Dispersal of Galerucella pusilla and G. calmariensis via passive water transport in the Columbia River Estuary

Dispersal of biological control agents and their subsequent population growth can be a major determinant of the success of landscape-scale weed control programs. Biocontrol agents must be able to disperse across the distances between patches of host plants in order to colonize and control their targets. The presence of three species of biocontrol agents for purple loosestrife (Lythrum salicaria L.): Galerucella calmariensis L. (Coleoptera: Chrysomelidae), Galerucella pusilla Duftschmid (Coleoptera: Chrysomelidae), and Nanophyes marmoratus Goeze (Coleoptera: Brentidae), on relatively remote islands in the Columbia River Estuary (CRE) indicate that these organisms have the ability to disperse across large expanses of open flowing water to colonize remote sites. Previous studies suggest that colonization of these islands by active flight is highly unlikely; therefore, some other dispersal mechanism must be responsible for colonization. A spatial database of all known biocontrol agent release sites for purple loosestrife within 68 river kilometers of our CRE study area was developed and field surveys for biocontrol agents were conducted. A GIS was used to model dispersal distances between biocontrol agent recovery sites and the nearest conspecific release site. Tidal water flow within the CRE was assessed as a potential dispersal mechanism across the modeled distances. The ability of the biocontrol agents to withstand submersion was evaluated in field tests. Our results indicate that it is highly likely that passive water transport has been responsible for some of the long-distance open-water dispersal that would have been necessary for colonization of the remote islands where biocontrol agents were recovered.