A multiyear year study of three plant communities with purple loosestrife and biological control agents in Virginia.
Purple loosestrife (Lythrum salicaria L.) native to Eurasia has become an invasive weed in North America since its introduction in the late eighteenth century. To control L. salicaria; Galerucella calmariensis L., Galerucella pusilla Duftschmidt, and Hylobius transversovittatus Goeze were released at Coeburn (Sites 1 and 2) and Big Island, Virginia in 1992 and 1999, respectively. The biocontrol agents and plant community parameters were studied for 11 and 14 years at Big Island and Coeburn, respectively. The Galerucella spp. and H. transversovittatus became established at both sites. After 8 years Galerucella spp. at Coeburn dispersed 1768 m with a mean of 221 m/year from the original release site. H. transversovittatus was found 400 m from the original release site at Coeburn after 14 years. Both Galerucella spp. were found in similar ratios at Coeburn. At Big Island the biocontrol agents did not spread due to the low L. salicaria density beyond the release site. At Coeburn Site 1 L. salicaria percent cover, May and fall stem density, seed capsules and total inflorescence length per m2 significantly declined by 88%, 98%, 97%, 94%, and 88%, respectively. While L. salicaria declined non-native and invasive species percent cover increased at Coeburn Site 1. Non-invasive species cover also increased but was lower than the invasive species cover. At Coeburn Site 2 L. salicaria was much denser and did not decline after 11 years. Invasive species were also lower in density than at Coeburn Site 1, possibly allowing L. salicaria to maintain its competitive advantage over other species at Site 2. No significant reductions occurred in percent cover, stem height, seed capsules or inflorescence length per m2 in any of the L. salicaria metrics at Big Island except the number of stems per m2 which declined. Deer herbivory was high at Big Island and may have reduced Galerucella spp. density and its impact on L. salicaria metrics. Percent cover of native, non-native, and invasive species significantly increased at Big Island with non-native species increasing at a faster rate. The biological control agents had a significant impact on L. salicaria at Coeburn Site 1 but not at Coeburn Site 2 or Big Island. While L. salicaria was reduced by the biocontrol agents at Coeburn Site 1, invasive species became more abundant. Non-native and invasive species increased and the quality of this plant community remained low. Future weed biological control programs should include habit restoration.