Philip W. Tipping

Biological control of Melaleuca quinquenervia: an Everglades invader

A massive effort is underway to restore the Florida Everglades, mainly by re-engineering hydrology to supply more water to the system at appropriate times of the year. However, correcting water flow patterns alone will not restore the associated plant communities due to habitat-transforming effects of invasive species, in particular the Australian wetland tree Melaleuca quinquenervia (Cav.) S. T. Blake (Myrtales, Myrtaceae), which has invaded vast areas and transformed sawgrass marshes into dense, biologically impoverished, structurally altered forest habitats. To address this threat, an invasive species reduction program was launched that combined mechanical removal and herbicidal control to remove mature trees with the release of specialized insects to suppress seed production and lower seedling survival. Melaleuca has now been removed from most public lands while biological control has limited its ability to regenerate and reinvade from nearby infestations often located on unmanaged privately held lands. This case illustrates how restoration of highly modified ecosystems may require both restoration of physical conditions (water flow), and suppression of high impact or transformative invaders, showing well the need to integrate biological control into conservation biology.

Initial Impacts and Field Validation of Host Range for Boreioglycaspis melaleucae Moore (Hemiptera: Psyllidae), a Biological Control Agent of the Invasive Tree Melaleuca quinquenervia (Cav.) Blake (Myrtales: Myrtaceae: Leptospermoideae)

Abstract Invasion of south Florida wetlands by the Australian paperbark tree, Melaleuca quinquenervia (Cav.) S.T. Blake (melaleuca), has caused adverse economic and environmental impacts. The tree’s biological attributes and favorable ambient biophysical conditions combine to complicate efforts to restore and maintain south Florida ecosystems. Management requires an integrated strategy that deploys multiple biological control agents to forestall reinvasion and to supplement other control methods, thereby lessening recruitment and regeneration after removal of existing trees. This biological control program began during 1997 when an Australian weevil, Oxyops vitiosa (Pascoe), was released. A second Australian insect, the melaleuca psyllid (Boreioglycaspis melaleucae Moore), first introduced during 2002, has also widely established. After inoculation of the psyllid in a field study, only 40% of seedlings survived herbivory treatments compared with 95% survival in controls. The resultant defoliation also reduced growth of the surviving seedlings. A weevil-induced decline at a site comprised mainly of coppicing stumps had slowed after a 70% reduction. Psyllids colonized the site, and 37% of the remaining coppices succumbed within 10 mo. The realized ecological host range of B. melaleucae was restricted to M. quinquenervia; 18 other nontarget plant species predicted to be suboptimal or nonhosts during laboratory host range testing were unaffected when interspersed with psyllid-infested melaleuca trees in a common garden study. Evaluations are ongoing, but B. melaleucae is clearly reducing seedling recruitment and stump regrowth without adversely impacting other plant species. Manifestation of impacts on mature trees will require more time, but initial indications suggest that the psyllid will be an effective supplement to the weevil.

Decline in exotic tree density facilitates increased plant diversity: the experience from Melaleuca quinquenervia invaded wetlands

The Australian tree Melaleuca quinquenervia (melaleuca) formed dense monocultural forests several decades after invading parts of Florida and the Caribbean islands. These dominant forests have displaced native vegetation in sensitive wetland systems. We hypothesized that native plant diversity would increase following recent reductions in density of mature melaleuca stands in south Florida. We therefore examined data on changes in melaleuca densities and plant species diversity derived from permanent plots that were monitored from 1997 to 2005. These plots were located within mature melaleuca stands in nonflooded and seasonally-flooded habitats. Two host-specific biological control agents of melaleuca, Oxyops vitiosa and Boreioglycaspis melaleucae, were introduced during 1997 and 2002, respectively. Also, an adventive rust fungus Puccinia psidii and lobate-lac scale Paratachardinapesudolobata became abundant during the latter part of the study period. Overall melaleuca density declines in current study coincided with two to four fold increases in plant species diversity. The greatest declines in melaleuca density as well as the greatest increases in family importance values and species diversity indices occurred in nonflooded as compared to seasonally-flooded habitats. Most pioneer plant species in study sites belonged to Asteraceae, Cyperaceae, Poaceae, and Ulmaceae. The rapid reduction in melaleuca density and canopy cover during the study period may be attributed to self-thinning accelerated by the negative impact of natural enemies. Densities of other woody plants, particularly Myrica and Myrsine, which were sparsely represented in the understory by a few suppressed individuals also declined during the same period, possibly due to infestation by the generalist lac-scale. These findings indicate that natural-enemy accelerated self-thinning of melaleuca densities is positively influencing the native plant diversity and facilitating the partial rehabilitation of degraded habitats.

Aboveground Biomass of an Invasive Tree Melaleuca (Melaleuca quinquenervia) before and after Herbivory by Adventive and Introduced Natural Enemies: A Temporal Case Study in Florida

Abstract Invasive plants can respond to injury from natural enemies by altering the quantity and distribution of biomass among woody materials, foliage, fruits, and seeds. Melaleuca, an Australian tree that has naturalized in south Florida, has been reunited with two natural enemies: a weevil introduced during 1997 and a psyllid introduced during 2002. We hypothesized that herbivory from these and other adventive organisms (lobate-lac scale and a leaf-rust fungus) would alter the distribution and allocation of biomass on melaleuca trees. This hypothesis was tested by temporally assessing changes in aboveground biomass components in conjunction with the presence of natural enemies and their damage to melaleuca trees. Melaleuca trees of different diameters representing the range (1 to 33 cm diam at 1.3 m height) within study sites were harvested during 1996, prior to the introduction of herbivorous insects, and again during 2003 after extensive tree damage had become apparent. Aboveground biomass, partitioned into several components (woody structures, foliage, fruits, and seeds), was quantified both times in Broward, Miami–Dade, and Palm Beach county sites located in south Florida. The two harvests within each site were performed in closely-matched melaleuca stands, and changes in biomass components were compared between years. Total biomass and woody portions decreased in Broward, whereas they increased in Miami–Dade and Palm Beach sites. Reductions in foliage (on all trees) and seed biomass (among seed-bearing trees) were greatest at Broward and least at Miami–Dade County site. Hence, overall seed and foliage production was severely reduced at the Broward site where both the natural enemy incidence and damage were more abundant compared to other sites. We therefore attribute the reduced foliar biomass and reproductive capability of melaleuca trees to infestations of natural enemies. These findings highlight the role that natural enemies can play in the long-term management of invasive tree species.

EVALUATING ACEPHATE FOR INSECTICIDE EXCLUSION OF OXYOPS VITIOSA (COLEOPTERA: CURCULIONIDAE) FROM MELALEUCA QUINQUENERVIA

Abstract One method of evaluating the impact of insect weed biological control agents is to exclude them from their host with insecticides, thereby enabling comparisons of host fitness between infested and non-infested plants. However, the insecticide must not positively or negatively affect the plant being protected. The insecticide acephate was tested for its effects on Oxyops vitiosa Pascoe and Melaleuca quinquenervia (Cav.) S. T. Blake. Saplings of M. quinquenervia were sprayed with concentrations of 0, 0.073, 0.36, and 0.73% a.i. acephate every 7, 14, and 21 days. A bioassay using leaves from sprayed plants and third instars of O. vitiosa found reduced defoliation up to 21 days after treatment at the 0.36 and 0.73% concentrations of acephate. There were minor phytotoxic effects on younger, more tender leaves at the 0.73% concentration of acephate which reduced leaf biomass. Acephate can protect M. quinquenervia foliage from O. vitiosa larvae at the 0.36% concentration and spraying every 14 days will not affect the plant.

Release and Establishment of Megamelus scutellaris (Hemiptera: Delphacidae) on Waterhyacinth in Florida

Summary More than 73,000 Megamelus scutellaris (Hemiptera: Delphacidae) were released in Florida over a 2 to 3 yr period at 10 sites in an attempt to establish sustainable populations on waterhyacinth, Eichhornia crassipes Mart. Solms (Commelinales: Pontederiaceae). Insect populations persisted at most sites including those furthest north and consecutive overwintering was confirmed in as many as three times at some sites. Establishment appeared to be promoted at sites with some cover or shading compared to open areas. Insects readily dispersed over short distances which made detection and monitoring difficult.

Specificity of Lepidelphax pistiae (Hemiptera: Delphacidae) to Pistia stratiotes (Araceae)

The delphacid Lepidelphax pistiae was found on water lettuce in Argentina. Nymphs and adults did not survive past seven days on 29 species of test plants, and no progeny were produced. In contrast, survival and reproduction were high on water lettuce. Results suggest that L. pistiae is monospecific to water lettuce.

Asymmetric impacts of two herbivore ecotypes on similar host plants.

1. Ecotypes may arise following allopatric separation from source populations. The simultaneous transfer of an exotic plant to a novel environment, along with its stenophagous herbivore, may complicate more traditional patterns of divergence from the plant and insect source populations.

Effect of Insect Density and Host Plant Quality on Wing-Form in Megamelus scutellaris (Hemiptera: Delphacidae)

ABSTRACT Megamelus scutellaris Berg (Hemiptera: Delphacidae) is a South American species that feeds on waterhyacinth, Eichhornia crassipes Mart. (Solms). This species exhibits significant wing dimorphism whereby fully winged adults (macropters) are capable of flight while those with reduced wings (brachtypters) are not. The wing form is determined by a developmental switch triggered by environmental factors including crowding, host plant quality, temperature, and photoperiod. This study examined the influences of insect density and host plant quality on M. scutellaris wing dimorphism, development, and biomass as well as their effects on E. crassipes. Two experiments exposed a single generation of M. scutellaris to lower and higher densities of conspecifics on low and high quality plants. The first experiment involved transferring second instars to test plants at loads of 50, 5, or 1 g of fresh weight plant biomass per nymph, which resulted in mean densities of 2, 15, and 69 nymphs, respectively, on both low and high quality plants. A second experiment exposed test plants to 2, 4, or 20 M. scutellaris adults for 7 days and allowed their progeny to develop into adults which ultimately produced densities of 0, 56, and 352 F1 adults, respectively, per low and high quality plants. No macropterous adults were produced in any treatment combination in either experiment. Several plant variables were affected by insect densities and plant quality including the mean relative growth rate, the change in leaf number, and the percentage of dead leaves on a plant. Megamelus scutellaris appeared to have a relatively high density threshold for macroptery within the range of host quality used in these studies. This may promote more brachyptery which, in turn, may increase the chances of this insect reaching damaging densities in field populations of E. crassipes.

The release and unsuccessful establishment of the Melaleuca biological control agent Fergusonina turneri and its mutualistic nematode Fergusobia quinquenerviae

The Australian tree Melaleuca quinquenervia is an invasive weed in wetland systems of Florida, USA. A biological control program targeting M. quinquenervia resulted in the simultaneous release of the gall-fly Fergusonina turneri and the nematode Fergusobia quinquenerviae. Fergusonina (Diptera: Fergusoninidae) flies are gall formers that exploit plants in the Myrtaceae through a mutualistic association with nematodes in the genus Fergusobia (Tylenchida: Neotylenchidae). With a limited number of founding individuals, a risk-spreading release strategy was employed in 2005 by liberating a total of 1996 adult flies across seven locations in southern Florida. However, all release efforts failed to establish a viable population at any of the sites despite variation in location. In an effort to increase founding population size and improve phenological synchrony, 1,432 individual flies and associated nematodes were released within a single M. quinquenervia stand during the early winter months of 2006–2007. The population of F. turneri and F. quinquenerviae persisted at the field site for between two and three generations and, in accordance with the ca. 2-month generation time, emergence of F generation flies peaked in March, May and July 2007. Population growth rate increased with each succeeding generation up to the F3, after which the population went extinct. Both the F1 and F2 generations expanded spatially when compared to the distribution of their respective parental generations. The field population failed to spread after the F2 generation, with F3 generation galls found entirely within the spatial distribution of F2 galls. The release of F. turneri and F. quinquenerviae represent the first obligate mutualism used in weed biological control. Factors contributing to the failure of these species to establish are discussed.