Ten-year impacts of the biological control agents Galerucella pusilla and G. calmariensis (Coleoptera: Chrysomelidae) on purple loosestrife (Lythrum salicaria) in Central New York State

More than a decade has passed since the biological control agents Galerucella pusilla and G. calmariensis (Chrysomelidae) were introduced into North America for biological control of the wetland weed purple loosestrife (Lythrum salicaria). This study provides an assessment of impact of these beetles at 36 sites in Central New York where they were released in 1994. Measurements of stand area, plant density, and plant height were made at the time of release and again in 2004. Plant heights were found to be significantly reduced, but there were no changes in stand area or plant densities, even when the presence or absence of beetles was included as a factor. The relative abundances of the two beetle species were also compared between 2004 and the last observation made in 1997. G. pusilla was found to be more abundant than G. calmariensis, but G. calmariensis had increased in relative frequency as compared to the earlier survey. Both species were more likely to be present in sites where larger releases were made in 2004. In addition to the 36 release sites, 22 non-release sites were surveyed for Galerucella and their damage. Only 27% of non-release sites were found to be occupied by Galerucella. Beetle abundance and damage levels were lower at these sites than at release sites. The results suggest that any regional, population-level decline in purple loosestrife in the Central New York region will require more time before it can be detected and that the biocontrol program could be expedited with human aided dispersal of Galerucella to areas not yet occupied.