Have you noticed stands of fresh bright purple plumes along
the roadside during your mid summer afternoon walks? This striking purple plant, purple loosestrife, matures fast,
multiplies prolifically, spreads like wildfire, and often crowds out native
species. This purple invader has
been swept into the category of nuisance species and is now a resident of the
Boquet River Watershed.
Purple loosestrife, Lythrum saliaria, an herbaceous
perennial weed is now considered one of the worst invasive non-native species in
North American wetlands. It
accidentally arrived as a traveler in European ship ballast in the early
1800’s. Sediments, sand and rocks
were dredged from the bottom of European bays and used to maintain the balance
of ships during the long voyages across the turbulent seas.
Upon reaching the Americas the ballast water and sediments were released
into the virgin waters of North America. Many
of the sediments used in the ballast contained purple loosestrife seeds, which
began the current epidemic. Purple
loosestrife also made its way into America through sheep and raw wool imports,
as herbal remedies, and as garden plants.
The spread of purple loosestrife across the United States
has been swift and continuous. The earliest known herbarium records are from
eastern Massachusetts, Long Island and the Hudson River Valley in New York, and
the Delaware River Valley in Pennsylvania and New Jersey.
Destruction of wetland and riverine ecosystems along with construction
and increased use of inland canals and road systems further facilitated the
inland spread of this invasive weed. From
the Hudson Valley it migrated northward to the southern end of Lake Champlain by
1889. It reached the northern end
of Lake Champlain four years later. By
the early1900s purple loosestrife had spread across the entire state of New
York. By the 1930’s purple
loosestrife was recognized as an invasive weed, when its extreme growth became a
nuisance in the floodplain pastures of the St. Lawrence River.
Purple loosestrife has now established dense mono-specific (single plant
species) stands throughout wetlands in the United States and adjacent Canada. Large stands of this invasive plant threaten the
biodiversity, habitat quality, and even some endangered species such as bog
turtle and dwarf spikerush. In New
York, purple loosestrife invasion is responsible for the declining abundance of
marsh-dependent birds, such as the black tern, least bittern, American bittern
and Virginia rail.
Over the last 50 years many efforts have been made to
control and limit the spread of purple loosestrife. The efforts include burning, mowing, hand-pulling, water
level manipulation, chemical application and biological control.
Mowing and burning do not result in permanent control.
Damaged stands can quickly regenerate new growth.
Water level manipulation should only be considered with heavy
infestations of loosestrife because it may alter the natural community
composition and threaten some desirable or native species.
Hand pulling is effective in small infestations, but care must be taken
to remove the perennial rootstock as well as the plants.
It is easiest to remove young plants from moist soil.
The plants should be pulled prior to seed production, in the early
summer. Plant materials can be
dried and burned. Chemical
application has demonstrated high effectiveness of control.
However, most chemicals are not target-plant specific, and the effects of
herbicides on ecosystems are not fully understood.
Another alternative is biological control. Biological control works by using a plant’s natural enemies against it. The selection of biological control agents includes vigorous testing using the target plant, plants related to it, plants that commonly grow near it, and economically important plants. After extensive trials, five insect species, which feed on purple loosestrife in Europe were approved and introduced as biological control agents by the US Department of Agriculture’s Animal and Plant Health Inspection Service. For large mature stands of purple loosestrife, biological control is considered as an effective control strategy. Thirty-five states (AK, CA, CT, DE, IA, ID, IL, IN, KS, MA, MD, ME, MI, MN, MO, MT, ND, NE, NH, NJ, NY, OH, OR, PA, RI, SD, TN, UT, VA, VT, WA, WI and WV), and all Canadian provinces have chosen to use biological control in the fight against purple loosestrife. The New York Department of Environmental Conservation has released two species of leaf-eating beetles (Galerucella pusilla and Galerucella calmariensis) to control purple loosestrife. Both larval and adult forms G. pusilla and G. calmariensis can cause significant damage to purple loosestrife plants. Adults emerge in the spring and start feeding on the leaves and buds of purple loosestrife immediately. Larvae feed at the tips of the plants, and destroy growing tissue. As the larvae mature they begin “window-pane feeding” on the leaves and stem and leave a “skeletonized” leaf. The effects of biological control are often not seen until 3-8 years after the release. It is believed by many biological control experts that Galerucella pusilla and Galerucella calmariensis may be able to reduce purple loosestrife density by about 90%. Also, once the beetle populations are established, the beetles will continue to control loosestrife on a long-term self-sustaining basis. The cost after release is relatively minimum.
