Beetles overcoming purple loosestrife infestations at Delaware Water Gap National Recreation Area

The flowering spikes of purple loosestrife (Lythrum salicaria), an invasive alien weed that degrades wetland plant communities and wildlife habitat, turn wetlands into purple carpets. To suppress Lythrum populations and allow native species to return to Delaware Water Gap National Recreation Area (Pennsylvania and New Jersey), we took an integrated pest management approach utilizing biological control agents. We expected to wait 5–10 years to see biocontrol results from the methods we are using, and it looks like we are right on schedule.

We released three groups of biocontrol organisms in the park beginning in 1999 to be continued through at least 2007. Galerucella calmariensis and G. pusilla beetles feed on young shoots and foliage; Hylobius transversovittatus beetles are root-borers; and Nanophyes marmoratus beetles are flower-feeders. We selected release sites based on location, wetland size and type, purple loosestrife infestation level, and the presence of special concern species. Cornell University and the New Jersey and Pennsylvania Departments of Agriculture provided the beetles.

Galerucella releases were made throughout the park from 1999 to 2004, with 22 sites receiving initial releases in 1999 or 2000 and eight key sites receiving supplemental releases in 2002 or 2003. The root-boring Hylobius beetles have been released at 10 sites (2000–2004) and the flower-feeding Nanophyes at five sites (2003–2006) (table 1). Additional Hylobius and Nanophyes beetles will be purchased and released in 2007.

Monitors visit release sites annually for up to five years to determine success or failure in establishing viable beetle populations following protocols developed at the New Jersey Department of Agriculture Beneficial Insects Lab. Galerucella beetles have established at 11 of 14 key sites and dispersed along the Delaware River and from other release sites. (Upper Delaware Scenic and Recreational River, Pennsylvania and New York, also has a biocontrol program and staff has observed Galerucella spreading along the Delaware there.) Nanophyes have not been recovered and establishment in the park is not confirmed. Hylobius adults are nocturnal and larvae feed within plants. Because of the secretive nature of this beetle, we have not monitored these root-borers.

We chose 14 sites to monitor effectiveness of the biocontrol organisms in suppressing purple loosestrife. We visit these sites at least once every two years, any time from late June to mid-July, when purple loosestrife plants should be well grown, flower buds present in healthy plants, and feeding damage evident. Purple loosestrife abundance, based on visual estimates of percentage of cover, is ranked as low (<25%), medium (25–75%), or high (>75%). Feeding damage is ranked on a scale of one (little or no damage) to five (damage severe and extensive) (fig. 1). We consider a site to be rated “control achieved” (i.e., management objective met) when purple loosestrife abundance is low and biocontrol beetles are established at the site. In 2005–2006, purple loosestrife abundance was low, accompanied by moderate to severe Galerucella feeding damage, at 7 of 14 monitoring sites (table 2).

Results of plot-based sampling at one site (fig. 2) indicate that the downward trend in purple loosestrife abundance, and the upward trend in beetle feeding damage between 2001 and 2006, is statistically significant. Furthermore, in the summer of 2006, we observed that most of our marshes were not purple (fig. 3). We conclude that biocontrol agents have contributed to a significant decline in purple loosestrife abundance at this site. Overall, our results to date are consistent with those seen at other sites in Pennsylvania and New Jersey, where Galerucella have dispersed to new areas and control has been achieved at release sites in 5–10 years.

—Jeffrey Shreiner, Biologist, Delaware Water Gap National Recreation Area; jeffrey_shreiner@nps.gov.

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This page updated:  29 May 2007
URL: http://www.nature.nps.gov/ParkScience/index.cfm?ArticleID=201&Page=1

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