Large-statured invasive grasses are among the most notorious invaders of wetland and riparian systems in North America. In eastern North America, European common reed (Phragmites australis) genotypes are super-abundant in most wetland habitats where native genotypes were once present. In the west, common reed invasion has occurred in only a few locations, but there is an abundance of suitable habitats. Specialist exotic aphids (Hyalopterus pruni) prefer to feed on native reed genotypes and cause significantly more damage than they do to the exotic genotype. A Gulf Coast common reed genotype, not previously known to be invasive, is expanding rapidly in riparian corridors and agricultural ditches and canals in the southwest, displacing native populations and possibly facilitating the spread of H. pruni into native stands. The objectives of my research are to 1) identify native, exotic, and Gulf Coast P. australis populations throughout the southwest US and use these data to build a GIS database for the management of exotic and Gulf Coast genotypes and conservation of native genotypes, 2) determine the distribution of (H. pruni) and quantify population sizes and subsequent damage to native and invasive genotypes in this region, and 3) evaluate the potential for the invasive Gulf Coast genotype to facilitate the spread of H. pruni into native P. australis populations.