Plant Pathogens as Biocontrol Agents in Native Hawaìn Ecosystems

Several examples, some spectacular, are available of successful applications of weed biocontrol approaches with plant pathogenic agents in agricultural and rangeland situations. The overall percentage rate of success of all such programs attempted cannot yet be documented, however. Both the "mycoherbicide" method, in which large quantities of inoculum are directly applied to target plants, and the "conventional" method, in which the pathogen is allowed to distribute itself from an introduction point, are used. Two successful biocontrol programs in Hawai`i using plant pathogens are the fungus Fusarium oxysporum control of prickly pear cactus (Opuntia ficus-indica), and Hamakua pamakani (Ageratina riparia) control by the fungus Entyloma ageratinae or E. compositarum. A third project, in which the fungus Colletotrichum gloeosporioides has been released on Koster's curse (Clidemia hirta), is currently under evaluation. Little precedent exists for management use of biocontrol with plant pathogens in native habitats with the goal of preservation of intact ecosystems, as distinct from agricultural applications, however. Certain limitations, both of a biological and an administrative or political nature, must be considered in National Park Service biocontrol programs. Perhaps the most important potential biological limitation may be the unavailability of suitable control agents. Suitable agents must be both host specific and sufficiently virulent to effectively control the target species. Conflicts between National Park Service and agricultural (including horticultural and rangeland) management objectives may also occur. Possibilities for success must be evaluated for each target species individually, taking into account all biological factors as well as other factors involved. Biocontrol offers certain potential advantages over herbicidal or mechanical methods, including longevity and self-dispersal of the agent in the environment; disadvantages include slower, often less perceptible, and less complete control as compared with herbicidal or