Establishment and impact of Galerucella calmariensis L. (Coleoptera: Chrysomelidae) on Lythrum salicaria L. and associated plant communities in Michigan

Purple loosestrife, Lythrum salicaria L. (Lythraceae) is an invasive wetland perennial plant of Eurasian origin that is widely established in North America and is considered a threat to native wetland flora and fauna. Two European beetles, Galerucella calmariensis L. and Galerucella pusilla Duft. (Coleoptera: Chrysomelidae) have been introduced and widely distributed in North America for biological control of Lythrum salicaria. Experimental releases of Galerucella spp. beetles were made in three locations in Michigan in 1994. In 1997 we initiated a project to rear, redistribute, and evaluate the impacts of Galerucella calmariensis in 19 additional sites throughout Michigan. G. calmariensis became established at 100% of the 24 release locations monitored in these studies and have persisted for up to seven years while G. pusilla apparently failed to establish. Large populations of G. calmariensis developed from each of the 1994 releases and caused 100% defoliation of L. salicaria. From 1995 to 2000, L. salicaria stem height was reduced 73–85%, percent plant cover was reduced 61–95%, and richness of nontarget plant species increased significantly at four out of five sites. By 2001, L. salicaria stem height and percent cover were reduced 38–81% and 32–74%, respectively, and nontarget plant species richness increased significantly at all five sites in contrast to the situation in 1995. Beetles have spread 3–10 km from these original release sites. Of the 19 additional sites monitored for 3–5 years post-release, 50% (4/8) of the 1997 releases have developed into large G. calmariensis populations and produced severe damage to L. salicaria. Thirty-three percent (2/6) of the 1998 releases have generated moderate impacts while all 1999 releases (5/5) remain without clear impacts. The successful establishment, spread, and impacts of G. calmariensis indicate the critical need for additional research on its role in the restoration of desirable plant communities in areas formerly dominated by L. salicaria.