Purple Loosestrife

History

Credit: USFWS Purple loosestrife (Lythrum salicaria).

Purple loosestrife (Lythrum salicaria) is a perennial plant with showy purple flowers that is native to Europe. During the 1800's purple loosestrife was introduced into North America by European settlers and sailors. The settlers brought the plants for their flower gardens and the sailors typically used soil that contained seeds from the plants in the ballast holds of their ships to weigh down the vessels for stability on the sea.

In time, the plant began to spread, primarily by seed dispersal, through the northern wetlands of the United States and Canada. It sounds hard to believe, but a single mature plant can produce up to 2.7 million seeds annually! Once purple loosestrife becomes established in a wetland it will typically spread rapidly and eventually prevent native plant species from growing. In time, the wetland often ends up as a "sea of purple." While aesthetically pleasing to the eye, the loss of plant diversity results in a loss of wildlife diversity.

Biological Control

For years, biologists at the Great Swamp Refuge attempted to control the plant by spraying it with the herbicide glyphosate. Normally, one or two biologists would spend from six to eight weeks each summer walking through the marshes with backpack sprayers. Despite the effort, it was very difficult to keep up with the tremendous seed production and germination of new plants.

During those years of spraying the refuge kept abreast of research that was being conducted on the possibility of biological control for purple loosestrife. Researchers were trying to determine if any of the insects that controlled purple loosestrife in Europe could be introduced into North America without causing their own problems. Although it was discovered that over 20 insects feed on purple loosestrife in Europe, researchers found that only five of those species completed their whole life cycle only on purple loosestrife. One could say that these five beetles were genetically "locked" into purple loosestrife. After the research was complete, the United States Department of Agriculture agreed to allow those five species of beetles into the United States for bio-control of purple loosestrife.

Credit: USFWS Galerucella spp.

In 1995, the Great Swamp NWR stopped spraying and began raising and releasing one of the five authorized beetles; Galerucella spp., a leaf-eating beetle. Although the refuge continued to release thousands of beetles each spring, for a number of reasons, the beetle population in the field increased slowly. The leaf-eating beetles normally lay their eggs in the developing stem tips and that is exactly the part of the loosestrife plant that white-tailed deer relish eating. Also, the refuge frequently received prolonged spring flooding that drowned the beetles' eggs and fall flooding that drowned the overwintering adults. And, even under normal circumstances, only about 20% of the adults will survive through the winter!

Although more beetles and plant damage, were observed each summer, the purple loosestrife was spreading in the absence of chemical control. By 1998, the refuge was wondering if the beetles could actually control loosestrife at the Great Swamp and if it would be necessary to start spraying again. However, in 1999, the beetles completely consumed a mass of loosestrife above a quarter acre in size. Encouraged, the refuge decided to continue with the bio-control program for another year. In 2000, the beetles consumed three acres and in 2001, they consumed 13 acres. Last year 40 acres were consumed and if this progression continues, the purple loosestrife at the Great Swamp NWR should be under control in one to two years!

Results of Efforts

Native cattails (Typha spp.), smartweeds (Polygonum spp.) are replacing the loosestrife. The refuge no longer needs to raise and release beetles and is shifting its invasive plant control efforts to Japanese barberry (Berberis thunbergii) and Japanese knotweed (Polygonum cuspidatum).