As many property owners and lake groups know, Eurasian water milfoil is a problem without a great solution. It’s an invasive plant that grows in dense mats in lakes throughout Wisconsin. It can be treated with chemicals to keep the growth down, but that comes with side effects as well as a hefty price tag. But some researchers think there could be a way to use tiny bugs called milfoil weevils as a biocontrol on some lakes. But the idea is more complicated than it seems.
In the top floor greenhouse at the University of Wisconsin Stevens Point…Amy Thorstenson is lifting the shade cloth from an aquarium tank. It’s full of bright green fronds of the notoriously invasive Eurasian water milfoil.
Thorstenson is a scientist with Golden Sands, a research and development company in Central Wisconsin. Right now she’s checking not on the milfoil, but on the tiny weevils that feed on it.
“Oops, there’s one that got loose from the milfoil," she said. "Come on…now I lost track of him.”
A lot rests on the shoulders of these small bugs the size of sesame seeds. They could be the key to developing a biocontrol method for Eurasian water milfoil.
“Eurasian water milfoil doesn’t know it, but it is unwittingly attracting its own enemy," Thorstenson explained.
Milfoil weevils are native to Wisconsin, and they typically feed on the native milfoil plants that are also found here. But if they’re raised on it, the weevils can learn to prefer the invasive kind, and eat those plants instead of the native variety. Thorstenson says that’s because the Eurasian milfoil actually provides better nutrition.
“They actually detect the sugars that Eurasian water milfoil exudes," Thorstenson said. She continued, "If an egg is laid on Eurasian water milfoil, the adult that hatches out of that will preferentially go back to Eurasian water milfoil to lay its eggs. So eventually the generation shifts over to feeding on Eurasian water milfoil.”
And if there’s enough of them in one place, those milfoil weevils can do significant damage to an infestiation of Eurasian water milfoil. Amy Thorstenston’s idea is that lake groups might harness the power of these insects, by raising them in tanks and stocking them in a lake that offers the right conditions. The bugs are so specific in what they eat, there’s little concern about them getting out of hand. And what better control for an invasive plant than developing a kind of natural predator?
“What we’re trying to get to is a process that volunteers can do, to raise their own weevils, start to finish, that’s what we’re trying to get to," said Thorstenson.
It’s a simple concept…that in reality is a little bit complicated. It turns out that milfoil weevils are in somewhat of a short supply.
“This weevil is widely found in lakes all across at least the northern part of the United States and Canada – it’s just not very abundant," explained Martin Hilovsky, President of Enviroscience. It's an ecological consulting company that does a lot of stream restoration, and wetland studies - and that has also gotten into the business of collecting and selling milfoil weevils, precisely because it’s not that easy for volunteers to go out and collect thousands of these bugs on their own.
“Frankly it’s very labor intensive." Hilovsky describes how each spring, Enviroscience hires teams of interns and college students to don snorkels and wetsuits, and spend dozens of hours hand picking weevils out of chilly northern lakes. He says it’s not as easy as you might think…to even find the weevils.
“There’s kind of like a squirrel effect – they’ve got good eyesight," Hilovsky explained. "When they see you, they scoot around to the back of the plant, so they effectively hide. You hope to be able to catch em on a sunny day, when their little carapuises glow like jewels, and you can see them very easily then.”
The process is time consuming, labor intensive, and weather dependent. And for several years Enviroscience filled a niche in collecting, propagating, and selling weevils to scientists and individuals. But then there was a setback.
“We were right at the end of the project, and just about ready to release the rearing methods to volunteers statewide," Thorstenson began. "So I was about to write out the full rearing manual, and then found out that Enviroscience backed out.”
Right as she was poised on the brink of publishing her volunteer handbook for weevil biocontrol, Enviroscience stopped selling the crucial bugs in Wisconsin. State regulations changed, saying weevils released in Wisconsin lakes couldn’t be just any weevils – they had to come from Wisconsin origin. Other states passed similar rules, and it just wasn’t feasible for the company to set up so many different collection operations, one for each state. So Enviroscience scaled back, and stopped selling milfoil weevils in Wisconsin.
“That was a big hiccup," Thorstenson said, "and trying to figure out how to get through it sounded like a lot of work for me!"
With no other companies raising or selling weevils in Wisconsin, weevil researchers like Amy Thorstenson were left with a big dilemma. She considered dropping the project.
“Part of me said well that would be the easy route. But then part of me said, then biocontrol in Wisconsin is dead. And lake groups will have no option but chemicals."
So instead, she went back to the drawing board, back to the greenhouse, and back to the lab.
Krista Kamke is a student researcher working in Amy Thorstenson’s lab. Wearing a pair of magnifying glasses, she’s checking for tiny yellow weevil eggs on a stem of Eurasian water milfoil. It’s one of the tedious steps now added to the process since researchers lost their easy source for weevils. Thorstenson says having to collect her own weevils in spring has introduced a new layer of work, and uncertainty.
“And we’re finding out what is involved with some of those logistics, because it is mass chaos in spring, trying to find the weevils in the numbers you need them at the exact time that you need them…”
Still, Thorstenson and her team are forging ahead, and maintaining hope, that this is a process that can work, and that volunteers can replicate it, at least mostly on their own.
“There’s still going to have to be a great degree of technical involvement to get them off to the right start," Thorstenson said. "Because if they try it and it flops, they’re going to feel really disappointed, but it’s also going to create a bad name for the rearing process, and say oh gee it doesn’t work. So that’s why giving the volunteers the right amount of technical training and working with them along the way to make sure that things are going well is going to be essential to making biological control a success at their lake.”
Thorstenson cautions that even under ideal circumstances, weevils won’t be the silver bullet that eliminates all Eurasian water milfoil in Wisconsin. Lakes need to have certain characteristics, like natural shoreline, for milfoil weevils to survive. And studies have been inconclusive as how reliable this method really is. But for Amy Thorstenson, the prospect of giving lake groups at least the hope of another option besides chemicals – is motivation enough.