PURPLE LOOSESTRIFE

Purple Loosestrife (Lythrum salicaria) is an invasive herbaceous perennial native to Eurasia in the loosestrife family, Lythraceae. They are not closely related to most of Ontario’s native “Loosestrifes” such as Swamp Candles (Lysimachia terrestris), which are instead in the same genus as Northern Starflower (Lysimachia borealis) in the Primrose Family (Primulaceae). However, the Swamp Loosestrife (Decodon verticillatus), a common plant in the marshes of Big Creek National Wildlife Area, is also in the Lythraceae family.

Purple Loosestrife plants can grow 1-2 m in height, and also spread by the roots, forming clonal colonies. In July, the flowers emerge, 1-2 cm in diameter on top of a stalk. The primary pollinators of Purple Loosestrife flowers are bees (Apidae). After pollination, a fruit capsule will develop, releasing dozens of tiny seeds in September and October. Since Purple Loosestrife has few natural insect predators in its introduced, North American range, these plants become very competitive in it’s preferred wetland habitat at the expense of native vegetation. In locations especially ideal for loosestrife growth, Purple Loosestrife can form dense, monospecific strands, with major reductions in native biodiversity.

BIOCONTROL EFFORTS 

Invasive species can be controlled through by a variety of mechanisms, including physical removal, chemical control, or biological control, the latter involves controlling invasive species by introducing populations of its natural enemies (which are also from the region of the invasive pest’s origin, having co-evolved with it). While physical and chemical control can work in small areas, biological control is usually the only method of sufficiently controlling invasive pests at the ecosystem level.

To combat the growing population of invasive loosestrife, the Agriculture Canada approved the release of 2 beetles, Neogalerucella calmariensis and Neogalerucella pusilla, in 1992. By 1998, monitoring results from over 50 sites have indicated that biocontrol efforts have shown success – with Purple Loosestrife populations reduced to small populations where the beetle is present. While Purple Loosestrife is unlikely to be eliminated from North America, the Neogalerucella beetles have reduced the invasiveness of Purple Loosestrife and associated negative impacts on native vegetation communities. These beetles are host-specific to Purple Loosestrife, having negligible impact on native plant populations. As of 2022, Neogalerucella beetles have been introduced to over 500 sites across Ontario! 

To monitor for the presence of Neogalerucella beetles, look at the foliage in early June. Holes in the leaves or the presence of eggs on the leaf underside are strong indicators that these insects are present. Try not to handle the plants, as this will cause the beetles to fall off the plant.

BEETLE ECOLOGY

Each generation of adult Neogalerucella beetles fly for 2 weeks, dispersing up to 2 kilometres. They overwinter in the soil, emerging in mid to late May, feeding on the foliage of Purple Loosestrife. In June or July, these beetles lay small batches of eggs, typically on the leaf underside. Each female is capable of laying around 500 eggs in her lifetime. Larval development takes around 4 weeks, with the subsequent  pupation process taking 2 weeks. The adults will continue feeding on the leaves until autumn, where they move to the soil to overwinter.

NATIVE PLANT ALTERNATIVES TO PURPLE LOOSESTRIFE

Purple Loosestrife was originally introduced to North America for horticultural purposes as a garden plant. It is prized for an ability to grow in wet soils, and vibrant display of purple flowers in midsummer. Native plant alternatives to Purple Loosestrife include Swamp Milkweed (Asclepias incarnata) – a beautiful perennial that produces large, purple to pink flowers in midsummer, while being a host plant to the Monarch Butterfly (Danaus plexippus). Other potential Purple Loosestrife replacements include Swamp Candles, Swamp Loosestrife, White Turtlehead (Chelone glabra), Swamp Coreopsis (Coreopsis palustris), Cardinal Flower (Lobelia cardinalis), and Great Blue Lobelia (Lobelia siphilitica). If you have dry or sandy soils in the garden, consider the use of Blazing Star (Liastris sp.), many which have a visually similar form and flowering stalk to Purple Loosestrife, especially Dense Blazingstar (Liatris spicata), which can also tolerate moist soils.

CONSERVATION MANAGEMENT ACTIVITIES 

BEETLE RELEASE AT HAMMOND HEMLOCK NATURE RESERVE (JUNE 2025)

The stewardship team at Long Point Basin Land Trust released beetles at Hammond Hemlock Nature Reserve to combat the invasive Purple Loosestrife (Lythrum salicaria) as part of a biological control program. Bundles of loosestrife stems containing Neogalerucella beetle larvae were placed directly into infested wetland areas. The aim of the beetle introduction is to decrease the invasiveness of Purple Loosestrife, so that native wetland vegetation can thrive and successful outcompete loosestrife plants.

Stewardship Manager, Ian Fife, assisted with the release and volunteer Owen Ridgen will continue to monitor beetle activity during site visits to track early signs of success.

Thank you to Donna MacKenzie of Ontario Beetles for guiding this important release and donating the beetle larvae. Your generous support is greatly appreciated.

 

 

---

The species you’ve just read about—and the ecosystems they depend on—need our protection. Support the work of the Long Point Basin Land Trust and help preserve the wild spaces of the Long Point Basin: https://longpointlandtrust.ca/donate

To view a directory of all species featured on our website, visit: https://longpointlandtrust.ca/featured-species

—
Written by Matthew Palarchio, HBSc in Environmental Sciences, Western University.

Photos L-R: Canva stock photo (Adult beetle), © Ontario Beetles (Larvae), Matthew Palarchio (Flowers)