The accidental or intentional introduction of exotic plants into aquatic systems, as into terrestrial systems, has resulted in the rapid expansion of populations of exotic species and a reduction in abundance and species richness of native aquatic macro-phytes. The ecological approaches that have been evaluated for the biological control of Eurasian watermilfoil include resource competition by other native macrophyte species, interference competition, both exotic and native insects, exotic herbivorous fish, and endemic pathogens. Competing aquatic macrophyte species can be planted to control nuisance species. Herbicides are used to control Eurasian watermilfoil in some states, but aquatic herbicides are illegal in other states. Most texts suggest going to the native range of a nuisance species to find a potential biological control agent, but finding a native agent of biological control in the exotic range of the pest may be a faster or more cost-effective means of biological control.