Novel Approaches to European Frogbit Detection and Management
European frogbit (Hydrocharis morsus-ranae) is a recently arrived invasive wetland plant in the Upper Great Lakes region. It is a floating aquatic, with small leaves reminiscent of lily pads. It can form dense vegetative mats on the water’s surface, blocking out sunlight and preventing the growth of other aquatic species. While frogbit is still uncommon in Lakes Huron, Michigan, and Superior, we found it growing at Munuscong Bay in our study plots in 2011. We reported its presence to the Michigan Department of Natural Resources, and it has since been put on the state’s watch list as a priority species.
Through qualitative observation and the analysis of a region-wide wetland plant dataset, we have concluded that frogbit invasion can be facilitated by the presence of invasive cattails. We suspect that the cattails, which are anchored in the sediment, buffer the floating frogbit from waves that would normally prevent it from establishing. Because frogbit is often found growing among cattails, it presents a significant challenge to invasive species managers. The current Best Management Practices (BMPs) for frogbit include herbicide treatment and physical removal. Both of these are hard to accomplish when the frogbit is in the midst of a dense cattail stand.
In 2016 we began working on a Michigan DNR-funded project addressing frogbit management in cattail. We set up an experiment at Munuscong Bay that combines the treatments of above-water and below-water cattail harvesting, frogbit hand-removal, and herbicide treatment. We hypothesize that the treatments that had cattails removed prior to herbicide application will have a much stronger impact on frogbit. This experiment is also giving us an opportunity to test the effectiveness of herbiciding cattails compared to above-water and below-water harvesting, which we have done in mesocosms but not in a field trial.
The deeper-than-expected water levels and sheer size of the site at Munuscong made the implementation of this project difficult, and we could not have set up the experiment without the help of staff from the Sault Tribe of Chippewa Indians and the Three Shores Cooperative Invasive Species Management Area. We are also collaborating with Dr. Jodi Brandt and Matt Unitis from Boise State University and Dr. Dennis Albert and Kate Wellons from Oregon State University to improve the remote sensing of frogbit and other wetland invasive species in the St. Mary’s River. Over the 2017 field season we will be collecting post-treatment experimental data from the Munuscong site, and we will continue to improve our remote sensing toolkit.