UNIVERSITY OF WISCONSIN-MADISON: Trophic Interactions & the efficacy of milfoil weevils in biocontrol of EWM in norther lakes


The Board of Regents of the University of Wisconsin System is sponsoring an AIRD grant to study four lakes in northern WI. The project will focus on estimating the correlation between relative fish abundance and weevil densities. Project activities include: 1) Sample fish using angling and minnow traps and estimate relative abundance; 2) Conduct fish diet analysis; 3) Sample invertebrates; 4) Hire a graduate student; 5) Data analysis; 6) Public education and outreach \2013 presentations at WI Lakes Convention, NALMS and other appropriate requested conferences. Project results will be published in an appropriate scientific journal; 7) Annual summaries and a final report. Project deliverables include: 1) Hired student; 2) Fish diet analysis; 3) Public education and outreach \2013 informational materials, presentations, media outreach, web-based materials, K-12 education, and undergraduate education; 4) Annual summaries and final report. Specific conditions for this project: Annual summaries and final report need Dept review and approval. WDNR Lakes Management Coordinator will be provided with an electronic (pdf or word) copy of annual summaries, final report, all education and outreach materials, and all data and maps from project.

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e tested the effectiveness of milfoil weevils (Euhrychiopsis lecontei) for reducing biomass of Eurasian watermilfoil (Myriophyllum spicatum, EWM) under natural lake conditions in a 3-year field experiment. In each of four lakes, we randomly chose two EWM beds for stocking and two beds as controls. A total of ca. 40,000 weevils were added to the eight stocked beds. During June and August 2013–2015, we measured plant diversity, biomass of EWM and native plants, and weevil abundance in the 16 study beds. Background weevil densities varied widely among beds and were often greater than the densities stocked. Weevil stocking had no significant effect on EWM or native plant biomass. Nevertheless, weevil damage to EWM was common and its extent appeared strongly related to observed densities of the weevil. ANCOVA results indicated that weevil density was a significant predictor of EWM biomass in both June and August, but not on growth during summer. Overall, our study found that weevil density is an important factor for predicting EWM biomass, while weevil density is likely affected by a large number of environmental factors. This work highlights the importance of carefully considering lake conditions that may influence the efficacy of stocking for biological control.
ABSTRACT: Eurasian water-milfoil (Myriophyllum spicatum L., henceforth “EWM”) is the most heavily managed nuisance submersed aquatic plant in the United States. EWM’s rapid spring growth and formation of dense surface mats inhibits native macrophyte communities, serves as poor-quality habitat for fish and macroinvertebrates, impacts recreation, and can clog water supply infrastructure. The milfoil weevil (Euhrychiopsis lecontei Dietz) has been associated with EWM declines in several states, though natural weevil densities are generally too small to effect control. Augmentative biocontrol has had varied success and fish predation may account for high weevil mortality. Weevils were augmented in 4 northern Wisconsin lakes in summer 2013. In summer 2014, I collected invertebrates associated with EWM plus 442 bluegill (Lepomis macrochirus Rafinesque) diet samples from the 4 study lakes. Overall, chironomids and oligochaetes were the dominant invertebrates associated with plants, while chironomids and Daphnia spp. constituted up to 27.2% and 24.0% of the fish diets, respectively. Milfoil weevils were found in 2.9% of diet samples examined. Weevil larvae were preyed upon more frequently than adults (94.2% of weevils consumed) and sometimes occurred in high numbers within single diet samples. Since the larval stage contributes the most to EWM damage, selective predation on this stage may limit its use as a control agent.
The main goal of this study was to explore diet selectivity of bluegill sunfish, particularly with respect to the milfoil weevil (Euhrychiopsis lecontei). This native weevil is ordinarily in low abundance and has been stocked for the purpose of controlling the invasive aquatic plant Eurasian water‐milfoil (Myriophyllum spicatum, EWM). Maintaining populations following stocking of the weevil depends upon controlling sources of mortality, such as losses to fish predation.
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Grant Awarded
Grant AIRD-100-14 awarded