Abundance and Distribution Levels of Milfoil Weevils on Lake

Minnetonka

by William Inglis

2004-2005

Introduction

• Eurasian milfoil, Myriophyllum spicatum, is an invasive species

Photo by: William Inglis

Introduction

• Eurasian milfoil has drastically changed ecology of many bodies of water by forming dense mats of vegetation

Current Methods of Control

• Mechanical harvesting• Aquatic herbicides• Only temporary solutions

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Effective Biological Methods

• Indigenous milfoil weevils, Euhrychiopsis lecontei

• Effective when weevil population density greater than 0.5 weevils per milfoil stem

Ideal Weevil Conditions

• Northern United States and southern Canada

• Summers in water on milfoil plants

• Over-winters in shoreline soil

Introduction

• Weevil larvae burrow through vascular tissue of milfoil meristem for nourishment. Larvae create chamber in milfoil stem where they pupate

• Once pupae evolve into adults, they exit stem

• Effective in limiting Eurasian milfoil because their development cycle disrupts nutrient supply to milfoil roots

Introduction• Predators such as sunfish feed

on weevils, reducing their population density

• Mechanical harvesting cuts and removes the top meristem of Eurasian milfoil where weevils develop

• Harvesting is a temporary control and is very expensive

Previous studies

• Observation of weevils in experimental tanks

• Chemical cues and milfoil weevil host choice

• Monitoring weevil and milfoil populations

• Lake factors that affect weevil densities

Goals 1

• To survey Eurasian milfoil weevils at nine sites on Lake Minnetonka, MN, to compare population densities and determine what factors affect weevil abundance in bays that had been harvested of Eurasian milfoil and those that had not

Methods

• Chose nine sites on Lake Minnetonka for sampling and requested Lake Minnetonka Conservation District (LMCD), to not harvest my sites for Eurasian milfoil during the course of my study

• LMCD continued harvesting in surrounding bays for all sites except Smith’s Bay

Map by: William Inglis

Methods• Sampled each site 3 times during

summer, collecting randomly 10 stems from 15 stations along a saved GPS grid at each site

• This was done by snorkeling and placing the stems in labeled zip lock bags

• Milfoil samples were placed in an iced cooler and were counted for weevils within 24 hours

Methods

• Under a lighted magnifying glass, I counted for weevil eggs, larvae, pupae, and adults.

• All insects were preserved in vials with 80 % ethanol, labeled by sampling date, site, and station.

• Data compiled to find average weevil density for each site• Weevil density levels at sampled sites were compared to LMCD

records of % acres harvested from each site with a linear regression

Results: Weevil surveyFigure 1: Percent Acres Harvested vs. Weevil

Density y = -0.2287Ln(x) - 0.2531

R2 = 0.7014

-0.050

0.200

0.450

0.700

0.0% 10.0% 20.0% 30.0% 40.0%

Percent of Site Harvested

Weevils per Stem

1st-total

2nd-total

3rd-total

Log. (1st-total)

Results: Weevil survey

Sites on LakeMinnetonka

2000-2003 Average percentharvested

Summer averageweevils/stem

Enchanted Island 9.90% 0.127Carman’s Bay 36.70% 0.027Lafayette Bay 18.70% 0.024Smith’s Bay 2.80% 0.536Wayzata Bay 24.10% 0.000Gray’s Bay 31.10% 0.191

St. Alban’s Bay 27.00% 0.019Gideon’s Bay 21.50% 0.000Veteran's Bay 12.20% 0.398

Conclusion

• Data show a clear inverse relationship between number of acres harvested and weevil population density

• This suggests that mechanical harvesting extracts weevils from milfoil beds, affecting population densities of entire bay

• This presents a strong reason for discontinuing mechanical harvesting

Further Study

• The results from my study will hopefully persuade the LMCD to stop harvesting so that weevil densities will rise and naturally control Eurasian milfoil.

Goals 2

• Designing and building this trap became the final goal of my study as a means of redistributing weevils into lakes with low population densities.

• Uracil, a compound that attracts weevils to meristem, was used to catch weevils in trap

Methods

• Took a 13 x 13 x 4 cm hermetic container and cut four holes in bottom for tubes, serving as points for weevils to enter the trap

• Cut blocks of Styrofoam and wrapped them around container for buoyancy

• Tied strings to all four corners of trap and attached a brick for anchoring

• Prepared bait by adding 10 mL of dH2O to 0.10 g of agar in a 50 mL beaker.

• In boiling bath, I added 0.01 g of uracil until uracil dissolved.

• Poured agar/uracil solution into pitri dish where it cooled.

Methods

• After agar solidified and became gel-like, I cut agar into thin strips, and placed them in a 13 cm long dialysis bag, tied off the ends of the bag, and placed them in the trap

• Tested trap at the lab in a tank of Eurasian milfoil with weevils.

• Tested at Smith’s Bay on Lake Minnetonka, the site with the most weevils

• Anchor trap and pull four E. milfoil stems through tubes in trap to serve as a latter for weevils

Results: Trap

Table 3: Weevils Caught in Trap

Trial Site Number weevils caught Uracil concentration

Lab Eurasian milfoil tank 1 0.015 g/ 1 mg agar

Smith’s Bay trial 1 0 0.010 g/ 1 mg agar

Smith’s Bay trial 2 1 0.015 g/ 1 mg agar

Conclusion

• The weevil trap was not as effective as anticipated

• Many variables involved with trap, and may be more effective with different uracil concentrations

Further Study

• Further trials should be done using different concentrations of uracil, to find a concentration that best attracts weevils to the trap. If the trap proves successful, further studies should include a control trap.

• Increasing diameter of entry tubes might increase chance of capturing weevils. Future development of this weevil trap could serve in eradicating Eurasian milfoil by redistributing weevils into lower density regions.

Acknowledgements

• Dr. Ray Newman for his expertise, laboratory, and curiosity

• Mrs. Lois Fruen for her ongoing support and motivation, and her inspiring passion for

science

• Team Research for the moral support, suggestions, and comradery

Abundance and Distribution Levels of Weevils on Lake Minnetonka

by William Inglis

2004-2005