Impacts of Two Introduced Ladybeetles, Coccinella septempunctata and Harmonia axyridis (Coleoptera: Coccinellidae), on Native Coccinellid Species at Mount St. Helens, Washington and in Southwestern Virginia

To assess changes in coccinellid communities subsequent to the introduction of two non-native species, Coccinella septempunctata and Harmonia axyridis, ladybird numbers were monitored over a 20 year period at Mt. Saint Helens, Washington and in three years between 1996 and 2004 in southwestern Virginia. Surveys conducted at Mt. Saint Helens included pre- and post- invasion years for both non-natives. At Mt. Saint Helens, C. septempunctata and H. axyridis both increased in abundance, accompanied by declining numbers of the native ladybird Adalia bipunctata at one of the two Washington sites and of the native ladybird Coccinella transversoguttata at both sites. Declines in these two native ladybirds are strongly correlated with increases in the two introduced species. Given the long study period at Mt. Saint Helens, the declines in these two natives appear to be real trends. Further research is required to determine if the introduced species have indeed been the direct cause of the declines, and to find the mechanisms of interactions. The arrival of C. septempunctata to Virginia predated the first year of this study likely by several years. Harmonia axyridis was detected in Virginia by 1993, three years prior to the first study year in this state. In Virginia, only one introduced species, H. axyridis, showed an increase during the study and this was at one site only. This increase was not accompanied by declines in native species. In fact, the native Coleomegilla maculata also increased at the site during this period. Lack of evidence for declines in native species in Virginia may simply indicate that native species were not affected by the two introduced species, or it may reflect lack of power in the data to detect changes. Alternatively, any changes to the ladybird community may have occurred prior to the start of this study.