Population Establishment, Dispersal, and Impact of Galerucella pusilla and G. calmariensis, Introduced to Control Purple Loosestrife in Central Ontario

As part of a biological control program against purple loosestrife (Lythrum salicaria), two species of exotic leaf-grazing beetles, Galerucella pusilla and G. calmariensis, have been introduced throughout North America. To assess the potential of these species to establish in an area approaching the northern limit of L. salicaria, we studied beetle population characteristics and feeding impact on L. salicaria at wetlands near North Bay, Ontario, Canada, where each beetle species was released on a separate site in 1995. Weekly counts of adults and larvae of each Galerucella species were made on tagged L. salicaria plants throughout the growing season from 1996 to 1998. At the G. pusilla release site (Site 1), the abundance and frequency of beetles decreased significantly (P < 0.05) from 1996 to 1998, life history and development patterns were inconsistent and incomplete, and dispersal activity was low. Over the same time period at the G. calmariensis release site (Site 2), the abundance of beetles increased significantly (P < 0.05), frequency remained stable, life history and development patterns were consistent and complete, and adults dispersed actively. In the third year following release, G. calmariensis exhibited a 17-week active period and produced three generations of adults at Site 2. Leaf area analysis on plants harvested in 1998 suggested that the impact of both species on L. salicaria was low. Feeding produced a 4.5 and 3.5% leaf tissue loss at Sites 1 and 2, respectively. The frequency of shot-hole feeding (specific to Galerucella spp.) was 32% at Site 2, where G. calmariensis was established, compared to 10% at Site 1 (where G. pusilla failed to establish) and at a control site. Based on these results, it appears that there is a potential for effective control of L. salicaria in central Ontario using G. calmariensis.