Near the shore of eastern Minnesota’s idyllic Long Lake, ecologist Jimmy Marty and fisheries specialist Tom Langer empty a trap full of small fish into a holding bucket in their boat.
Beyond them, the tops of large, rectangular white boxes protrude above the surface. Around the boat, the shallows abound with native plants like water lilies and pondweed.
And invasive Eurasian watermilfoil.
The boxes are screened-in “exclosures” to keep hungry panfish from the resident milfoil weevils, tiny insects that eat the aggressive water plant. They are the centerpiece of a long-running milfoil-control project that benefits from a new program of the University of Minnesota’s Minnesota Aquatic Invasive Species Research Center (MAISRC), in the College of Food, Agricultural and Natural Resource Sciences.
Called Externships, the program pairs aquatic invasive species researchers in four community projects with MAISRC-affiliated researchers. Fortified by their input, the community partners conduct applied research in invasive species management designed to shift the balance against persistent pests statewide. Besides milfoil, externships target mystery snail and rusty crayfish, starry stonewort, and common carp.
“Milfoil is very competitive,” says project leader Marty, a self-described “plants guy” with the civil engineering firm Emmons and Olivier Resources, Inc. “It grows fast and forms mats that shade out native vegetation.”
Joining Marty and Langer, who works for the Carnelian-Marine-St. Croix Watershed District, on the milfoil project are Daniel Larkin, a professor in the University’s Department of Fisheries, Wildlife and Conservation Biology; and MAISRC statistician Alex Bajcz.
The externship appealed to Marty and Langer because researchers like Larkin and Bajcz would ensure that their experiments to test the exclosures’ potential would be designed to yield reliable data that could also guide similar efforts in other lakes.
“Jimmy Marty has a ton of knowledge and expertise about plant ecology,” says Larkin. “Our role is helping him think more about experimental design to get the most out of his work.”
No fish allowed
Finding their trap teeming with fish delighted Marty and Langer. They had already found the exclosures barren of fish. Now they knew it wasn’t for lack of fish, but because the exclosures were doing their job.
The milfoil project began in 2017, when the plant was first discovered on 17 of Long Lake’s 108 acres.
Five years of herbicide-focused management reduced it to less than one acre. But while the chemical treatments were effective, Marty and Langer concluded that continued suppression would likely rely on high-intensity herbicide management.
As an alternative, "The Minnesota Department of Natural Resources suggested using exclosures as a lower-intensity management option and helped fund their construction,” Marty says.
Previous research at the University had found that the weevils ate milfoil, but the results were variable. One likely complication was that panfish were eating the weevils.
But with exclosures protecting them, weevils can safely feast on the milfoil around them. However, the openings in the exclosure screens allow weevils to escape. No weevils are added to the exclosures; instead, the experiment tests the ability of existing weevil populations — whose presence Marty and partners had previously confirmed — to expand and inhibit milfoil growth in and near the exclosures or, possibly, throughout the lake.
The hope is that weevils that leave the exclosures are replaced through breeding inside the exclosures, which could potentially produce three new generations of weevils in a summer. Once hatched, the larvae burrow down into the milfoil stems, eating their insides as they go. Adults live outside the plants, usually eating leaves near the tips of the stems.
To capture the most comprehensive data, the team has set out groups of three exclosures at five locations around the lake.
“We’re trying to answer a question that may have big ramifications,” adds Langer. “This is a critical point in the trajectory from chemical to biological management.”
The experiment
Marty and his colleagues are performing two types of monitoring. Researchers with Golden Sands in Wisconsin will measure weevil abundances in the exclosures and along transects — straight lines leading outward from the exclosures — to estimate how far the weevils will spread through the lake.
They will also monitor the amount of damage to milfoil around the lake, using the "point intercept" method. This involves throwing a rake into the water at locations all over the lake. The researchers then identify the dredged-up plant species and rate their abundance on a scale of one to three.
“We’ve been doing that in June and August for seven years,” says Marty.
The team expects weevil numbers and milfoil damage to be highest near the exclosures and taper off as the distance increases.
Assessing milfoil growth all over the lake is essential, Bajcz says.
“Far from the exclosures, they’ll see the background rate of damage,” he explains. “If you don’t know the background rate, you don’t know where the bottom is.” Not knowing the “bottom” rate of damage in any lake would make it impossible to assess how much of a difference the weevil exclosures make.
The experiment is due to wrap up this August. When the data come in, the weevils’ value can be assessed.
“Jimmy’s project is an opportunity to improve the understanding of the exclosure strategy, and to find out if it works and how to optimize it,” says Larkin.
And Minnesota may just be the best place to do it.
“I’ve lived in seven states,” says Bajcz. “None take aquatic invasive species as seriously as Minnesota — by far.”
Make a gift to support aquatic invasive species research through the Minnesota Aquatic Invasive Research Center Fund.
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