The submersed plant, Eurasian water milfoil is considered to be the worst aquatic weed in the US, occurring in more than 30 states. The first biocontrol work for this plant begain in 1967 in Yugoslavia. Because many of the possible biocontrol insects from its home range already occur in the US, the major recent research emphasis has been on pathogens to control this plant.
As for insects, a flower-eating weevil, Phytobius leucogaster, was evaluated in the 1970s, but failed to establish field populations (when moved from California to Florida).
However, new biocontrol work with an indigenous American aquatic weevil, Euhrychiopsis lecontei, shows promise. In experimental tank tests, weevil herbivory resulted in a 50% decline in water milfoil biomass and up to 100% of the plants were damaged. Further research confirms that this native weevil is a water milfoil specialist, preferring Eurasian water milfoil to all other plants. The first instar weevil larvae emerge from eggs laid on the meristem of water milfoil and feed on meristematic tissue. Later instars feed on the stems and burrow into them; adults feed on leaves and stems. The efficacy of field releases of this insect currently is being evaluated.
Other possible biocontrol candidates for Eurasian water milfoil include a naturalized pyralid moth, Acentria ephemerella, and a native chironomid midge, Cricotopus myriophylli. The caterpillar of Acentria has been associated with milfoil declines in New England and Ontario; studies show it has a "high preference" for Eurasian water milfoil, but it also eats many other species of aquatic macrophytes.
Cricotopus myriophylli, the native midge, has been associated with Eurasian water mifoil declines in the Pacific Northwest and British Columbia. The highly specific larvae eat the water milfoil meristem.
More detailed information on these insects is at this Web site, which is maintained by Dr Raymond Newman, University of Minnesota Department of Fisheries and Wildlife. We thank he and his associate, Lynn Maher, for the use of these photographs.