Controlling nature naturally

Beetles battle beautiful invasive plant

By SANDY LONG

UPPER DELAWARE RIVER REGION — Few would disagree that purple loosestrife (Lythrum salicaria) is an attractive plant. Carving its way into the marshes and wetlands of the Upper Delaware region, the plant produces a tall stalk tipped with vibrant purple flowers. As groves of the resilient invader become established, a beautiful lilac blush creeps over the landscape. But this blush belies a harsher reality, as the invasive alien perennial weed drives out native vegetation, and earns the plant its nickname “marsh monster.”

Although delicate in appearance, a purple loosestrife plant is a powerhouse, capable of producing up to three million seeds per year. The lightweight seeds disperse easily on the wind and exhibit very high germination rates by remaining viable for many years. They are also transported by animals, vehicles and stormwater runoff.

The plants can spread vegetatively from pieces of roots and stems, making them extremely difficult to eradicate once established. Such variables contribute to its success in establishing solid stands of purple loosestrife, which reduces biodiversity, affecting factors such as nutrient cycling and wildlife uses such as feeding and shelter. In addition, the plant can disrupt water flow and reduce forage opportunities.

What’s being done?

To control this purple perpetrator without the use of herbicides and chemicals, the National Park Service (NPS) has begun instituting biological controls in the form of a small beetle. The reddish-brown Galerucella beetle is approximately the size of a grain of rice. Such biocontrols provide a means for reducing insects, diseases and weed pests through the use of natural enemies, such as parasitoids, pathogens, predators and weed feeders.

Galerucella beetles are an example of weed feeders that target a very specific host—purple loosestrife. The European beetles were introduced to North America in 1992 as part of a five- to 15-year program to control purple loosestrife and results have been very encouraging. Adult beetles emerge in spring and begin feeding on the new leaves and shoots of the plant. Females lay 300 to 400 eggs per year and the larvae feed on bud, leaf and stem tissue. Ultimately, the plant weakens and succumbs as a result of this damage.

Natural Resource Specialist Don Hamilton of the National Park Service’s Upper Delaware Scenic and Recreational River said that the agency began releasing the beetles in 2004 at a loosestrife-infested site along the Delaware River below Pond Eddy. The beetles have expanded their range and evidence of their leaf-feeding behavior can be observed along the river’s edge.

The NPS currently obtains beetles at no cost from the New York State Department of Environmental Conservation (DEC), which also uses Galerucella beetles in controlling purple loosestrife, and has released another 3,000 to 4,000 beetles this year. Transects are monitored annually by assessing the number of plant stems and counting egg masses and larvae in the plots. So far, the beetle population is thriving, and native plants such as water willow are making a slow comeback.

Farther down the river, the Delaware Water Gap National Recreation Area has been practicing this biocontrol since 1999 through an integrated pest management program. The successful program, which has indicated a downward trend in purple loosestrife abundance with a corresponding upward trend in beetle feeding damage, was featured in a recent issue of the NPS’s Park Science magazine. To learn more, visit www2.nature.nps.gov/ParkScience and click on the “Highlights” button at the right.

As for the eventual loss of the aesthetic qualities contributed by the plant, it’s a matter of weighing the benefits of its elimination against the harms it causes. “Despite its attractiveness, purple loosestrife displaces plants that are more beneficial to wildlife,” said Hamilton. “We’re happy to see evidence that the beetles are getting established.”

Other initiatives

In conjunction with other agencies, the NPS has a variety of additional natural resource projects and studies underway. Following is a representative sampling:

Vegetation Mapping: The culmination of a three-year effort, this project will focus on seven to 10 of the 61 invasive plant species in the region. Plants studied will be selected for modeling efforts to predict habitat types, rate of spread and other information.

Macroinvertebrate Biomonitoring: Macroinvertebrates (mostly aquatic insects) are sampled at specific locations within the Delaware River to assess biological health and water quality and to document change over time. The monitoring is done in cooperation with the Delaware River Basin Commission.

Prioritization of High Quality Riparian Plant Communities along the Delaware River: This project is being performed in cooperation with the Pennsylvania Chapter of The Nature Conservancy and local landowners. Its goal is to prioritize high-quality riparian plant communities in the 100-year floodplain of the Pennsylvania portion of the Upper Delaware River Basin in order to target future conservation efforts along the main stem Delaware River.

For additional information contact Hamilton at don_hamilton@nps.gov or by calling 570/729-7842.