Control Options for Eurasian Milfoil

Once a group such as Bonner County recognizes the threat Eurasian Milfoil poses to water resources and human health, the next step is to review the control options available to manage this noxious weed and determine the best mix of tools to provide control.

Bonner County staff did a considerable amount of research prior to selecting the tools to be used during the summer of 2006. There are chemical, mechanical and biological control options available to aquatic plant managers. When selecting the appropriate tool, the County looked at all options, the effectiveness of each option, the potential impacts of each option and the economics of each option.

Biological control options for Eurasian Milfoil are at this point severely limited.

Triploid grass carp are the most widely used biological control tool for aquatic weeds. This fish consumes aquatic vegetation and is commonly used in to control weed growth in golf course pond and irrigation deliver systems. This approach is acceptable in those cases where the objective is minimal growth. Grass carp are not considered a viable option for large lake systems for milfoil control. This fish has a very low feeding preference for Eurasian Milfoil and studies have found it will eat native beneficial plants first. In addition, in order to achieve control, fish would have to be stocked at a rate that would suppress all aquatic plant life in the water body. For this reason, many states such as Idaho will not allow this tool to be used in this type of system. The environmental impact of the fish would be dramatic.

There have been cases where an aquatic weevil have causes a reduction in Eurasian Milfoil growth in a lake system. States like Vermont have done over 15 years of research and operational release of these insects. A number of researchers in other states have also looked at this as a control option. At this point, this insect biocontrol agent is not a viable option. There has not yet been a documented case where adding insects has cause a reduction in Eurasian Milfoil populations. In lakes where there have been naturally occurring declines, researchers now understand that this is cyclical. The milfoil comes back as weevil populations drop. Research has also found and documented the fact that predation on weevils by fish is the mechanism that keeps them from being a viable option in the field.

Aquatic plant harvesting is often something that is promoted as an alternative. In fact, while aquatic herbicides can be used with no impact on fish or invertebrates, aquatic plant harvesters remove and kill significant amounts of both of these. A study (copy attached below)conducted by the Wisconsin DNR found that harvesting 18 acres of milfoil removed over 33,000 fish and 2 million invertebrates. Aquatechnex is one of the larger operators of this equipment in the Western United States. We had to stop operations in many areas because we recognized this impact on fish and insect populations. In many areas in the Pacific Northwest, endangered salmon species are present and their young are the fish being captures by this equipment. Lake Pend Oreille is home to the Bull Trout, an endangered species that would also be threatened by aquatic plant harvesting operations. Lastly, aquatic plant harvesting targeting Eurasian Milfoil often selects for that species. Milfoil can grow back much faster than native aquatic plants. When areas are harvested, the remaining plants grow and replace the native growth and it speeds up the colonization of a system by this invasive weed. Other technologies such as diver dredging and bottom barriers can be effective, but are cost probative on the scale required here.

Aquatic herbicides are much like the drugs we rely on and consume to maintain our health. They go through years of testing and required an approval from the federal EPA for this use. Products that result in the claims many opponents to treatment bring up (human health impacts, fish kills) do not survive the registration process. Sonar herbicide for example has been cleared by the US EPA and the US Food and Drug Administration to be present in delivered drinking water at up to 150 parts per billion, the treatments proposed for Bonner County will be in the 20 ppb range and below, well below this established safe level. There are many substances (salt, caffeine, vinegar) that are orders of magnitude more toxic that the products considered for Bonner County that we consume each day without a thought.

US EPA approved aquatic herbicides were selected for this project because they can selectively target this weed without the impacts caused by harvesting and other methods. They are cost effective and will reduce populations of this noxious weed to the point where other options may be viable.

Related Attachments

Biocontrol Factors to Consider.pdf 46KB
Weevil Need to Know.pdf 41KB
Impacts of Harvesting Paper.pdf 48KB
Harvestingpaper.pdf 498KB