Notes from the Northwoods: Can Native Bugs Take Out Invasive Plants?

by AnnaKay Kruger

UW-Madison undergraduate Joe Bevington rakes the bottom of Manson Lake for Eurasian water milfoil. Photo: AnnaKay Kruger
UW-Madison undergraduate Joe Bevington rakes the bottom of Manson Lake for Eurasian water milfoil. Photo: AnnaKay Kruger

Joe Bevington leans over the side of the boat and eyes the dense weeds in the water below us, watching green, long-feathered arms of Eurasian water milfoil move indolently with the current. The UW-Madison undergraduate wields a long rake that he drags along the lake bottom, twirling tendrils of plant matter around it like spaghetti. He and Jim Miazga, a UW-Stevens Point undergrad, amass piles of slimy milfoil in the bottom of the boat. The damp smells of lake sediment and wet foliage permeate the air.
Susan Knight, the scientist directing this research project, mans the oars as we paddle gently around Manson Lake in pursuit of the milfoil weevil, a small, native beetle that is being monitored for it’s efficiency as a biological control. The bug eats native species of milfoil and, Knight hopes, may have an appetite for the invasive variety.
Eurasian water milfoil grows toward the surface of a lake. Photo: Emily Hilts
Eurasian water milfoil grows toward the surface of a lake. Photo: Emily Hilts

Though the weather this morning over Manson Lake is fair, the hulking shadow of a storm haunts the horizon, and we work quickly in the hopes that we might beat the impending downpour. There are four designated sample sites around the lake that have been programmed into our GPS, each of which we mark upon arrival with a series of portable orange buoys that indicate the outer edge of a bed of Eurasian milfoil.
Steven's Point undergraduate Jim Miazga rests momentarily between taking Eurasian water milfoil samples on Manson Lake.  Photo: AnnaKay Kruger
Steven’s Point undergraduate Jim Miazga rests momentarily between taking Eurasian water milfoil samples on Manson Lake. Photo: AnnaKay Kruger

The fast-growing invasive plant is relatively easy to spot; its long stalks often grow tall enough to break through the lake surface, swiftly becoming a tangled and unsightly mess of weeds. Unchecked populations of Eurasian water milfoil can disrupt native plant growth and hinder shoreline activities like boating and fishing. No plants in our sample areas appear to have breached the surface, but their feathery leaflets are visible just beneath it, waving to and fro in tandem. Joe and Jim alternate taking tip-counts between sites, a process that entails counting individual plants in order to determine population density. While they work, I am made useful as a data scribe and a bagger of discarded plant material.
During the forty-five minute drive between Trout Lake Field Station and Manson Lake, Knight provides me with a brief summary as to what their business is with Eurasian water milfoil anyway. She explains that milfoil is a nuisance commonly controlled with herbicides, but that this tactic has raised concerns among lakeshore property owners who are wary of putting chemicals into their water. To combat this plant overgrowth without herbicides, some lake associations began releasing populations of a native herbivorous beetle, called milfoil weevils, into their lakes in hopes that the insects would act as a biological control against troublesome milfoil plants. These associations requested grants from the Wisconsin DNR in order to help pay breeding companies to provide the insects. Reluctant to fund such an expensive endeavor without conclusive evidence that it would be effective, the DNR consulted Knight and colleague, John Havel, to assess how effective milfoil weevil truly is in controlling Eurasian water milfoil under natural conditions.
Susan Knight paddles the boat through a sample area on Manson Lake.  Photo: AnnaKay Kruger
Susan Knight paddles the boat through a sample area on Manson Lake. Photo: AnnaKay Kruger

A tiny creature that overwinters on land in leaf litter and soil, the milfoil weevil emerges back into the lake once the ice recedes and takes up residence in the nearest bed of milfoil. Here their larvae hatch and grow, burrowing their way through the network of milfoil stems, causing the plants to weaken and die. In theory, these insects would act as highly successful control agents, but what researchers have found is that the conditions have to be ideal in order for them to be effective. Havel, a professor of biology at Missouri State University, explains that dense populations of milfoil weevil in the proper conditions are certainly capable of controlling Eurasian water milfoil. “The trouble is,” he says, “that it requires a number of things to come together and be right for them to work.” With the final year of the study underway, it has become clear that weevil populations are highly variable between lakes and may even vary quite a bit between individual milfoil beds. So while this particular biological control has potential, it appears that a lot of pieces have to fall into place for it to be a worthwhile investment.
The milfoil plants that Bevington and Miazga sample from Manson Lake today will be thoroughly examined and processed back on station to determine the ratio of plant damage to weevil population density, which is the biggest indicator as to whether or not the weevils have made any significant progress.
Sampling is an arduous process primarily because of the many steps involved, and despite our best attempts to beat the encroaching storm, it arrives and lets loose a torrent of rain as we steer ourselves back into shore. The sky is an ominous color and thunder claps in the distance, but we still have two more sample sites to complete before the day is out.
Whether or not it’s raining we are forced to tether the boat to the nearest shoreline and seek shelter off the water at the first sign lightning. However, we are well compensated for the inconvenience with the awesome vision of the mountainous storm front crawling toward us, alive with bright white tendrils of lightning. Between episodes of lighting we scramble to get the final samples done before the inclement weather can drive us away. We’re glad to be rid of the storm when we finally finish sampling, but the sky overhead teases us on our way home with rays of sunshine and tantalizing glimpses of blue.
 
 
 
 
 
 

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