Biological Control of Eurasian Watermilfoil (Myriophyllum spicatum) using the Native Milfoil Weevil (Euhrychiopsis lecontei)
Thorstenson, Amy L.
University of Wisconsin-Stevens Point, College of Natural Resources
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The native milfoil weevil, Euhrychiopsis lecontei (Dietz), shows potential to be effective biological control of the nuisance aquatic macrophyte Eurasian watermilfoil, Myriophyllum spicatum L., in controlled conditions, but field application has shown mixed results. This graduate project was Phase 1 of an anticipated, larger, long-term study to better understand how to use this biological control agent. Two subprojects are described in Chapter I and Chapter II, with supplementary data presented in Appendices 1 and 2, respectively. Baseline data collected for Phases 2 and 3 of the long-term study are presented in Appendix 3. Chapter I defines habitat requirements for overwintering success, using multivariate (discriminate analysis) and univariate statistical methods to identify habitat variables that best define weevil hibernation habitat at three lakes in Portage County, Wisconsin: Thomas Lake, a glacial seepage lake; Springville Pond, an impoundment of the Little Plover River; and McDill Pond, an impoundment of the Plover River. Weevil presence and weevil quantity were evaluated relative to numerous habitat variables. Depth of duff material was positively correlated with weevil quantity on Springville Pond, but was inconclusive with multivariate statistics. Percent cover of leaves was positively correlated with weevil quantity on Thomas Lake, but was inconclusive with multivariate statistics. On all three lakes, weevils were never found at sites with zero cm of duff, such as bare sand or mowed, raked lawns. Although not entirely conclusive, the results suggest that management activities that remove duff material from the shoreline, such as mowing and raking, may be disadvantatgeous to weevils. On Thomas Lake and Springville Pond, distance from water was negatively correlated with weevil quantity. Weevils were most common at 2 – 6 m from shore, but located as far as 8.3 m from shore. Discriminant analysis on Thomas Lake also identified height above water as a significant variable with positive correlation with weevil presence, suggesting that weevils occur more often at higher (and thereby drier) sites. The combined results suggest that higher sites nearer to shore, possibly with more duff material, correlate positively with weevil presence. Chapter II develops a method for rearing large numbers of weevils to make biological control a practicle option for lake groups. The best chamber type for outdoor weevil rearing was 370-L 'Freeland poly-tuf' stock tanks. A 9.6 fold average return rate was produced from four stock tanks stocked initially with 0.19 weevils/L and two Eurasian watermilfoil stems per weevil. Weevils were fed an additional 2.28 milfoil stems per weevil initially introduced at 21 days, and 2.35 milfoil stems per weevil initially introduced at 42 days. An average of 672 weevils per tank was produced.