FOGL Volunteers Search for Wild Milfoil Weevils in Green Lake

by Gayle Garman, FOGL President

We’ve been getting many comments this summer (2012) from cross-lake swimmers that the Eurasian milfoil in Green Lake has gotten worse. Members of Green Lake rowing and kayaking clubs confirmed swimmers’ observations.

Milfoil is an invasive weed, with its roots in the muddy lake bottom and long, stringy stems that reach for the sunlight at the water’s surface. The stems are surrounded by whorls of feathery leaves and as summer progresses, the milfoil stems grow longer to get more sunlight. When these stems are near the surface they can wrap around oars, paddles and swimmer’s legs, greatly reducing enjoyment of water activities. In the past, the Parks Dept kept a large floating “mowing machine” on Green Lake to mow the top off milfoil plants before rowing competitions. The problem with mowing is that any piece of milfoil stem that floats away can start another plant; so mowing helps spread the milfoil over the entire lake.

Imagine our excitement when we learned there is a native beetle that eats this milfoil and that can be an effective biological control on its growth! There are even commercial labs that culture these beetles, or milfoil weevils, for sale (similar to selling ladybugs to gardeners). But cultured milfoil weevils are much more expensive than ladybugs; before you buy them, you want to know they will live happily in your lake, hopefully establishing a reproducing population. The best indicator of whether a lake will support a population of milfoil weevils is whether there are already wild weevils in the Lake. This is why we had wild milfoil weevil hunts on August 10 and 24.

Linda Noble and Ben Hall organized volunteer snorkelers and Garet Munger and Jeannine Florance did the lab work, checking milfoil samples collected by the snorkelers for indications of damage from weevil larvae, that burrow into and eat the milfoil stems, and for adult weevils. This is very exacting work, because the larvae are inside the stem, and adult weevils are only 1/8 inch long.

Several promising stem samples were collected, and one sample was sent to the King County laboratory for micro-photography and forwarded to experts for evaluation. Unfortunately, the experts all agreed that while this sample showed damage from the larva of a boring insect, it didn’t have the exact characteristics that would indicate a milfoil weevil. This doesn’t prove there aren’t milfoil weevils in Green Lake, only that we didn’t find any.

Because the adult weevils swim to shore as the water cools and spend the winter in leaf litter, FOGL won’t look for weevils again this year. If you would like to know more, come to a Friends of Green Lake meeting or contact Project Lead Garet Munger. Photos courtesy of Garet Munger and Richard Fleming.
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