lake | Targeted removal

Cons: Costly up front and continuing expenses to operate | Removes only the top few feet of plants below the surface | Does not kill or remove entire plant and roots | Multiple harvesters likely required to control the amount of acreage of Candlewood's milfoil.