|Posted by [email protected] on October 30, 2012 at 9:40 AM|
Since there is no way to completely eradicate Eurasian watermilfoil from a lake once it has been introduced, control efforts must instead focus on: controlling newly introduced infestations, preventing further spread of the plant, or reducing the nuisance level of the problem. Some methods are more appropriate for well-established populations, while others are better suited for those that are recent introductions.
The State of Vermont is concerned about the impacts certain milfoil control methods could have on the environment. Bottom barriers, all mechanically powered devices (harvesters, hydrorakes), herbicides, and biological controls require permits from the Department of Environmental Conservation before they can be used to control nuisance aquatic plant growth. Contact the Department to determine if your proposed control method requires a permit and to obtain permit applications.
Bottom Barriers, specially made sheets of materials such as fiberglass, polypropylene, or polyvinyl chloride (PVC), anchored to a lake bottom will prevent plant growth by blocking sunlight. Bottom barriers are most appropriate to control growth in localized areas such as in swimming areas, around docks or to create boat lanes out to deeper water.
With diver operated suction harvesting, scuba divers use suction hoses powered by a surface compressor to selectively remove milfoil from the lake bottom. Although too labor intensive on a large scale, this method has proven to be highly successful at combating newly established infestations in Vermont lakes.
A hydrorake removes plant roots and shoots by raking the lake bottom. Any removed material must then be deposited on shore. Hydroraking has had limited use in the state but is most practical for providing short-term relief from dense milfoil infestations.
When done properly, pulling milfoil plants by hand is highly effective for controlling small, newly introduced milfoil populations.
Mechanical harvesters cut off the milfoil (and any other plants) below the water surface, gathering the cut material as they move through the plant bed. Milfoil roots are not removed in this process. Mechanical harvesting, like lawn mowing, merely reduces the height of plant growth temporarily in order to make the lake more usable. Removing the plant material from the lake does however, prevent the plants from contributing to the sediment which rapidly accumulates under dense aquatic plant beds. Mechanical weed harvesting has been used on several heavily infested Vermont milfoil lakes.
Other potential milfoil control methods include rotavating, chemical herbicides and biological controls such as grass carp.
Rotavating involves a machine that "tills" the lake bottom, dislodging both the roots and stems of the plant. Plants are either collected by a mechanical harvester or, if conducted in the late fall, are allowed to wash ashore and dry over the winter. This method has yet to be used in Vermont.
There are a number of federally registered aquatic herbicides that control Eurasian watermilfoil. Considerations for use include cost, the potential need for repeated applications, and product label restrictions that prevent their application in lakes used as water supplies
Biological controls such as insects, bacteria or fungi that will impact milfoil are in the experimental stages only. Their use as a milfoil control method may prove to be the control of the future. The use of plant-eating fish such as the grass carp, a native to China, is currently illegal in Vermont.
The VTDEC has been working with the watermilfoil weevil Euhrychiopsis lecontei since 1989. The weevil was responsible for at least one Eurasian watermilfoil decline in Vermont, in Brownington Pond in Brownington. It has been found naturally occurring in many other milfoil-infested lakes in Vermont. The weevil, a native aquatic insect, has shown promise as a potential biological control agent for Eurasian watermilfoil, and is currently the subject of ongoing research.