Melaleuca, Cajeput Tree (Melaleuca quinquenervia)(Cau.) S.T. Blake.
Melaleuca is characterized in Florida by a rapid growth rate, efficient reproduction, and the ability to invade a wide variety of habitats (Meskimen 1962). This exotic tree grows along roadsides, on ditchbanks, in mesic prairies, in sawgrass marshes, and on lake shorelines. When established, trees form dense stands that are nearly impenetrable (Center and Dray 1986). More than 4,000 trees/ha is not uncommon in melaleuca forests. Although small mammals seem to use these forests, species diversity in wet prairie-marsh ecosystems with dense monocultures of melaleuca decreases by 60-80% (Austin 1978, Woodall 1978, Mazzotti et al. 1981). Schortemeyer et al.(1981) reported that only 10% of the bird species in melaleuca stands actually fed there and only 1.5% of their activity involved nesting in these trees. Wildlife experts fear the exclusion of native vegetation by melaleuca will reduce deer abundance in the endangered Florida panther's (Felis concolor coryi) limited range (Grow 1984). Melaleuca is feared to replace some of the area that is occupied by the native pond cypress Taxodium distichum var. nutans that had expanded into many areas of southern Florida because of lack of competition from other native plants (Myers 1983).
The effect of melaleuca on native populations can be evaluated by determining the benefit and losses from development of a method that completely eradicates the malaleuca (Turner 1984). Information on the economic impact from the elimination of melaleuca was compiled (Diamond et al. 1991) as part of an effort to measure the economic impact if the species is placed on the Florida Prohibited Aquatic Plant List. Such considerations include its impacts on Hendry County that is still using the species in its landscaping plan, on public health (skin and respiratory allergic reactions), on the residents and tourist trade, and on the displacement of native species. Opinions about the effects of the loss of the trees on beekeeping and on honey production and the pollination of the vegetable crops remain divided; however, no harm was anticipated from the use of the species for woodchip and mulch. Benefits of removal included the long-term protection of regional water tables, maintenance of tourism at the current minimum worth of $4.2 million in direct receipts and as much as $145 million/year in regional expenditures, reduced expenditures for allergy treatment by $0.5- 2.0 million, and reduced eradication expenses of $370--$2,079/ha (Diamond et al. 1991).
Good methods for the control of this species are not available. However, the range of this plant is limited, and prospects for the identification of a biological control agent are good because no native species is closely related to melaleuca (Balciunas and Center 1991). Fire does not kill melaleuca trees, which is almost perfectly adapted to fire. It has a thick, spongy bark that insulates the cambium. The outer layers of bark are flaky and burn vigorously. The leaves and small branches are killed, but dormant lateral buds on the trunk germinate within weeks (Ewel 1986). Moreover, burning causes massive seed release and germination. A burned melaleuca can release millions of seeds that are dispersed short distances by wind and water (Ewel 1986).
Melaleuca becomes established more readily on sand than on marl but can survive on any disturbed soil in southern Florida. It tolerates extended flooding, moderate drought, and some salinity (Ewel 1986). The limited mobility of the seed probably confined the invasion of melaleuca thus far. Resource managers should concentrate on eliminating seed sources nearest the pine-cypress ecotones into which melaleuca is preadapted to spread, rather than expending resources on pockets of melaleucas near other, less susceptible habitats (Ewel 1986).
The melaleuca seedlings in test plots had lower than 50% survivorship in five of the eight communities where it was experimentally planted. Survivors grew best in two disturbed ecosystems (a severely burned pine-cypress ecotone and a drained forest) and in one undisturbed community (a dwarf cypress forest; Myers 1984). Melaleuca is capable of invading the zone between pine (Pinus spp.) and cypress (Taxodium distichum) forests in southern Florida and of displacing the cypresses (Ewel 1986). The probability of selecting the right combination of environmental conditions for the increases of established melaleuca seedlings increases with time and with the number of dispersed propagules (Ewel 1986).
Thayer et al. (1990) in their Melaleuca Management Plan reported that the major potential threat by this species is its harm to southern Florida's water supply. This is ironic because the principal reason for its original introduction was the drying up the Everglades (Thayer et al. 1990). The control and management of this species on federal lands in southern Florida is expensive in time and resources (Doren 1991). The cost of controlling melaleuca in three major areas (the Big Cypress National Preserve, the East Everglades, and the Loxahatchee National Wildlife Refuge) is significant (Burkhead 1991; Maffei 1991; Molnar et al. 1991). Cochran (1992) applied a benefit-cost analysis to the introduction of melaleuca and determined that the eradication of melaleuca would greatly benefit the state's economy.
The most successful control of melaleuca is pulling by hand and injecting or girdling the trunks and injecting the herbicide imazapyr. Basal soil treatments with the herbicides hexazinone or tebuthioron have provided effective control with little damage to surrounding vegetation; however, none of these herbicides is presently fully registered for use in standing water (Schardt and Schmitz 1990). An assessment of the methodology and efficiency of the eradication of melaleuca with herbicides was recently conducted in Florida (Trimmer and Teague 1991). A combination of herbicidal, mechanical, and environmental controls are proposed for trees, seedlings, and seed germination. Cassani (1986) discussed the arthropods that may be used as potential control agents and Center and Balciunas (1988) discussed the role of insects in the management of melaleuca.
A weevil, Oxyops vitiosa, was imported from Australia and is in quarantine in Gainesville, Florida, as a biological control agent of melaleuca. The evaluation and clearance of this insect could take from 9 months to 6 years (Leist 1993). If it passes all tests, it may be released in 1994.
