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Weevils

September 18, 2020 by Ted Rulseh

Just imagine a lake where you’ve owned property for decades, or visited on vacations regularly, suddenly becoming infested with a nuisance weed, one that spreads rapidly and grows densely enough to impede favorite pursuits like fishing and canoeing.

That’s what can happen when Eurasian water milfoil moves in and gets out of control. The plant’s bad effects are widely known; perhaps less so is the depth of pain those who have loved a lake feel when its character changes for the worse. And some point, the pain turns to anger intense enough to provoke a declaration of war.

It came to that point last year on the Buckatabon Lakes in Vilas County. I attended the lake association’s annual meeting, where frustration about the milfoil boiled over, to the point where those in attendance voted to explore the creation of a lake district with the power to levy taxes for lake improvements, including invasive species management. 

You know a problem is serious when people are willing to consider a new layer of government and a new line on their property tax bills. At any rate, the folks on the Buckatabon Lakes are attacking the milfoil aggressively.

Once Eurasian water milfoil invades a lake, it is usually there for the long term. Eradication isn’t possible; what’s left is control. If it’s not too widespread, it can be pulled out by hand, such as by divers. Otherwise it can be cut with a weed harvester, or killed with herbicides.

The Buckatabon association is trying a biological approach: releasing weevils that attack the milfoil. The weevils used to be available commercially, but then the company the offered them closed up shop. So the Buckatabon association is breeding them and released a batch last month.

Is it a model other milfoil-infested lakes in Oneida and Vilas counties can follow? Time will tell, but many lake lovers would like to see this natural control method work, so as to avoid or at least limit the need to treat the weed with chemicals.

These weevils feed and reproduce only on water milfoil plants, including the native milfoils, but they especially like the Eurasian variety. They live under the water throughout their lifecycle — egg, larva, pupa, adult. Scientists have looked at other insects for milfoil control, but so far the weevils show the most promise.

They bore through the stems and eat the interior tissue (cortex). This makes the plant less buoyant and reduces its stores of energy-supplying carbohydrate, so that it might be less able to survive the winter. In turn, a weaker milfoil population creates openings for more beneficial species to take holds, researchers say.

The weevils live naturally in many Wisconsin and Minnesota lakes that have native or Eurasian milfoil. There is some empirical evidence of milfoil declines caused by the weevils. They are not a cure-all — at best they are a form of control. Results of experiments suggest it takes as many as 100 to 200 weevils per square meter of bottom to cause the milfoil to decline, according to the University of Minnesota. 

Are these weevils part of a milfoil solution for your lake? Maybe the experience at Buckatabon will eventually supply some clues.

Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected]



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