Battling the Invaders
As of January 2007, twenty-nine waterbodies in Maine are known to be infested with invasive aquatic plants. Variable water-milfoil is the most widespread of these invaders, accounting for twenty-four of the infestations. A hybrid of variable-milfoil is found in two additional waterbodies. The remaining three are all single infestations of small ponds by a single invasive species: Hydrilla in Pickerel Pond in Limerick, curly leaf pondweed in West Pond in Parsonsfield, and Eurasian water-milfoil in an unnamed quarry pond in Scarborough.
| Invasive Aquatic Plants Known to be in Maine | |
|---|---|
 Variable water-milfoil infestation in the Songo River at Sebago Lake State Park |
 Eurasian water-milfoil infestation in a Scarborough quarry pond photo by: Don Cameron |
 Culry-leaf pondweed has infested West Pond in Parsonsfield photo by: Ann Bove |
 Hydrilla infestation in Pickerel Pond in Limerick |
Once an infestation has been confirmed, rapid response is crucial. The prospects for eradication (or barring that, effective management at minimum risk to the aquatic ecosystem), is greatly increased by swift, well planned, and properly executed controls. In developing an invasive aquatic plant management plan, one of the most important questions to be answered is “How, exactly, is the invasive plant infestation to be controlled?” The principal approach in Maine—used primarily by groups currently involved in battling variable milfoil (or its invasive hybrid)—is “manual control.” Manual control methods may alternately be referred to as “non-chemical,” “physical” or “mechanical” methods. The three primary manual control methods currently being used in Maine are: manual harvesting, benthic barriers, and suction-assisted harvesting.
Maine has taken a cautious approach to the use of aquatic herbicides to control invasive aquatic plants. Herbicides, like all pesticides, pose a definite degree of risk for people, for fish, and for the integrity of the aquatic ecosystem which depends on that body of water. Though aquatic herbicides are seen by state officials as an “effective tool,” it is the state’s position that the “benefits of using herbicides rarely exceed the risks of very real adverse ecological impacts.” Therefore “it is only in extraordinary circumstances that the Maine Department of Environmental Protection (DEP) will support the use of herbicides.”1 In recent years, the DEP has approved and overseen the use of aquatic herbicides in two specific instances--the Hydrilla infestation in Pickerel Pond in Limerick, and the Eurasian water-milfoil infestation in the unnamed gravel pit in Scarborough. For more information on the use of aquatic herbicides in Maine, please see “The Use of Herbicides to Control Invasive Aquatic Plants: Questions and Answers More Questions) in the Fall 2006 issue of the Water Column
IMPORTANT! – All invasive aquatic plant control projects are subject to regulation under Maine’s Natural Resources Protection Act. Before planning any control project, contact the Maine Department of Environmental Protection for specific permit requirements. All native aquatic plants are strictly protected by Maine law.
Manual Control Methods
Below is a brief overview of the three primary manual control methods currently being used in Maine: manual harvesting, benthic barriers, and suction-assisted harvesting. More detailed information on each method is provided by clicking on the link at the end of each section. Accounts of ongoing manual control projects that are utilizing these methods may be reviewed in the Case Studies section.
Manual Harvesting (or Manual Removal)
Jim Chandler, using manual harvesting to
control variable milfoil in Lily Brook,
surfacing with a bag full of milfoil.
Most of the variable milfoil management efforts currently underway in Maine involve a combination of manual control methods. Nearly all of these projects involve at least some use of the method known as manual harvesting. Manual harvesting is a useful technique for removing scattered individual plants and controlling small infested patches. With manual harvesting, plants and their root systems are individually removed from the infested area, collected, and transported away from the waterbody for disposal. As even tiny plant fragments can generate new plants, it is very important when using manual harvesting, that every attempt is made to remove all plant and root fragments from the project site.
The means by which the plants are approached, handled, and even the way in which they are disposed of may vary, but the basic concept remains the same. Think “weeding the garden by hand (or with hand tools).” Now think “weeding the garden under several feet of water.” This should give you a pretty good sense of the work. Depending on the water depth, the work is done by waders, boaters, snorkelers and/or SCUBA divers. Though manual harvesting is a labor intensive process, if done with care it is a “species selective” technique that causes minimal impact to other native species in the vicinity of the control activity. However, despite the level of care and thoroughness, it is nearly impossible to see and remove every stem and root fragment in the infested area. For this reason, ongoing monitoring of management sites and routine control activity is essential. For more information on manual harvesting, please see “Battling Invaders with Our Bare Hands” in the Winter 2007 issue of the Water Column.
Benthic Barriers, (also called Benthic Mats or Bottom Barriers)
In the news: WCSH Channel 6 ran a story on June 2, 2007 reporting on Pleasant Lake - Parker Pond Association's work controlling variable water-milfoil with benthic barriers. Follow this link for the full story and video:Volunteers In Casco Continue Battle Against Milfoil
Photo by Nikki Leam
Team installing benthic barriers
to control variable milfoil in Lily Brook.
Placement of benthic barriers is another labor intensive, but effective, method for controlling invasive milfoils. Benthic mats are particularly useful in treating small to moderate sized patches of dense growth. They are used to suppress invasive plant growth in high use areas such as public swimming areas. If depths are sufficient, benthic barriers may also be used to clear and define plant-free boating channels through infested areas, reducing plant-boat contact and thereby minimizing the potential for boats to spread the infestation. Controlling larger infestations with benthic barriers is possible, but given the labor and materials involved, larger control projects are generally done incrementally in stages, and may in some cases take several years to reach the desired result.
Benthic barriers may be constructed in various shapes and sizes, using a variety of materials and systems for weighting the mats down. Their basic function, however, is to lay “flat” on the bottom of the lake, pond or stream, covering the infested area, preventing plants underneath from receiving sunlight, thereby killing them. (Returning to the garden analogy . . . think mulch.) The mats are left in place long enough to kill the plants (generally four to six weeks, though in some cases, the mats may be left in place for longer periods). Manual harvesting is often used in tandem with the placement of benthic barriers to control any “outliers” and plants that find their way out from under the mats around the edges. One significant advantage with the use of benthic barriers is that the plants in the treated area are, by and large, killed. The “almost impossible” challenge of extracting every root hair from the substrate (as is necessary to completely kill a plant through manual harvesting) is largely eliminated when this method is properly employed. One disadvantage is that benthic mats are not "species selective” and may cause “collateral damage” to any native flora and fauna that do not have the means to escape out from under the mats.
Suction Assisted Harvesting (also called Diver Operated Suction Harvesting or Suction Dredging)
Little Sebago Lake Association
has developed a floating work
station to support their suction
assisted harvesting activity.
The LSLA crew call their retrofitted
pontoon boat "Hippo."
Suction assisted harvesting is the least frequently used, of the three manual control methods now employed in Maine. It is a relatively expensive and cumbersome control option. However in certain circumstances--such as large, widespread infestations--suction assisted harvesting is proving to be an important management tool. Groups in Maine utilizing this method have shown enormous industry and innovation in developing the required technology and techniques. As the fine-tuning of the process proceeds and more “rigs” come on line, it is likely that the use of suction assisted harvesting in Maine will expand.
Suction assisted harvesting is “manual harvesting” (see above) with the added advantage of a highly efficient way to get the plants to the surface where they are collect for disposal. Rather than swimming the plants to the surface in mesh bags, divers extract plants by hand as above, and then feed the plant material directly into a suction tube for rapid transport to the work platform at the surface (generally a pontoon boat or barge). From the hoses, the plants and any sediments clinging to the plants, are pumped through some form of strainer system, then piled or bagged. The sediment laden water that comes along with the plants is either returned directly to the waterbody, or (better) is put though another system that removes sediment particles or allows them to settle out.
Plant fragmentation is a concern with all of these manual control methods, but with diver operated suction harvesting the potential for fragmentation is moderately high. Use of careful technique and fragment barriers can significantly reduce the creation and escape of fragments from the work area.
Battling Variable Milfoil in Maine: Case Studies
Lake Auburn, Androscoggin County
Middle Range Pond, Androscoggin County
Lake Arrowhead (Little Ossipee Flowage), York County
Rapid Response
Any invasive aquatic plant management efforts at the local level should be seen as adjunct to, and working in concert with, any “rapid response” activity taking place at the state level. Maine’s Rapid Response Plan for Invasive Aquatic Plants, Fish and Other Fauna was formally adopted in January 2006. The plan is intended to ensure that appropriate protocols, trained personnel, equipment, permits, and other resources are “ready to go” to contain or eradicate newly detected illegal aquatic plant or animal introductions as they are reported to or discovered by state agency personnel. Though Maine’s Rapid Response Plan officially pertains to “newly detected” infestations only (i.e. infestations confirmed after the final approval of the plan in 2006), the document contains information useful to any control effort, and is, therefore, a good resource for those engaged in managing “older” (pre-2006) infestations as well.
The plan is available on line at www.maine.gov/dep/blwq/topic/invasives/invplan.htm.
1. Keynote Presentation at the Seventh Annual Maine Milfoil Summit by Commissioner David P. Littell, Maine Department of Environmental Protection. Click here for the text of the commissioner’s speech or see the link on the Commissioner’s Corner of the Maine DEP website at www.maine.gov/dep/oc/commissioner.htm
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