Purple loosestrife biocontrol agent aggregation and overwintering are influenced by tidal inundation

Previous studies have concluded that biological control of Lythrum salicaria may not be possible in tidal environments because the biological control agents are easily removed from their host plants by tidal action. In this study, we examined the spatial relationships between L. salicaria, its biocontrol agents, and the agents' feeding damage in relation to elevation using Spatial Analysis by Distance IndicEs (SADIE) at three sites within the Columbia River Estuary (CRE), USA. We then compared the abundances of overwintering Galerucella pusilla adults recovered along an elevational gradient at two sites within the CRE. Galerucella pusilla aggregated most frequently at elevations above mean high water; however, clusters of feeding damage tended to be evenly spread across all elevations. We found significantly more overwintering beetles at high elevations than low elevations, despite similarities in the availability of overwintering habitat (number of dead stems). Our results indicate that although biological control of L. salicaria may be possible at higher elevations in tidal estuaries such as the CRE, it will not likely be an effective means of controlling infestations at low elevation areas. An approach that integrates multiple management strategies to control L. salicaria is therefore recommended for tidal ecosystems.