Long-awaited Weevils Go To Work in Cedarville Bay
The term "snug as a bug in rug" was illustrated as 15,000 milfoil weevils were carefully placed in heavy mats of weeds in Cedarville Bay Thursday, June 21. Soon after, the tiny aquatic insects started their new job of controlling an invasive plant called Eurasian watermilfoil in the bay.
The insects were planted on milfoil by technicians from Enviro- Science of Ohio. The places where weevils were planted are now marked by orange, bullet-shaped buoys, and boaters are asked to avoid those areas if possible. Mackinac County Marine Deputy Tom Sherlund will help inform boaters about the purpose of the buoys, said Robert Smith, a member of the Les Cheneaux Watershed Council, which coordinated the weevil planting.
The weevil project was selected by the organization as a natural, non-invasive way to control the explosive growth of Eurasian watermilfoil that is choking the bay, residents say. Frequent complaints about Cedarville Bay weeds, which get caught up in boat propellers throughout the summer and cause motors to seize, prompted the group to consider different control and removal methods, including weevils, chemical herbicides, and mechanical removal. Amilfoil weevil is an aquatic insect the size of a sesame seed, with an appetite for milfoil. They have been used successfully to control large beds of Eurasian watermilfoil around the Great Lakes.
At Cedarville Bay, public support was greatest for the weevil method, and several donations have been made toward the project. The Watershed Council raised approximately $24,000 needed for the purchase, planting, and monitoring of the project for one year. The second and final year of the weevil project will cost approximately $10,000. The weevils are expected to naturally establish a population after that, and results should be seen gradually over the next three years.
The weevils have an affinity for Eurasian watermilfoil and actually prefer to feed on the invasive Eurasian watermilfoil over other milfoil species, Mr. Smith said. They will not entirely wipe out Eurasian watermilfoil mats, but reduce the density of the plant growth, making it less of a threat to native plants competing for the same ground.
"Before planting weevils, the team surveyed for all plant species and characterized the Eurasian watermilfoil density," Mr. Smith said. "They consider our beds to be as dense as any they have ever encountered."
Mr. Smith took technicians Courtney Marquette and Missy Kudla to the sites where the insects were planted, as well as a "control site" where weevils were not planted. The sites will be compared later in the year to assess the effectiveness of the weevils. Les Cheneaux Watershed Council President Christine Perreault accompanied them.
The group was thrilled to learn Cedarville Bay has its own population of native weevils that will enhance the planting effort, Mr. Smith said. They could even see a line in the vegetation where the native weevils have already been grazing.
"The technicians showed us multiple examples of eggs, larvae, and adults," Mr. Smith said. "It appears that our native weevils have been grazing seriously for about two weeks. We're hopeful that the high numbers of weevils can now keep up with the rapid Eurasian watermilfoil growth."
Mr. Smith took several photographs of the milfoil beds, now showing bright red flowering tips near the water's surface that contrast with the dark, brownish green foliage below. Eurasian watermilfoil easily spreads because if one piece is torn off, for example by a boat motor propeller, and carried to another body of water, it can grow there and eventually root and establish itself in the lake bottom.
Technicians planted in two locations on the bay. The two treated sites and one control site have all been marked with floats. Three generations of weevils should be produced this summer, Mr. Smith said, "and this should give us an excellent population by fall."
The team will return in late August or early September to assess the impact of the weevils, and issue a report on their findings. Mr. Smith has also taken aerial photographs of Cedarville Bay's submerged vegetation mats and will shoot more pictures as the project continues, to provide visual comparisons of before and after treatment.
Adult weevils will live on the stems of Eurasian watermilfoil, and will insert their eggs into the milfoil. When the eggs hatch, weevil larvae remain in the stem of milfoil, where they bore out the stem and weaken it to the point the plant collapses and dies. In the fall, the insects move to the shoreline and burrow in leaf litter there.