The milfoil weevil photograph used with permission - Dr. Ray Newman

The Milfoil Weevil in Washington

The milfoil weevil, Euhrychiopsis lecontei, has been associated with declines of Eurasian watermilfoil (Myriophyllum spicatum) in the United States (e.g. Illinois, Minnesota, Vermont, and Wisconsin). Researchers in Vermont found that the milfoil weevil can negatively impact Eurasian watermilfoil by suppressing the plants growth and reducing its buoyancy (Creed and Sheldon 1995). In 1989, state biologists reported that Eurasian watermilfoil in Brownington Pond, Vermont had declined from approximately 10 hectares (in 1986) to less than 0.5 hectares. Researchers from Middlebury College, Vermont hypothesized that the milfoil weevil, which was present in Brownington Pond, played a role in reducing Eurasian watermilfoil (Creed and Sheldon 1995). During 1990 through 1992, researchers monitored the populations of Eurasian watermilfoil and the milfoil weevil in Brownington Pond. They found that by 1991 Eurasian watermilfoil cover had increased to approximately 2.5 hectares (approximately 55-65 g/m²) and then decreased to about 1 hectare (<15 g/m²) in 1992. Weevil abundance began increasing in 1990 and peaked in June of 1992, where 3 - 4 weevils (adults and larvae) per stem were detected (Creed and Sheldon 1995). These results supported the hypothesis that the milfoil weevil played a role in reducing Eurasian watermilfoil in Brownington Pond.

Another documented example where a crash of Eurasian watermilfoil has been attributed to the milfoil weevil is in Cenaiko Lake, Minnesota. Researchers from the University of Minnesota reported a decline in the density of Eurasian watermilfoil from 123 g/m² in July of 1996 to 14 g/m² in September of 1996. Eurasian watermilfoil remained below 5 g/m² in 1997, then increased to 44 g/m² in June and July of 1998 and declined again to 12 g/m² in September of 1998 (Newman and Biesboer, in press). In contrast, researchers found that weevil abundance in Cenaiko Lake was 1.6 weevils (adults and larvae) per stem in July of 1996. Weevil abundance, however, decreased with declining densities of Eurasian watermilfoil in 1996 and by September 1997 weevils were undetectable. In September of 1998 weevil abundance had increased to >2 weevils per stem (Newman and Biesboer, in press).

Based on observations made by researchers in Vermont, Ohio and Wisconsin it seems that having 2 weevils (or more) per stem is adequate to control Eurasian watermilfoil. However, as indicated by the study conducted in Cenaiko Lake, Minnesota, an abundance of 1.5 weevils per stem may be sufficient in some cases (Newman and Biesboer, in press). 

In Washington State, the milfoil weevil is present primarily in eastern Washington and occurs on both Eurasian and northern watermilfoil (M. sibiricum), the latter plant being native to the state (Tamayo et. al. 1999). During the summer of 1999, researchers from the University of Washington determined the abundance of the milfoil weevil in 11 lakes in Washington. They found, that weevil abundance ranged from undetectable levels to 0.3 weevils (adults and larvae) per stem.  Fan Lake, Pend Oreille County had the greatest density per stem or 0.6 weevils (adults, larvae and eggs per stem). The weevils were present on northern watermilfoil. These abundance results are well below the recommendations made by other researchers in Minnesota, Ohio, Vermont, and Wisconsin of having at least 1.5 - 2.0 weevils per stem in order to control Eurasian watermilfoil. The photograph of a milfoil weevil egg is used with the permission of  L. Jester.

To date, there have not been any documented declines of Eurasian watermilfoil in Washington State that can be attributed to the milfoil weevil, although Creed speculated that declines of Eurasian watermilfoil in Lake Osoyoos and the Okanogan River may have been caused by the milfoil weevil. In Minnesota, Cenaiko Lake is the only lake in that state that has had a Eurasian watermilfoil crash due to the weevil; other weevil lakes are yet to show declines in Eurasian watermilfoil. 

Researchers in Minnesota have suggested that sunfish predation may be limiting weevil densities in some lakes (Sutter and Newman 1997). The latter may be true for Washington State as sunfish populations are present in many lakes in the state, including those with weevils. In addition, other environmental factors that may be keeping weevil populations in check in Washington, but have yet to be studied, include over-wintering survival and habitat quality and quantity (Jester et. al. 1997; Tamayo et. al., in press). Although the milfoil weevil shows potential as a biological control for Eurasian watermilfoil more work is needed to determine which factors limit weevil densities and what lakes are suitable candidates for weevil treatments in order to implement a cost and control effective program.

Literature Cited

Creed, R. P., and S. P. Sheldon. 1995. Weevils and watermilfoil: did a North American herbivore cause the decline of an exotic plant? Ecol. Applic. 5:1113-1121.

Jester, L. L., M. A. Bozek, S. P. Sheldon, and D. R. Helsel. 1997. New records for Euhrychiopsis lecontei (Coleoptera: Curculionidae) and their densities in Wisconsin lakes. Gr. Lakes Entomol. 30: 169-176.

Newman, R. M., and D. D. Biesboer. A decline of Eurasian watermilfoil in Minnesota associated with the milfoil weevil, Euhrychiopsis lecontei. J. Aquat, Plant Manage. In Press.

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? J. Freshwat. Ecol. 12:225-234.

Tamayo, M., C. W. O’Brien, R. P. Creed, C. E. Grue, and K. Hamel. 1999. Distribution and classification of aquatic weevils (Coleoptera: Curculionidae) in the genus Euhrychiopsis in Washington State. Entomol. News 110:103-112.

Tamayo, M., C. E. Grue, and K. Hamel. The relationship between water quality, watermilfol frequency, and weevil distribution in the State of Washington. J. Aquat. Plant Manage. In Press.

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Contact Kathy Hamel by e-mail at kham461@ecy.wa.gov