By: Dr. Sigrid Resh, Coordinator,

Keweenaw Invasive Species

Management Area, and Research

Assistant Professor, Michigan

Technological University (scresh@mtu.edu); Erin Mauk, completed Wildlife Ecology and Conservation undergrad degree, Michigan

Technological University (ermauk@mtu.edu)

Purple loosestrife (Lythrum salicaria), a perennial wetland plant native to Asia and Europe, has established itself as an invasive species throughout much of the Midwest. In August, with its pink to purple spike-like flower heads, it is particularly noticeable along roads and in wetlands in the Keweenaw Peninsula. This attractive yet invasive plant thrives in a variety of wetland soil conditions, allowing it to crowd out native species, thus drastically altering wetland plant communities and ultimately creating unsuitable habitats for fish and other wildlife. In addition to its purple spike-like flower heads, it has square stems, opposite or whorled leaves, and can grow up to 6 feet tall. It reproduces by seed and by resprouting from the perennial rootstock.

Since 2017, the Keweenaw Invasive Species Management Area (KISMA) has been targeting this wetland invader using weapons so small that they can rest on your fingertip. These tiny but powerful weapons are known as Galerucella calmariensis and G. pusilla, most commonly referred to as purple loosestrife leaf-eating beetles or biocontrol beetles. These critters eat away at the leaves and flower buds of purple loosestrife, inhibiting the plant’s growth and its ability to produce seeds, thus limiting the plant’s spread. These beetles are host-specific, meaning they will not target desirable native species. These beetles have been in use for decades in the US, and our beetles were collected from wetlands in Ontonagon county.

The northern boardwalk of the Nara Nature Preserve was a purple loosestrife infestation site (see photo on right from August 2017). However, after beetles were released in 2017, 2019, and 2021, the original infestation has been considerably reduced (see photo from the same location from August 2023). While the beetles are not eliminating the purple loosestrife, they are preventing monocultures. The beetles are a natural predator of purple loosestrife; they will keep the population under control, as natural predators of our native plant species tend to do. Basically, the beetles are forcing purple loosestrife to share the ecosystems they invade with other plants.

The Sturgeon Sloughs and associated rivers are another site where beetles were released around 2010 by the Great Lakes Indian Fish & Wildlife Commission. Purple loosestrife clumps are still scattered throughout the area but are not monocultures. The beetles are good at finding their host plant and can spread about a half mile a year, so we are now finding beetles on purple loosestrife stems along Highway 41 between Chassell and Houghton and the connecting side roads.

According to the article Biological Control of Purple Loosestrife published in BioScience by Malecki et al. (1993), the adult beetles emerge from the soil in mid-late May and begin feeding on new leaves. They lay their eggs on stems and leaves in June through and die shortly after. Their larval offspring hatch in late July through August and feed on flower buds, leaves, and stems while they make their way down to the soil, where they overwinter and develop into their adult form for another round of dispersal, eating, and mating by late May to June of the following year. Because the beetles overwinter in the leaf litter at the base of the loosestrife plants, if you see signs of beetles, do not disturb the plant roots. Signs of beetles include holes in the leaves, active beetles in late May and early June, and egg galleries of 2-10 eggs on leaves and stems in June.

Besides biocontrol, manual methods of management for purple loosestrife include:

1. Digging out as much of the root mass as is possible. This is effective for small patches.

2. For larger patches, cutting off flower heads before they go to seed. This will stop them from spreading, but the purple loosestrife plant will keep growing back until the roots are removed or beetles colonize them.

3. With either manual management scenario, it is crucial that roots are either dried in the sun on pavement (not the soil), and seedheads are bagged and disposed of at the landfill as purple loosestrife root fragments can re-establish. However, flowerheads do not seem to develop into seed unlike thistle flowers (personal observations by Sigrid Resh), so those can be dried in the sun or bagged and taken to the landfill.

A common lookalike to purple loosestrife is the native fireweed (Chamaenerion angustifolium), who’s flowers can also be seen along roads and in moist to drier sites. The flowerheads are located at the top of the plant, similar to purple loosestrife, but the individual flowers bloom progressively from bottom to top creating a tapered shape instead of a spike. This native plant is a home to many pollinators.

Additional Resources

• Collecting beetles for release is possible. Beetle collection and rearing techniques can be found here: https://www.yorkswcd.org/photogallery/Invasives%20Files/Beetle%20Rearing%20Protocol.pdf

• For more information on identifying this species, please refer to the Michigan Invasive Species Information Network (MISIN) website: http://www.misin.msu.edu/facts/detail/?id=32.

• Visit KISMA’s website for more information on purple loosestrife identification, management, and native alternatives: https://www.mtu.edu/kisma/aquatic-wetland-species/purple-loosestrife/.

• To report purple loosestrife in your area, please email KISMA directly with the GPS location or just a map showing the location. KISMA contact: Sigrid Resh, KISMA Coordinator, kisma.up@gmail.com.