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Results of herbicide treatment encouraging thus far

The results are encouraging thus far from a herbicide treatment of invasive milfoil in Great Meadow Stream, 7 Lakes Invasive Aquatics Program Director Sharon Mann reports.

The stream, which flows from North Pond to Great Pond, was treated June 16 with the low-risk herbicide ProcellaCOR. Since the treatment, 7 Lakes Alliance’s invasive aquatics dive crew has monitored the response of native and invasive plants to the herbicide on a biweekly basis.

The effects of the treatment were noticeable at two weeks. By the fourth week, no invasive milfoil was observed growing in the stream or the mouth of the stream, which has historically been the most concentrated invasive milfoil infestation.

The herbicide is a synthetic growth hormone that essentially tells the milfoil to grow rapidly until the plants starve themselves to death. The invasive milfoil plants literally disintegrated into slime and sediment.

Mann cautioned the fight against invasive milfoil is not over. The herbicide treatment was meant to greatly reduce the footprint of invasive milfoil in Great Meadow Stream. Mann said that purpose has thus far been fulfilled beyond expectations.

A similar treatment was applied in Annabessacook Lake in 2019. Invasive milfoil took two years to grow back there. When it did, it was found mostly in areas with a history of high density.

Similar results are expected in Great Meadow Stream, underscoring the need to be prepared financially and with manpower for the inevitable regrowth, Mann said. If and when regrowth occurs, it most likely will be in the mouth of the stream, where invasive milfoil roots intertwine with native spadderdock and bulrush roots, Mann said. Several areas of black milfoil that had not disintegrated and appeared to have viable roots were found in the mouth of the stream. New England Milfoil, a seasoned milfoil removal contractor, is on standby to remove these patches if they regrow in the fall this year. 7 Lakes’ dive crew will also be prepared.

Three native plants – native milfoil, spadderdock and elodea – appeared to be initially negatively impacted by the herbicide treatment. The herbicide caused some irregular growth such as spiraling and leaf curling, and some plant death. Fortunately, the native plants began to recover four weeks after the treatment and are expected to make a full recovery.

7 Lakes’ dive crew has observed no harm to wildlife during post-treatment surveys. A nesting pair of loons produced one healthy chick and the stream is incredibly active with juvenile fishes, insects, turtles and crayfish.

Anecdotally, Mann, who has been diving in Great Meadow Stream for five years, says she saw more schools of juvenile fishes in Great Meadow Stream four weeks after the treatment than she ever had before. She suspects removing large, dense patches of invasive milfoil has provided more space for fish to navigate more efficiently. The juvenile fish include yellow perch, landlocked alewife, smallmouth bass and chain pickerel, along with adult pumpkinseed sunfish. An abundance of healthy mussels have also been noted filter-feeding throughout the stream.

Depending on regrowth in the spring of 2023, the Maine Department of Environmental Protection, which held the permit for the treatment, may pursue “spot treatments” using the same herbicide in key areas. There are no plans for another full-scale treatment in the 19-acre area treated in 2022.



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