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WSU/Pend Oreille Extension introduced the Sense of Place series in 1999, with a focus on local landscape and natural history of Pend Oreille County, Washington. A partnership with the Kalispel Tribe of Indians Natural Resources Department has allowed us to expand this program through EPA funding to include more classes, a newsletter and this website.

Milfoil Weevils

and the Pend Oreille River


by Jan Rice

Eurasian milfoil (Myriophyllum spicatum) affects water quality, fish habitat and production and recreation in the Box Canyon Reservoir. In 2001, with funding from Public Utility District #1 of Pend Oreille (PUD), the County milfoil management program embarked on an experimental study to examine the milfoil weevil (Euhrychiopsis lecontei) as a potential biocontrol agent. This weevil is native to the U.S. and occurs naturally in this area on northern milfoil. The objective of this project was to augment the number of milfoil weevils that occur in a section of the Pend Oreille River to see if this would help locally reduce the amount of Eurasian milfoil. Until PUD’s experiment, this strategy had never been attempted in a river environment. As is typically the case in biological control, these weevils were not expected to eradicate the milfoil, only to reduce the amount to a more tolerable level.

The experiment began in 2001 with divers collecting weevils from Eloika Lake and Eurasian milfoil from the Pend Oreille River. These were brought back to an outdoor lab set up in the PUD yard. One of the objectives for the first year was to quantify the number of weevil eggs and larvae that were being released. To accomplish this, Jan Rice of the Pend Oreille County Weed Board and volunteers from the Newport/Priest River Rotary Club checked the milfoil stems in the tanks every 3-5 days. They collected adult weevils (to be returned to the original tank after monitoring) and examined each stem under a microscope to count the number of eggs and note larval damage. Then divers “released” these infested stems twice a week by attaching bundles of them to milfoil growing in the study site. Weevil rearing and releasing continued until the fall of 2001 when female weevils normally stop laying eggs and prepare to overwinter on land next to the edge of the river.

The project was repeated in 2002. During both years, plant samples were taken from the study site to determine which plant species were present, the relative biomass of each species and the number of weevils present. At the end of the second year, plant species richness had increased and Eurasian milfoil was no longer the only plant encountered in many samples. However, the relative biomass of each species had not changed and Eurasian milfoil was still the dominant plant. Despite releasing an estimated 1,600 weevils over the two year period, there was no significant difference in the number of weevils counted in 2000 – 2002.

Milfoil weevils under study in lakes

By Paula Rudberg Lowe (from Waterline Newsletter, Washington State Lake Protection Association)

The milfoil weevil (Euhrychiopsis lecontei) is sometimes an effective biocontrol agent against Eurasian milfoil. Environmental Specialist Jenifer Parsons, Washington State Department of Ecology (Ecology), began studying and rearing milfoil weevils in June 2002. She, along with a Department of Agriculture intern, will continue to study weevils this summer.

The milfoil weevil has been implicated in causing declines of Eurasian milfoil populations in Midwestern and Northeastern states. This weevil is native to the northern part of the U.S., including Washington. The weevil’s native host is the native northern milfoil (Myriophyllum sibiricum), however, if the weevil is reared on Eurasian milfoil then it will prefer that species over northern milfoil. Weevils spend their entire life cycle on milfoil, while adults will eat leaves, most damage is done by larvae that mine into the stem causing a reduction in plant buoyancy.

Last summer Parsons collected the naturally occurring milfoil weevils in Stan Coffin Lake near Quincy to raise them in freshwater aquariums, which are housed in the Fish and Wildlife Department buildings in Yakima.

The tiny weevils were collected by snorkeling in Stan Coffin Lake. Weevils were placed in aquariums where the adult weevils laid eggs on the growing tips of fresh milfoil. The eggs hatched in three to six days and the larvae ate the growing tips, then burrowed into the milfoil stems. At the end of the rearing period, the eggs and larvae were counted and released to Mattoon Lake near Ellensburg. Parsons continued the cycle through the summer and by August, she had released nearly 3,000 weevils of all life stages.

Parsons reports that she has not seen evidence of the weevils establishing in Mattoon Lake, but it may take a few years for the weevils to build to a level to be able to control the milfoil. Weevils in Stan Coffin Lake were in densities know to control Eurasian milfoil growth, and the collected weevils were on northern milfoil because there is so little Eurasian milfoil in the lake.

Parsons hopes that the results of this study will be successful so she can advise lakefront property owners interested in using these weevils to control Eurasian milfoil in their lakes.

The study will continue this summer. Parsons also plans to inventory several lakes in King County to look for a test lake there and to check for natural weevil populations.

Questions about the project may be addressed to Jenifer Parsons, 509-457-7136 or jenp461@ecy.way.gov.

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