Status and Impacts of the Melaleuca Biological Control Program by C

Status and Impacts of the Melaleuca Biological Control Program by C. S. Silvers, USDA-ARS Invasive Plant Research Laboratory he biological control program for Melaleuca quinquen- THE BIOLOGICAL ervia (melaleuca), an invasive tree in South Florida, CONTROL AGENTS began in the mid 1980s with the hunt for natural enemies T The first melaleuca biological in the tree’s native range in Australia. More than a decade later, control agent, the Australian two insects have successfully run the gauntlet of quarantine- melaleuca snout weevil Oxyops based host specificity testing and emerged as promising biolog- vitiosa, was released in 1997 ical control agents in the fight to tame melaleuca. Six years after (Center et al., 2000). Both adult the first biological control agent release, feeding by the two and larval stages of the weevil are insects is having a dramatic effect on melaleuca throughout foliage feeders, preferring tender southern Florida. new leaves (Purcell and Balciunas, The melaleuca biological control program is spearheaded by 1994), although the majority of the USDA Agricultural Research Service (ARS) Invasive Plant weevil damage is a result of larval Research Laboratory (IPRL) in Fort Lauderdale, and relies heavily feeding. Larvae feed externally, on support from a number of other agencies, including the Army skeletonizing leaves by scraping Corps of Engineers, the South Florida Water Management District tender tissue from the surface. (SFWMD), the University of Florida, the Florida Department of Weevil-damaged leaves and Environmental Protection (DEP), the Departments of branch tips dry out, become brit- Environmental Resource Management (DERM) in Broward and tle, and break off. Miami-Dade Counties, and Australia’s Commonwealth Scientific The Australian psyllid (“SILL- and Industrial Research Organisation (CSIRO). As documented in id”), Boreioglycaspis melaleucae, the Melaleuca Management Plan was the second melaleuca biologi- (Laroche, 1999), the goal of the cal control agent introduced into program was to complement South Florida, in 2002 (Wood and removal efforts of land managers by Flores, 2002). Like the weevil, the slowing the spread of remaining psyllid feeds on leaves and the infestations, thereby reducing the immatures (nymphs) do most of risk of new invasions and reinva- the damage (Purcell et al., 1997). sion of treated areas. To accomplish But unlike the weevil’s chewing this, biological control agents were mode of feeding, the psyllid uses sought that would its piercing-sucking mouthparts to [from top] Psyllid adult; Psyllid reduce melaleuca flow- penetrate the leaf surface and feed nymphs; Psyllid flocculence. ering and therefore seed on sap within the plant’s phloem. The psyllid prefers to feed on production, and that tender foliage but will feed on older leaves as well, especially would inhibit seedling when psyllid populations are high and the amount of fresh foliage growth and regrowth on is limited. Psyllid nymphs secrete honey-dew, which hardens into cut stumps. This article small crystalline droplets. They also excrete white, waxy filaments describes how the two that look like cotton when large quantities build up on plant sur- biological control agents faces. These white filaments, called flocculence, make detection of already introduced psyllids in the field easy despite their small size. Additional signs for melaleuca of psyllid feeding include leaf discoloration, which changes from management yellow to red or brown, leaf desiccation and, ultimately, leaf drop. are meeting and even surpassing AGENT ESTABLISHMENT AND SPREAD program goals. Both the melaleuca weevil and psyllid have readily estab- lished at release sites, particularly when releases coincided with a [from top] Oxyops new flush of growth on the trees. The one notable exception is weevil adult; that weevil populations failed to establish in permanently aquatic Oxyops weevil larvae; Branch on right habitats (Center et al., 2000). This is because the weevil spends has been damaged by part of its life cycle in the soil, falling to the ground to pupate. If weevils. the area is flooded for an extended period of time, the pupae 8 SPRING 2004 drown. In contrast, the life cycle of the psyllid is completed entire- within 3 hours and all abandoned the ly in the tree canopy where it is not affected by flooding. plants within 32 hours. While adult wee- After initial release and establishment, melaleuca biological vils may rest temporarily on native plants, control agents have the potential to spread unaided throughout sustained feeding or oviposition (comple- melaleuca infested regions because both weevil and psyllid adults tion of development) on native species has fly. Studies on the weevil estimated an average rate of spread of 0.6 not been observed after three years of field miles per year (Pratt et al., 2003). At that rate, weevil populations assessments. Consistent with quarantine were estimated to take 20 years to saturate Florida’s melaleuca testing results, minor weevil feeding does infestations. To expedite spread, a weevil collection and redistrib- occur on the Australian bottlebrush trees ution program was begun, organized by researchers at the IPRL, Melaleuca (=Callistemon) viminalis and Psyllid flocculuence and feeding damage on melaleuca flowers. funded by DEP and DERM, and powered by labor from Student M. rigidis. Conservation Association/AmeriCorps interns and the UF Because melaleuca exhibits terminal Cooperative Extension Service. The weevil redistribution program growth, with new vegetative and repro- has so far collected almost 300,000 weevils and released them at ductive buds emerging at branch tips, the 150 sites throughout southern and central Florida. Weevils are biological control agents’ feeding prefer- now present in at least half of the state’s counties reported to have ence for new tips was predicted to hinder melaleuca infestations. both growth and flowering of trees. In an Much smaller and lighter than weevils, psyllids are dispersing ongoing study of biological control agent as far as 6.8 miles per year, with an average rate of spread of 4.3 impacts on melaleuca saplings, trees that miles per year (Paul Pratt, unpublished). To expedite the land- were not protected with insecticides and scape level impacts of psyllid populations, a collection and redis- growing under drier, west coast conditions Undamaged melaleuca flower (left) and weevil-damaged flower. tribution program also is underway for this insect. As of the fall of increased in height only 9.8% in 23 2003, more than 450,000 psyllids have been released at 26 sites months and produced no flowers. Trees growing in wetter, east in Florida. The psyllid now appears to be as ubiquitous as the coast conditions increased in height by 22.6% during the same peri- weevil, if not more so. od and produced an average of 0.3 flowers per tree. In contrast, trees that were protected from the biological control agents with IMPACTS insecticides and growing under drier conditions increased in height In addition to the introduction and establishment of agents, a more than 100% and produced an average of 4.6 flowers per tree. critical phase of a biological control program is follow up research Finally, trees growing under wet conditions and protected with on post-release activity of the agents. Scientists at the IPRL are insecticides were able to increase their height by 127.2% and pro- currently quantifying impacts of melaleuca biological control duce an average of 34 flowers per tree during the study period. agents on the target tree A separate study found that weevil feeding alone can reduce and surrounding vegeta- flowering and subsequent seed production by as much as 80% on tion, and determining how mature melaleuca trees. Similar studies have shown that insect dam- to most effectively integrate aged melaleuca trees are 36 times less likely to reproduce than biological control with con- undamaged trees. For those few damaged trees that did reproduce ventional control methods in the study, the size of the flowers and number of seed capsules as detailed in the Melaleuca were greatly reduced as compared to undamaged trees. Management Plan. An ongoing study to evaluate the integration of biological con- As part of the approval trol agents with mechanical control shows that insect feeding on and permitting process for melaleuca stump regrowth (coppices) reduces plant biomass by release, both the melaleuca more than 55% as compared to those protected from the insects. Melaleuca on right was exposed to biocon- weevil and psyllid under- Similarly, in a separate study conducted in a cattle pasture, the com- trol insects while trees on left were protect- went rigorous laboratory bination of biological control and occasional mowing reduced the ed with insecticide applications. Trees were studies to insure they density of coppicing stumps by approximately 80% in less than five of similar size when study began. would damage only years. melaleuca and pose no threat to desirable plants or native vegeta- Herbivory by the biological control agents also is proving to tion (Balciunas et al., 1994; Purcell et al., 1997; Wineriter et al., significantly reduce seedling and sapling survivorship. One study 2003). Following release and establishment of the two agents in found that feeding by the psyllid alone resulted in as much as 65% Florida, garden plots and field studies were conducted to confirm seedling mortality after just three generations of the insect (~4 that the insects’ specificity

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