Eurasian Watermilfoil.  (Myriophyllum spicatum L.)

Eurasian Watermilfoil is the first truly aquatic plant to be included among Idaho’s noxious weeds list. This invasive and troublesome weed has infested several waterways in Ada County. In an effort to control this invasive plant, the Idaho State Department of Agriculture has made $4,000,000.00 dollars available for request. A selection committee composed of various legislators; weed superintendents, and members of the Department of Agriculture, have reviewed the applications and awarded $44,085.00to the Ada County Department of Noxious Weed Control.

If you know of, or are responsible for, a water-body that may be infested with Eurasian Watermilfoil, we encourage you to contact us regarding possible treatments. Please include the following information, and we will evaluate the area you are concerned with.

Request for Inspection Form

Synopsis of  Eurasian Watermilfoil:

Eurasian Watermilfoil an invasive plant introduced from Eurasia and probably North Africa. This is a perennial plant that often over-winter’s as a pale brown plant that lies on the bottom of the waterway. In our experience, it can also stay green throughout the winter.  In the spring it returns to a dark green color and continues its growth.

Recommended treatments:

Prevention:

It is unknown at this time exactly how the waterways in Idaho have been infested. In some cases, we suspect fragments of Eurasian Watermilfoil is clinging to waterfowl and being carried from pond to pond, where it roots in the lake bottom soil.

There are also unconfirmed reports that this plant is sold as an ornamental plant for fish-ponds and aquariums. In these cases, it is important not to dump aquarium vegetation in large ponds hoping to increase the habitat for the fish.

Probably the most important step that might be taken to prevent infesting a water-way with Eurasian Watermilfoil, is to clean boats and their trailers of all vegetable matter when leaving a waterway. Preferably the boat, bilge, and trailer will be inspected before it is launched in another waterway as well.

Biological controls:

At this time there are two types of biological controls.

Triploid Amur: (Grass Carp) (Ctenopharyngodon idella)

These sterile carp are fish that will eat vegetable matter is a pond or lake. They will eat the Eurasian Watermilfoil, but it is the their least favorite food, and if there are any surviving native or desirable plants, they will be consumed to eradication before any of the Eurasian Watermilfoil is controlled. These grass carp are very expensive, and stocking numbers must be high for them to give acceptable level of control of the Eurasian Watermilfoil. It is imperative that the fish be used only where there is no chance of them escaping into natural waterways. The treatments that make the fish sterile may not always be effective, and these fish are not native to Idaho’s waters.

At this point in time, Ada County Noxious weed Control is not recommending these fish in any waterways. We feel that the fish out weigh their benefit, by destroying the desirable vegetation that will compete with the Eurasian Watermilfoil. These fish are difficult to remove, and their presence makes it impossible to revegetate the waterway.

The Northern Watermilfoil stem-boring weevil: (Euhrychiopsis lecontei (Dietz))

This biological control is a native weevil that shows a strong preference for Eurasian Watermilfoil over the Northern Watermilfoil. Water temperatures, and fish predation appear to have the largest impact on how well this weevil controls the Eurasian Watermilfoil. Where the weevil is found, fish consume them to the point that it isn’t practical to expect them to adequately control Eurasian Watermilfoil.

Ada County Noxious Weed Control is concerned that there are no reliable sources to obtain these insects that do not have a high risk of introducing a parasitic snail commonly called the New Zealand Mud Snail. If these snails were to be introduced in Idaho, it is likely that they will do more damage to our environment than the Eurasian Watermilfoil.

Dredging and mechanical controls:

Dredging is done using specialized equipment and under water divers who will mechanically collect the Eurasian watermilfoil in a specialized boat and physically remove the plant from the water. When conditions allow, this method is effective in small areas, but in silt or clay lakebeds, is not effective, in as much as the divers cannot see the milfoil to remove it when mud obscures their vision. Dredging is typically very expensive and does not offer long-term control without modification to the lakebed to prevent re-infestation.

Chemical controls:

There are several approaches to herbicide applications and many factors that will offer some control or possibly eradicate Eurasian Watermilfoil. These products are labeled for aquatic applications, and have different characteristics that make them more or less suitable for use in a given situation. Licensed professional applicators need to be consulted before an aquatic herbicide is used in Idaho. Specifically, there is a Short Term Activity Exemption, issued by the Department of Environmental Quality, with strict guidelines to be followed before herbicides may be used.

The commonly selected herbicides that control Eurasian Watermilfoil are:

Aquatic labeled 2,4-D:  2,4-D is formulated many ways depending on the intended application. Important considerations before using 2,4-D include length of time that the water must be held before use as irrigation, and what are the types of desirable vegetation that may be damaged during treatment. Stream or current flows have a large impact on the effectiveness of this product.

Aquatic diquat products:  These products are not systemic herbicides and will not control the roots of Eurasian watermilfoil. Diquat is good for minimizing the volume of Eurasian Watermilfoil in a water-body, often before other control measures are used.

Aquatic triclopyr:  This systemic herbicide can offer complete control, if not eradication, of Eurasian Watermilfoil because it will control the root mass, as well as the top growth. This systemic product has very long irrigation restrictions, so water must be held longer than is usually practical.

Floridone:  This product has very few restrictions on water usage but is very slow acting.

Endothall:  This contact herbicide is effective on Eurasian watermilfoil, but has several water use restrictions, that may make it a less desirable choice in our waterways and ponds.

Copper compounds: Depending on the brand purchased, Eurasian Watermilfoil is some times listed among the plants controlled. These products are not usually recognized as the most effective products for Eurasian Watermilfoil control. in most cases, these copper compunds are designed to control algae, and their effect on aquatic plants is limited. So far, we are not recommending copper based herbicides be used to control Eurasian Watermilfoil.

Blue  pond colorants.   These products are water colorants, used to lessen the amount of sunlight that penetrates the water column. By limiting the light available, plant growth is reduced.  Treatments with these products seldom offer the level of control required for Eurasian watermilfoil.

In all efforts to control Eurasian Watermilfoil, it is imperative that all label instructions, guidelines and restrictions be followed exactly. Although some of these products use similar active ingredients to terrestrial herbicides, their formulations and instructions for use, are different. All safety instructions, and limits on who may apply them, must be followed exactly.

Alternative revegetation:

Ada County Noxious Weed Control hopes to continue this project next year with an aggressive revegetation program designed to prevent or minimize the level of re-infestation by Eurasian Watermilfoil. At this time we are investigating the types of plants that may improve the habitat for native fish and wild life that use these waterways, and reduce the level of re-infestation by Eurasian Watermilfoil.

If you have questions about Eurasian Watermilfoil control, please contact:

Ada County Noxious Weed Control

975 E. Pine

Meridian  ID,  83642

(208) 577-4646