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The Weeders Digest - Articles > Biological Control of Eurasian Watermilfoil

Biological Control of Eurasian Watermilfoil


Eurasian watermilfoil (Myriophyllum spicatum L.) is an exotic aquatic plant that was introduced to North America between the late 1800's and the early 1940's (Aiken et al. 1979, Smith and Barko 1990). It grows rapidly and tends to form a dense canopy on the water surface, which often interferes with recreation, inhibits water flow, and impedes navigation (Grace and Wetzel 1978, Smith and Barko 1990). Therefore, there is much interest in developing safe, cost-effective control measures for this nuisance species. Currently, herbicides or mechanical harvesting are most often used to control watermilfoil infestations. These methods can provide relief from the nuisances caused by milfoil. As is the case with terrestrial weeds, control often must be done annually and sometimes more than once per season. These controls can be expensive ($150 to $2000 per acre annually in Minnesota). There is also concern that the methods may harm certain non-target organisms (e.g. Cooke et al. 1993, Nichols 1991). Therefore, investigation of other appoaches is desirable.

Biological control (or biocontrol) is one possible tool that deserves further consideration. Biocontrol offers several potential advantages over conventional methods, including reduced cost, long-term effectiveness, and little or no negative impacts on other aspects of aquatic systems. Several aquatic insects have been associated with declines of Eurasian watermilfoil (see Sheldon and Creed 1995), providing the impetus for research into biocontrol of Eurasian watermilfoil. Current efforts in Minnesota and elsewhere are focused on the native milfoil weevil, Euhrychiopsis lecontei, which has been associated with natural declines of Eurasian watermilfoil (e.g., Creed 1998) and has shown potential in controlled experiments in the field (e.g., Creed and Sheldon 1995, Sheldon and Creed 1995), and experimental tanks (Newman et al. 1996).

The milfoil weevil is native to North America and is a specialist herbivore of watermilfoils. Once exposed to the exotic Eurasian watermilfoil, the weevil prefers Eursasian over its native host northern watermilfoil (M. sibiricum) (Solarz and Newman 1996). Adult weevils live submersed and lay eggs on milfoil meristems. The larvae eat the meristem and bore down through the stem, consuming the cortex, and then pupate (metamorphose) lower on the stem (Sheldon and O'Bryan 1996a). Develop from egg to adult occurs in 18-30 days at summer temperatures (Newman et al. 1997, Mazzei et al. 1999). The consumption of meristem and stem mining by larvae are the two main effects of weevils on the plant and this damage can suppress plant growth (Creed and Sheldon 1993a, 1995), reduce root biomass and carbohydrate stores (Newman et al. 1996) and cause the plant to sink from the water column (Creed et al. 1992). Although the weevil has been quite effective at some sites, it has not been effective at other sites. Currently, we cannot predict when, where and how the weevils will or will not be effective. The aim of our work is to improve our understanding so we can predict effects and appropriate circumstances for use of biocontrol.

One extention of this research is this web site, which aims to provide timely information on biocontrol of Eurasian watermilfoil, as well as to provide a centralized collection of web links to existing information on the ecology and management of other aquatic plants. Follow the links below for more specific information.

For Further Information Contact:

Department of Fisheries, Wildlife & Cons. Bio.

R.M. Newman
Department of FWCB
University of Minnesota
St. Paul, MN 55108 USA
e-mail: RNewman@umn.edu


Last updated: 24 February 2003

R.M. Newman

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