Comparison of Three Physical Management Techniques for Controlling Variable-leaf Milfoil in Maine Lakes

Variable-leaf watermilfoil (Myriophyllum hetervphyllum) is a nuisance aquatic plant in the northeastern United States. It grows in thick mats, often out-competing native vegetation, clogging boat motors, and deterring people from swimming and other water-related activities. We implemented three physical management techniques--hand removal, cutting, and benthic mats--on eight infested lakes in Maine to determine the most effective method to control this aquatic invasive. All three treatments resulted in significantly lower plant regrowth than the control. No significant differences were found among the three treatments in plant regrowth or among lakes in percent regrowth of variable-leaf watermilfoil. The costs of hand removal and cutting were roughly one-third the cost of installing benthic mats, although mat installation required less time to implement than the other two mediods. Cutting was found to be unrealistic in practice because of difficulties in implementation. In conclusion, determining the most effective management technique for an area depends on the extent and density of the infestation. Our results suggest benthic mats provide a cost-effective option for thick, extensive infestations, whereas hand removal is more efficient in areas with small, high-density infestations or for selective removal in sparsely infested stands of mostly native macrophytes. Hand removal would also be useful during management surveys when individual plants or small clusters of variable-leaf watermilfoil are detected. Based on our study we suggest that benthic barrier and hand-removal methods are the most effective nonmechanical management techniques for lake associations and governmental agencies to incorporate into their management plans.