Purple Loosestrife and biological control

Staff Writer
The Chronicle Express
Though a striking flower, Purple Loosestrife is highly invasive in wetlands.

Purple loosestrife (Lythrum salicaria) can a create strikingly beautiful sea of purple, but choked out by the sea of purple was a wetland; a unique ecosystem that hundreds of species of plants, mammals, birds, fish, reptiles, amphibians, and insects rely on for survival. 

Purple loosestrife is an invasive species that is native to Europe, Asia, and northern Africa. It was likely introduced on multiple occasions to different parts of North America beginning in the 1800s. Settlers brought it for their flower gardens and as an herbal remedy. It is also very likely that seeds were present in the soil used to weigh down vessels on European ships that provided stability on the ocean. Since its introduction, purple loosestrife has spread across the United States and much of Canada, preferring areas with wet soils such as marshes, ponds, lakesides, along streams and rivers, and in ditches, but it is also capable of establishing in drier soils. 

Wetlands are biologically diverse and productive components of our ecosystem and purple loosestrife is a very aggressive competitor that can rapidly outcompete native and rare plant species, degrading and taking over wetlands, significantly diminishing their value to wildlife and ability to provide several other ecosystem services including water quality improvement. Additionally, purple loosestrife can clog drainage ways and ditches, negatively impacting adjacent land. In general, the control of invasive species presents many obstacles and factors to consider, but wetlands and waterbodies require a very high level of evaluation and precaution when developing management plans, and in the case of purple loosestrife, biological control presented as a safe and effective management option.

Biological control or biocontrol is a management strategy that integrates the use of natural predators or pathogens to manage pest or invasive species populations. The goal of biocontrol is to reacquaint invasive species with their native predators or pathogens to restore a more natural mechanism of long-term control. Biocontrol is a method of control, not eradication and controls the population of an invasive species or pest by reducing the population below a level that causes significant harm or damage.

It is important to ensure that a potential biocontrol agent doesn’t have the capacity to become invasive itself or attack other beneficial or native plants and insects. Before a biocontrol agent is approved for field release, intensive research and safety testing is performed to confirm that the potential agent is specific to the pest or invasive species that needs to be controlled. Biocontrol agents are also tested to make sure they pose no threat to human health, crop production, or beneficial organisms. 

Research for biocontrol agents to control purple loosestrife began in 1986 and after extensive review, approved biocontrol agents were released in New York State in 1992. In 1997, scientists from Cornell University partnered with Montezuma National Wildlife Refuge in Seneca Falls, NY in an effort to control over 1,500 acres that had become dominated by purple loosestrife. At the refuge, from 1887 to 1993, the local extinction of 1,000 breeding pairs of black terns coincided with the increasing infestation of purple loosestrife. Not only was the infestation impacting local wildlife and migrating birds, but it had significantly impacted overall functioning of the wetland ecosystem. In July of 1997, more than 20,000 Galerucella, loosestrife-specific, leaf-feeding beetles were released at the refuge. Over the years, the biocontrol agents became established at the site, and throughout North America, and loosestrife is now kept at a manageable level, or a population that no longer impacts functioning of the ecosystem. 

Even though there are biocontrol agents in place for control of purple loosestrife, if you have it on your property, it is still a good practice to actively manage it. Before management efforts, make sure that it is purple loosestrife and not a native look-alike including blazing star (Liatris spicata), Cardinal flower (Lobelia cardinalias), or ironweed (Vernonia novaboracensis).

Mechanical Control:For small infestations of younger plants (1-2 years), hand-pulling can be effective. Plants older than 2 years can be dug out with special attention to remove the entire root system. Removal should occur prior to flowering or before seeds develop to ensure that seeds are not dispersed or spread. All plant parts should be bagged and disposed of in a landfill or burned. Clean clothing, gear and equipment to prevent spread of seed or plant fragments. Do not cut or mow as these methods will only increase the spread of the plant since they can sprout vegetatively from fragments. However, if digging or pulling is not an option, flowers can be cut and properly disposed of to prevent seeds from producing more plants, as each mature plants can produce over 2 million seeds per year. Management will require multiple years to completely remove any adult plants (over 2 years old) and exhaust the seed bank. After successful removal, revegetation with native plant species is recommended.

Chemical Control: If an infestation is in a dry, upland location on your property, glyphosate or triclopyr based herbicides can be used for small infestations, applied to individual plants by selective hand spraying. Seek professional consultation for infestations in wet soils or near waterbodies. Broadcast spraying is not recommended as non-target species can be harmed. Be sure to follow product labels carefully. Management will require multiple years to completely remove any adult plants (over 2 years old) and exhaust the seed bank. After successful removal, revegetation with native plant species is recommended.

Identifying Purple Loosestrife:

• Average height of 5 feet, but can vary anywhere from 3 to 5 feet

• Stems or stalks are pubescent (have hairs) and four-sided and sometimes five or six-sided; may appear woody at the base of large plants

• Leaves are primarily arranged opposite (paired across from each other on the stem) or in whorls (three or more equally spaced leaves at a node) of three to each other; are lanceolate (lance, awl, or sword-like) in shape; leaf margins (outside edge) are smooth, with no teeth or serrations

• Individual flowers have 5 to 7 purple (occasionally pink) petals with yellow centers, occurring in a densely flowered spike; bloom June to September and stalks remain standing throughout the winter

• As flowers begins to stop, capsules containing many tiny seeds appear in their place; each mature plant can produce up to 2.7 million seeds annually; seeds are easily spread by water, wind, wildlife, and humans. Remember to clean, drain, dry your watercraft, clean all watercraft and hiking gear and equipment, and brush off your shoes and boots to help prevent the spread of invasive species!