Mattoon Lake, located near the town of Ellensburg in Central Washington, was selected as a site to study Eurasian watermilfoil control using weevils native to North America.

Aquatic Plant Monitoring


Freshwater aquatic plant monitoring is conducted by the Department of Ecology’s Environmental Assessment Program. The monitoring program’s purpose is to track aquatic plant community changes in lakes and rivers throughout the state, concentrating on invasive non-native species such as Eurasian milfoil. A high priority is to check the success of weed control projects funded by the Aquatic Weed Management Fund. In addition, targeted research and control projects have been undertaken as time allows. The aquatic plant monitoring program was established as part of the Freshwater Aquatic Weed Program in 1991. This program provides expertise on aquatic plant issues and a source of grant money (see Aquatic Weed Management Grants) for local invasive aquatic plant management projects.

What are invasive non-native plants?

Invasive non-native plants have been introduced to Washington either accidentally or on purpose by humans. They evolved in other parts of the world and are brought to this region without natural enemies such as plant-eating insects and diseases that normally keep their growth in check in their homeland.

The aquatic invasive plants found in Washington were all originally brought here as ornamental plants for aquariums or water gardens. These ornamental plants tend to be naturally hardy and able to withstand the tough growing conditions found in aquariums. Thus, when they are introduced to Washington’s waters, they often thrive and out-compete native plants.

See our Native Plants for Aquatic Gardens and Aquariums publication for recommendations on what native plants can be used as alternatives to non-native ornamental water garden and aquarium plants.

State Noxious Weed Board publications include recommendations for the use of non-invasive upland ornamental plants.

What Ecology’s Environmental Assessment Program is doing about freshwater aquatic invasive non-native plants

Washington state has a State Noxious Weed Control Board that, among other things, maintains lists of the most threatening invasive non-native plant species. (For more information on the list and aquatic plants included see Overview of Noxious Freshwater Weeds in Washington.

The listed aquatic weeds that are monitored by Ecology include:

Amorpha fruticosa (indigobush) description distribution
Butomus umbellatus (flowering rush) description distribution
Cabomba caroliniana (fanwort) description distribution
Egeria densa (Brazilian elodea or egeria) description distribution
Eichhornia crassipes (water hyacinth) (water hyacinth) description distribution
Elaeagnus angustifolia (Russian Olive) description distribution
Epilobium hirsutum (hairy willow herb) description distribution
Fallopia japonica (Japanese knotweed) description distribution
Glyceria maxima (reed sweetgrass) description distribution
Hydrilla verticillata (Hydrilla) description distribution
Hydrocharis morsus-ranae (Europeans frog-bit) description distribution
Iris pseudacorus (yellow flag iris) description distribution
Ludwigia hexapetala (water primrose) description distribution
Ludwigia peploides (floating primrose willow) description distribution: one wetland in King County
Lysimachia nummularia (creeping Jenny) description distribution
Lysimachia vulgaris (garden loosestrife) description distribution
Lythrum salicaria (purple loosestrife) description distribution
Myriophyllum aquaticum (parrotfeather) description distribution
Myriophyllum heterophyllum (variable leaf milfoil) description distribution
Myriophyllum spicatum (Eurasian milfoil) description distribution
Nasturtium officinale (watercress) description distribution
Nymphaea odorata (fragrant waterlily) description distribution
Nymphoides peltata (yellow floating heart) description distribution
Phalaris arundinacea (reed canarygrass) description distribution
Phragmites australis (common reed) description distribution
Potamogeton crispus (curlyleaf pondweed) description distribution
Sagittaria graminea (grass-leaved arrowhead) description distribution
Sagittaria platyphylla (delta arrowhead) description distribution
Schoenolectus mucronatus (ricefield bulrush) description distribution: one wetland in Clark County
Tamarix ramosissima(saltcedar) description distribution: none in database; see description.
Typha angustifolia (narrow leaf cattail) description distribution
Typha domingensis (southern cattail) distribution
Typha X glauca (hybrid cattail) distribution
Utricularia inflata (bladderwort) description distribution

Also see Invasive Nonnative Freshwater Plants for additional information on many of these species. Note, wetland and shoreline plants included in this list likely are more widely distributed than our database (below) would indicate since we generally only monitor waterbodies with public boat access.

Several research projects have been undertaken to further our knowledge of aquatic plant distribution and to investigate various control methods for aquatic weeds. These are described in more detail below:

Washington State Aquatic Plant Survey

Every year, we conduct site visits to identify aquatic plants, evaluate plant community structure, and detect the existence or potential for problems, particularly as they relate to invasive non-native aquatic plants. We also survey lakes where Aquatic Weed Management Fund grants projects have occurred. Results of these surveys can be accessed at the Lakes Module of the Washington Coastal Atlas.

Online results are current through October 2014.

Please note that our focus is on lakes with public boat access. The sites are not selected randomly, so data cannot be used to make inferences about the overall condition of Washington's lakes.

Class A aquatic weed eradication projects

Class A noxious weeds are defined as non-native species whose distribution in Washington is still limited. Preventing new infestations and eradicating existing infestations are the highest priority. Eradication of all Class A weeds is required by law. The full list of Class A weeds is at the Noxious Weed Control Board website.

Some Aquatic Weed Fund monies are spent each year in efforts focusing on eradication of Class A aquatic weeds. The plants targeted so far have been hydrilla, variable leaf milfoil, and flowering rush.

Hydrilla: Hydrilla (Hydrilla verticillata) is considered one of the worst aquatic weeds in the world. In Washington, only two connected lakes have been found with this species; Pipe and Lucerne Lakes in King County. Hydrilla was first identified in these small joined lakes in 1994. Work began in 1995 to eradicate the population from both lakes. No Hydrilla has been found in either lake since 2006. Since more than 5 years have passed with no Hydrilla found, we consider this population eradicated from Washington. However, continued monitoring takes place each year to be on the safe side. Ecology funded much of the eradication effort, however King County Lake Stewardship staff handled all of the work either themselves or contracted it out. See Project Reports and Plans: Hydrilla Eradication Project for additional information.

Variable leaf milfoil: Variable leaf milfoil (Myriophyllum heterophyllum) was added to the class A noxious weed list in 2008. This resulted from advances in milfoil genetic analysis that confirmed the presence of this plant in five lakes in Pierce and Thurston Counties. Through the Aquatic Weed Fund, Ecology has provided money to the county noxious weed control programs to support eradication efforts including herbicide and diver hand pulling. We have monitored remnant populations of the variable leaf milfoil in each lake during all years of treatment. As of 2013, the status of each lake was as follows:

Flowering rush: Flowering rush (Butomus umbellatus) was added to the class A noxious weed list in 2009. At the time, we thought there were only two populations in the state, one in Silver Lake, Whatcom County and one along a small stretch of the Yakima River in Benton County. In the summer of 2009 substantially more flowering rush was found along the lower Yakima River, and in 2010 it was found in Lake Spokane and Little Falls Reservoir on the Spokane River, the Columbia River at the mouth of the Yakima, and one patch was found in the Pend Oreille River. By 2014, those populations had continued to expand, and it was found in 9-Mile Reservoir, upstream of Lake Spokane. Control efforts are continuing with variable success. In addition, aquatic weed management funds are being used to help fund biocontrol research. Efforts to control the spread of this plant will continue.

Research projects

Use of 2,4-D for selective control of an early infestation of Eurasian watermilfoil in Loon Lake, Washington

The Use of 2,4-D for Selective Control of an Early Infestation of Eurasian Watermilfoil in Loon Lake, Washington (PDF) describes a project that was undertaken in 1998 and 1999 on Loon Lake in Stevens County. It was summarized and published in the Journal of Aquatic Plant Management. The following abstract is taken from this publication:

A patchy distribution of Eurasian watermilfoil (Myriophyllum spicatum L.) in Loon Lake was treated with the herbicide 2,4-D during July 1998. Aquatic plant biomass and frequency data were collected before treatment, and six weeks and one year after treatment. Aqueous concentrations of 2,4-D increased to 1 to 2 mg/l within one day of treatment, and were below detection limits by one week after treatment. Macrophyte data were analyzed to assess the herbicide's impacts on Eurasian watermilfoil as well as the rest of the aquatic plant community. Results showed a significant decrease in Eurasian watermilfoil biomass and frequency in treated areas 6 weeks after treatment, which continued through the one year post-treatment samples. No other plant species were significantly affected by the herbicide application.

Impact of endothall on the aquatic plant community of Kress Lake, Washington

Kress Lake is a small lake formed in an abandoned gravel pit in Cowlitz County (southwest Washington). This project was undertaken in 2000 to investigate the herbicide endothall's impact on both the noxious weed Eurasian watermilfoil and the native plant community. Monitoring continued until 2003 and the results were published in the Journal of Aquatic Plant Management in 2004. In 2005 additional plant frequency data were collected and were not published, but are described below.

The Impact of Endothall on the Aquatic Plant Community of Kress Lake, Washington. is a paper at the Aquatic Plant Management Society website. The following is the abstract from the publication:

A dense mat-forming population of Eurasian watermilfoil (Myriophyllum spicatum L.) was interfering with fishing and recreation in a small western Washington lake. A low concentration (1.5 mg/L active ingredient) of the herbicide endothall formulated as Aquathol® K was used in 2000 to attempt to selectively control the Eurasian watermilfoil. Aquatic plant biomass and frequency data were collected before treatment, ten weeks after treatment and during the growing season for 3 additional years. Macrophyte data were analyzed to assess the herbicide's impacts on Eurasian watermilfoil as well as the rest of the aquatic plant community. Results showed a significant decrease in Eurasian watermilfoil biomass and frequency 10 weeks after treatment. The Eurasian watermilfoil continued to be present, but at a significantly reduced level through the remainder of the study (three years after treatment). Of the native plant species, large-leaf pondweed (Potamogeton amplifolius Tucker.) frequency and biomass was significantly reduced after treatment. Common elodea (Elodea canadensis Rich.), muskgrass (Chara sp. Vallaint.) and bladderwort (Utricularia sp. L.) all increased significantly after treatment.

In 2005 additional plant frequency data were collected following the same methods used in the original study. A table comparing the 2005 data to the pretreatment data from 2000 is below.

Table: Percent frequency of common aquatic plants in Kress Lake pretreatment and five years post treatment. Significant differences indicated in bold (p<0.05).

    % present  
  Jun-00 July-05 p-value
Coontail (Ceratophyllum demersum) 0 26 0.000
Musk grass (Chara sp) 25 52 0.000
Common elodea (Elodea canadensis) 2 79 0.000
Eurasian milfoil (Myriophyllum spicatum) 84 76 0.204
no plants 14 1 0.001
Large leaf pondweed (Potomogeton amplifolius) 35 21 0.029
Bladderwort (Utricularia sp) 1 33 0.000

There was no significant difference between the frequency of Eurasian milfoil in 2005 and 2000, so it returned to essentially pretreatment levels.

The big leaf pondweed frequency was still at significantly reduced levels compared with before treatment. The other common native species, however, were all still found at significantly higher frequencies in 2005 than before treatment.

Use of Diquat to control Brazilian elodea in Battle Ground Lake, Washington

The aquatic invasive non-native plant egeria (Egeria densa Planch.) in Battle Ground Lake (Clark County) was treated with the herbicide diquat in 2003. The purpose of this project was to monitor plant community changes and water quality before treatment and up to three years after treatment. There was a significant reduction in egeria frequency and biomass after the herbicide treatment. The diquat spread throughout the lake and persisted at low levels in the water column for at least two weeks after treatment. There was a slight decrease in surface water dissolved oxygen and water transparency after the herbicide treatment due to increased algae growth.

The Impact of Diquat on Macrophytes and Water Quality in Battle Ground Lake, Washington is a study published in the Journal of Aquatic Plant Management in the January 2007 issue.

Use of fluridone to control Eurasian watermilfoil and Brazilian elodea in Loomis Lake, Washington.

In 2002 Loomis Lake (Pacific County, coastal southwest Washington) was treated with the slow-acting systemic herbicide fluridone to control both Eurasian watermilfoil (Myriophyllum spicatum) and Brazilian elodea (Egeria densa). We monitored the aquatic plant community before treatment and for three years after treatment. The Department of Fish and Wildlife monitored the fish community during this same time period. A paper presenting results from both studies has been published in the Journal of Aquatic Plant Management (January 2009 issue). The abstract is below:

Lommis Lake

Loomis Lake, a long narrow shallow lake on the coast of Washington State, had a submersed plant community dominated by the invasive non-native species Eurasian watermilfoil (Myriophyllum spicatum L.) and egeria (Egeria densa Planch.). In 2002, the whole lake was treated with the liquid formulation of the aquatic herbicide fluridone (1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone). We monitored aquatic plant frequency of occurrence and biomass before herbicide application (2002) and for 3 years after the treatment (2003 to 2005). The fish population was assessed one year prior to herbicide treatment (2001) and three years post treatment (2005). Prior to domination by invasive macrophytes, the lake had a diverse native plant community with low-growing species in the deep water providing open water. During that time the lake supported a stocked rainbow trout (Oncorhynchus mykiss Walbaum) and warmwater fishery.

As invasive macrophytes took over, the native plant richness decreased, the trout stocking program ceased, and small yellow perch (Perca flavescens Mitchill) dominated the fish community. The herbicide treatment resulted in a significant reduction in frequency (86% for egeria, 84% for Eurasian watermilfoil) and biomass (98% for egeria, 99% for Eurasian watermilfoil) of the invasive species for three years. The native submersed plant community was also significantly reduced for the study duration. We attributed this to fluridone use at a non-selective rate and poor light penetration caused by wind induced sediment entrainment. After treatment the growth of largemouth bass (Micropterus salmoides Lacepede) and pumpkinseed sunfish (Lepomis gibbosus Linnaeus) increased. In addition, the abundance of small yellow perch decreased while abundance of larger pumpkinseed sunfish increased.

Effect of Fluridone on Macrophytes and Fish in a Coastal Washington Lake. Article in the Journal of Aquatic Plant Management. was a study published in the Journal of Aquatic Plant Management in the January 2009 issue.

Use of triclopyr to control Eurasian watermilfoil in Capitol Lake, Washington.

In 2004 Capitol Lake (Thurston County, City of Olympia) was treated with the selective herbicide triclopyr to control a burgeoning population of Eurasian watermilfoil (Myriophyllum spicatum). Aquatic plant data were collected by Ecology before treatment, and by Thurston County after treatment. The final data were collected in 2006. If you would like additional information on the project, please contact Jenifer Parsons.

Biological control of Eurasian milfoil including rearing and release of the milfoil weevil, Euhrychiopsis lecontei.


The aquatic invasive non-native plant Eurasian milfoil (Myriophyllum spicatum) has been in Washington State since at least 1965. Since that time it has been introduced to more than 100 lakes and reservoirs and many major rivers including the Columbia, Okanogan, Snake and Pend Oreille Rivers (distribution map).

When the distribution of an invasive non-native species reaches the proportions of Eurasian milfoil, biological control agents are often sought to aid in their management. (Biological control is the use of natural enemies such as insects and diseases to reduce the damage caused by a pest population such as an invasive non-native plant.)

Several insects and diseases have been observed to damage Eurasian milfoil growth. The one that has received the most attention to date is the milfoil weevil (Euhrychiopsis lecontei) (a beetle in the family Curculionidae). This weevil is native to the northern part of the United States, including Washington (Tamayo and Grue 1996). The weevil’s native host is the native northern milfoil (Myriophyllum sibiricum), however, if the weevil is reared on Eurasian milfoil it will prefer it over northern milfoil. The weevils spend their entire life cycle on milfoil. The adults eat leaves on the growing tips, and larvae mine into the stem causing a reduction in plant buoyancy. An excellent summary of milfoil weevil life history and research is available through the milfoil biocontrol website.

Weevil Augmentation Project

A study of Eurasian watermilfoil, macroinvertebrates and fish in a Washington lake describes a study undertaken between 2002 and 2008, a milfoil weevil rearing, augmentation and monitoring project. The abstract from the paper follows:

This study was undertaken to gain experience propagating the milfoil weevil (Euhrychiopsis lecontei Dietz) and to monitor the macrophyte, macroinvertebrate and fish communities at a milfoil weevil augmentation site between 2002 to 2008, in a small lake in central Washington State. The milfoil weevil propagation was time consuming, but not difficult. Over the course of the project, monitoring showed a significant decrease in the frequency and biomass of Eurasian watermilfoil (Myriophyllum spicatum L) and no change or a slight increase in frequency and biomass of other macrophytes at the augmentation site. The milfoil weevil took 5 years to establish in the lake, during which time a midge (Chironomidae) population started to control Eurasian milfoil growth. The fish community changed from one dominated by stunted pumpkinseed sunfish (Lepomis gibbosus L) to a more balanced community of predator and prey fish. Fish diet analysis indicated that fish predation likely influenced herbivorous macroinvertebrate populations. This study supports the theory that fish and macroinvertebrate herbivores influence lake trophic interactions, affecting primary productivity as macrophyte growth.

Additional information on milfoil weevils in Washington:

During the later 1990’s Ecology’s Aquatic Weed Management Fund partially supported research on the milfoil weevil in Washington conducted at the University of Washington. Several papers have been published as a result of this work. (See list of publications below.)

As a result of the research conducted by the University of Washington, Ecology, and others, naturally occurring populations of the milfoil weevil are known from the following locations in Washington:

County Lake Name Year of milfoil weevil observation
Chelan Entiat Lake 1993
Chelan Wapato 1993
Douglas Pateros Lake 1993
Ferry Curlew Lake 1993, 1996, 1997, 2008
Grant Burke Lake 1999, 2002, 2005-2010
Grant Caliche Lake 2005
Grant Canal Lake 1996, 1997
Grant Corral Lake 1998, 1999
Grant Evergreen Lake 1997, 1998, 1999
Grant Priest Rapids Res 2004
Grant Quincy Lake 2003
Grant Stan Coffin Lake 1997, 1999, 2002 - 2005
Grant Warden Lake 1997, 1999, 2001, 2003
King Bass Lake 1999
King Meridian Lake 1999
King Sawyer Lake 1996, 1997, 1998, 1999, 2013
King Star Lake 1999
King Washington 2009
Kittitas Lavender Lake 2003-2012
Kittitas Mattoon Lake 2007, 2008
Lincoln Fishtrap Lake 1996, 1997
Okanogan Osoyoos Lake 2010, 2012
Pend Oreille Davis Lake 1999
Pend Oreille Eloika Lake 2001, 2002
Pend Oreille Fan Lake 1997, 1999
Pend Oreille Horseshoe Lake 2010
Pend Oreille Sacheen Lake 1999
Spokane Badger Lake 1997, 1999
Spokane Chapman Lake 1999
Spokane Williams Lake 1996, 1997, 1999

In 2002, we monitored weevil activity in Stan Coffin Lake into the fall.. The weevils were still evident, though in reduced numbers, in mid-October with a water temperature of 55˚ F (13˚ C). By November 1, 2002, the weevils were very difficult to locate with only one weevil found in 20 minutes of snorkeling; the water temperature was 43˚ F (6˚ C).

Weevil rearing:

Additional information on how we raised milfoil weevils is available in the weevil rearing document.


Divens, M. 2002 Washington Department of Fish and Wildlife, Spokane Office. Personal communication.

Hanson, T., C. Eliopoulos, and A. Walker. 1995. Field collection, laboratory rearing and in-lake introductions of the herbivorous aquatic weevil, Euhrychiopsis lecontei, in Vermont: Year 2. Vermont Department of Environmental Conservation, Waterbury, VT. 41 pp.

Lord, P.H., J.G. Wells, and A.L. Armstrong. 2003. BFS Technical Report #21: Establishing a connection: a survey of Eurasian water-milfoil (Myriophyllum spicatum), its insect herbivores and fish in eight Madison County lakes. Suny Oneonta Biological Field Station; Cooperstown, NY. 46 pp.

Newman, R. 2003. University of Minnesota. Personal communication.

Sutter, T. J., and R. M. Newman. 1997. Is predation by sunfish (Lepomis spp.) an important source of mortality for the Eurasian watermilfoil biocontrol agent Euhrychiopsis lecontei ? Journal of Freshwater Ecology 12: 225-234.

Tamayo, M. and C. Grue. 1996. Evaluation of the native status in North America for the weevil Euhrychiopsis lecontei. University of Washington cooperative Fish and Wildlife Research Unit. Seattle, WA

List of publications on weevils in Washington:

Tamayo, M. and C. Grue. 2004. Developmental performance of the milfoil weevil (Coleoptera: Curculionidae) on watermilfoils in Washington State. Environ. Entomol. 33(4): 872-880.

Tamayo, M., C. Grue and K. Hamel. 2004. Densities of the milfoil weevil (Euhrychiopsis lecontei) on native and exotic watermilfoils. Journal of Freshwater Ecology 19(2): 203-211.

Tamayo, M., C. Grue, and K. Hamel. 2000. The relationship between water quality, watermilfoil frequency, and weevil distribution in the State of Washington. Journal of Aquatic Plant Management 38: 112-116.

Tamayo, M. 2003. Developmental performance, abundance and habitat of the milfoil weevil, Euhrychiopsis lecontei, in Washington State. PhD Dissertation. University of Washington Cooperative Fish and Wildlife Research Unit. Seattle, WA

Population dynamics of curly leaf pondweed and Eurasian watermilfoil during one year in the Columbia River (Crescent Bar)

The objective of this project was to see if the non-native plant curly leaf pondweed (Potamogeton crispus) was a significant component of the plant community and if so, to see how the population changed over the growing season.

Crescent Bar is located on the Columbia River in central Washington. It is about 20 miles downstream of Wenatchee in Grant County. It is a resort area with a small marina and boat access located on the protected side of an island formed by a large sand bar. It is a heavily used recreation area, particularly in spring and summer months. Sampling stations were located along three transects, one at the north end of the island close to the main river, the second was near the boat launch/marina and the third was on the back (east) side of the island.

Sampling began in early April 2005, and continued monthly through late October 2005. Sample points were located approximately 20 meters apart along the transects and returned to each visit by using a GPS (Global Positioning System). At each point samples were collected by tossing a sampling rake four times and recording the species collected with each toss.

Below is a table of the percent of rake samples where the most common species were present.

% frequency
Date Curly Leaf pondweed Eurasian milfoil American waterweed
(Elodea canadensis)
(Ceratophyllum demersum)
04/05/2005 18 11 50 23
05/06/2005 30 14 55 18
06/07/2005 48 18 55 23
07/08/2005 43 23 75 32
08/05/2005 16 30 57 27
09/19/2005 0 36 70 20
10/20/2005 0 32 55 32

These data indicate that at least in this rather confined area of the Columbia River, the curly leaf pondweed was at its greatest frequency in June and July, then declined to the point of not being collected in any of the samples by fall (though it could still be found as a rare small plant in places other than the sample points, so it wasn’t completely gone). This is similar to what has been found in the Midwest (Catling and Dobson 1985), although the die-off appeared to happen a little later in the summer here. This could be due to cooler water temperatures in the Columbia River than in areas studied in the Midwest (Madsen 2005) (surface water temperature was; early June 14° C, early August 20° C). The Eurasian milfoil was most frequently collected toward the end of the summer, but its distribution was patchy. The dominant plant throughout the summer was American waterweed (Elodea canadensis), a common native species. Another native plant, coontail (Ceratophyllum demersum), was common and tended to dominate in the deeper water. In general, the plant growth in the study area was quite dense, especially in July and August, but it was a mix of the above four common species and other less common native plants. It was not a monoculture of any one of them.


Catling, P.M., I. Dobson. 1985. The biology of Canadian weeds 69 Potamogeton crispus L. Canadian Journal of Plant Science 65: 655-668

Madsen, J.D. 2005. Mississippi State University. Personal communication.

Who to contact for more information on this page

For more information about aquatic plants, algae, and lakes, see the Water Quality Program.