A field study was conducted to assess population dynamics and long-term effects of the biological control agents Galerucella calmariensis and G. pusilla (Coleoptera: Chrysomelidae) on Purple loosestrife, Lythrum salicaria, and non-target native plant communities in Minnesota. Five Galerucella spp. release sites in central and southern Minnesota were studied between 1995 and 2006. Galerucella spp. populations peaked between three and five years after successfully establishing at all five release sites. As a result, purple loosestrife densities, height and flowering were reduced across all sites. After the initial peak in Galerucella spp. densities, all sites saw a decline of Galerucella spp. abundance in response to the reduction in purple loosestrife abundance. In some sites, the reduction of Galerucella spp. allowed the purple loosestrife population to rebound. Galerucella spp. populations rebounded with increasing loosestrife abundance at most of the sites. However, some insect populations have declined precipitously after reaching high densities and have not recovered. The degree of purple loosestrife and Galerucella spp. population cycling may be dependent on the severity of original loosestrife infestation. Sites with monocultures of purple loosestrife tended to cycle more than sites with a higher diversity and abundance of plant species. This suggests that greater diversity and abundance of other plants contribute to the suppression of loosestrife, likely through increased competition. Our results suggest that Galerucella spp. can provide effective control of purple loosestrife and increase plant species richness and abundance.