Biocontrol of Forest Weeds
Although island ecosystems are stable if left undisturbed, they are particularly susceptible to disruption by aggressive introduced species. Thus, invasion by alien species, is the most urgent problem faced by resource managers of natural systems in Hawaii. Whereas most introduced plants have created little or no problem, certain species have shown the ability, sometimes unpredictably based on their ecological relationships in their native habitats, to spread aggressively into native habitats, displacing native species. Some such alien invaders have spread beyond practical control by conventional means, leaving biocontrol as the only feasible approach to curtailing their spread (Gardner, 1990, 1992; Gardner and Davis, 1982; Gardner and Smith, 1992; Markin and Gardner, 1993; Markin et al., 1992; Gardner et al., 1995). Rather than the elimination of such invaders, mitigation of the aggressiveness of such species through biocontrol, permitting the natives to compete successfully, is often the goal of successful biocontrol.
Operating under an interagency agreement among five state and federal agencies, the National Park Service contructed and operates a fully certified foreign insect quarantine facility, located at approximately 4,000 ft. elevation in Hawaii Volcanoes National Park on the island of Hawaii (Gardner and Smith, 1985). As such, it is thought to be the only quarantine laboratory established by a non-agricultural land managing agency for the purpose of biocontrol research primarily applicable to natural systems, as distinct from economic crops (including range and pasture species) or ornamentals.
The foreign insect quarantine facility located in Hawaii Volcanoes National Park (left). The facility has a moat and sealing doors (right), and otherwise meets the specifications for full certification.
Whereas the use of insects as biocontrol agents is a well-established practice, the use of plant pathogens in classical biocontrol of forest weeds is a relatively new approach. The recent availability of a plant pathogen containment facility by the Hawaii Department of Agriculture has expanded the capacity for weed biocontrol research.
The Hawaii Department of Agriculture foreign plant pathogen containment facility. The complex includes enclosed laboratory and non-quarantine greenhouse space (left), and a quarantine greenhouse (right).
Occasionally, a disease or insect already present in the environment (i.e., not intentionally introduced) is found to exert a significant controlling impact on a target forest weed. Sometimes it is possible to encourage the agent and direct it toward the target in areas where control is desired. In such cases it is not necessary to undergo the involved process of bringing the organism into the state and working with it under quarantine prior to its eventual release. Examples of biocontrol projects utilizing pathgens already present in Hawaii are (1) the control program for kahili ginger (Hedychium gardnerianum) (see link below), in which a host-specific strain of the common plant pathogenic bacterium Pseudomonas solanacearum is used against this escaped ornamental where it is invading native forests (Anderson and Gardner, 1996, 1997). (2) The presence of Botryitis cinerea attacking fruit of Myrica faya (see the link below). (3) The apparent controlling effect of a combination of a rust fungus and an eriophyid mite for introduced fuschia in Hawaii Volcanoes National Park (Gardner, 1987). (4) The attack of ivy gourd (Coccinia grandis), an aggressive introduced vine invading lower-elevation forests, by a powdery mildew fungus, Oidium sp., that had not been intentionally introduced (Gardner, 1994), as shown below:
Healthy growth of ivy gourd covering vegetation on a roadside in Windward Oahu.
Defoliated ivy gourd following infection with an unidentified species of powdery mildew (Oidium sp.). Infection may be seasonal, i.e., more apparent and severe during certain seasons than at others.
Closeup of ivy gourd leaves infected with powdery mildew, an obligate parasitic fungus, which, as the name implies, appears as superficial white powder on the upper leaf surface. Infected leaves typically die and fall from the plant.
Myrica faya (fayatree)
Rubus ellipticus (yellow Himalayan raspberry)
Rubus argutus (prickly Florida blackberry)
Hedychium gardnerianum (kahili ginger)
Ulex europaeus (gorse)
Miconia calvescens (miconia)
Passiflora mollissima (banana poka)
Psidium cattleianum (strawberry guava)