Can a species of beetle help control this invasive flowering plant?
Purple loosestrife, Lythrum salicaria, may look pretty, but it is a serious invasive species in Wisconsin, Minnesota and throughout North America. It takes over wetlands and destroys habitat for many native plant and animal species. Wetlands in the Lake Superior Reserve, like those in many places around North America, have been invaded by this aggressive species.
Native to Europe, purple loosestrife has been shown to decrease biodiversity in North American wetlands when at high density. Researchers have considered many tactics for controlling the invasive species. One of those ideas has been biological control.
Biological control means utilizing another species—in this case, the leaf skeletonizing beetle—in an attempt to control the purple loosestrife. Biological control is a common approach to controlling invasive species because one reason invading species are often so dominant in their introduced habitat is that the habitat doesn’t include native species that would normally keep them in check.
Researchers initially released the skeletonizing beetles, Galerucella pusilla and G. calmariensis, in an effort to control purple loosestrife found around Pokegama Bay in 2002-2004, well before the establishment of the Lake Superior Reserve in 2010.
In recent years, we’ve conducted plant cover surveys to assess any long-term changes in purple loosestrife abundance in the areas where the beetles were introduced. On top of assessing loosestrife abundance, we wanted to know whether the introduced beetles were actually useful in controlling the species. To do that, we conducted an exclusion experiment—we built mesh-covered cages around some purple loosestrife plants that allowed them to grow but kept the beetles out. This allowed us to gauge any growth differences between loosestrife that’s accessible to the beetles and loosestrife that isn’t.
Our surveys showed that purple loosestrife remains in relatively low and controllable abundance in the study area, and that the beetles likely contribute to controlling the invasive species. We’re continuing to monitor purple loosestrife within the Reserve and how the leaf-skeletonizing beetles affect it.