Executive Summary
The tree Melaleuca quinquenervia (Cav.) S. T. Blake (Myrtaceae) is native to the coastal wetlands of eastern Australia. Introduced into Florida in the late 1800s, it is highly invasive in a variety of wetland habitats and now infests at least 182,000 ha, including environmentally sensitive areas like Everglades National Park. The tree is allergenic, highly flammable, and alters hydrologic regimes. Its tendency to form dense monocultures severely threatens regional biodiversity. Currently, mechanical and herbicidal killing of mature trees often induces a massive seed release from the serotinous capsules. Germination of these seeds produces a circumferential stand of saplings around the dying trees, thus necessitating extensive follow-up procedures. An interagency task force has developed an integrated management plan to deal with these complications. This plan calls for the use of traditional control measures (mechanical & herbicidal) to remove existing stands from critical wetlands along with implementation of biological control to prevent reinvasion of cleared areas and to minimize spread onto public lands. Biological control will play a major role in the long-term management of M. quinquenervia invasions by curbing seed production and, perhaps, by causing direct tree mortality.
Comparative studies done in Australian and Florida melaleuca forests show few differences in standing biomass or litter throughfall, suggesting that productivity is similar in both adventive and native areas. Trees in the US, however, allocate more than twice the proportion of total biomass into regeneration, which explains the more invasive character of Florida populations. Capsule and seed production is over 10-fold higher in the US compared to Australia. Furthermore, high flower bud abortion caused by direct damage from bud-feeding insects in Australia results in lower numbers of capsules per unit of infructescence length (3.2 capsules/cm in Australia vs. 8.2 capsules/cm in Florida). We have now also demonstrated an indirect effect of herbivory by the weevil Oxyops vitiosa Pascoe, where seed production by affected trees has been severely curtailed. Thus, sustainable control of melaleuca seems dependent upon restraining seed production, which is a plastic trait vulnerable to manipulation. The theoretical basis for the integrated management plan thus appears to be sound.
Melaleuca seeds tend to be more viable and germinate more frequently in the US than in Australia (9.1% vs. 3.3%, and 7.8% vs. 2.8%, respectively). In the US, seed rain is continuous year round with about 268 million seeds falling per hectare per year. Similar data are now being collected in Australia but are not yet available for comparison. About 3% of buried seeds remained viable at dry sites after more than 1.5 years in the soil. However, seeds placed at wet sites germinated quickly, so soil seed banks are probably non-existant in aquatic habitats but provide a significant source of reivasion under drier conditions.
The Australian melaleuca snout beetle Oxyops vitiosa was released during 1997. Habitats with abundant young foliage on the trees, short hydroperiods, intermediate stages of melaleuca invasion, and dry winter conditions engendered field-colony development. Transect sampling estimated the population at more than 2000 adults and 22,000 larvae during October 1998, one year after release of 3300 larvae at a 8-ha pasture near Estero, Florida. By June 2000, numbers swelled to over 100,000 adults and nearly 64,000 larvae. Adults decreased during December 2000 after the field was cut, but the larval population exceeded 210,000. Adults and larvae are continuously being collected from this site and relocated. Approximately 50,000 weevils were released at 98 sites by the end of 2000, and populations of O. vitiosa are now widely established in southern Florida. Tip dieback is becoming evident, trees are appearing defoliated due to the lack of replacement foliage, and some smaller trees are apparently dying. Furthermore, trees that have suffered the severest damage for the longest time are no longer producing flowers. Dispersal has been slow, however, and wide scale infestion remains dependent upon manual redistribution.
Evaluation of a melaleuca sawfly (Lophyrotoma zonalis) determined that it was host-specific to melaleuca. However, two toxic peptides were identified in the larvae. These peptides have been implicated in livestock poisonings by related sawfly species in Europe, Australia, and South America so plans for the release this agent are on hold until the risks associated with these toxins can be more fully assessed.
Host range studies were completed with the Australian psyllid, Boreioglycaspis melaleucae, and a proposal for field release was submitted to the USDA Animal and Plant Health Inspection Service (APHIS) during March 2000. The petition was reviewed and the interagency Technical Advisory Group (TAG) recommended release of the psyllid during June 2000. The U.S. Fish and Wildlife Service completed a Biological Assessment (BA) during June 2001 in which it concurred with the TAG recommendation. Issuance of a permit is now awaiting completion of an Environmental Assessment by APHIS. These psyllids feed on the phloem and heavy nymphal feeding kills saplings.
The family Fergusoninidae includes only the genus Fergusonina, which contains flies that, in mutualistic associations with nematodes, induce galls on several species of Australian Myrtaceae. Both leaf bud and flower bud galls are produced. Injection of nematodes (N = 30) into leaf bud tissue suggested that the nematodes, not the flies, initiate gall formation. Female flies deposited juvenile nematodes along with fly eggs into apical regions of developing buds inducing formation of hypertrophied, uninucleate plant cells before the fly eggs hatch. Fly eggs hatched after 44 days when external gall morphology became more pronounced. These galls act as nutrient “sinks” preventing the plant from diverting nutrients away from damaged shoots to undamaged buds, as would normally occur. The available photosynthate nutrifies the gall, which feeds and shelters both flies and nematodes. This competition between galls and meristems for available photosynthetate will further curtail flower production and enhance traditional control measures.
Comparisons of DNA sequences from flies and nematodes showed that most attack only a single host plant species. Molecular analysis resulted in clear resolution of species and the relationships among them. Results show that the degree of host specificity of fergusoninids is high, with most species associated with a single host plant species. Information from these studies will be used together with host specificity screening to support a request for release of the M. quinquenervia fly/nematode system in Florida.
Host range studies have demonstrated the specificity of the Fergusonina sp. associated with M. quinquenervia in Australia. This fly is now being tested in quarantine on native species of Myrtaceae. Controlled studies revealed that flies make wounds with their ovipositors that leave characteristic scars that can be used to diagnose and detect ovipositional activity. Ovipositional specificity should be the only testing needed because the host is selected entirely by the adult female during the process of oviposition. The resultant immobile larvae remain within the gall, unable to switch hosts.
The rust fungus Puccinia psidii was discovered causing severe tip damage to melaleuca trees in Florida. Host range studies indicated that Eugenia carissoides, and Pimenta dioca were highly susceptible to this rust, but most native Myrtaceae were immune or resistant.