Min B. Rayamajhi

Herbivory alters resource allocation and compensation in the invasive tree Melaleuca quinquenervia

Abstract.  1. Plants may compensate for the effects of herbivory, especially under favourable growing conditions, limited competition, and minimal top‐down regulation. These conditions characterise many disturbed wetlands dominated by introduced plants, implying that exotic, invasive weeds in these systems should exhibit strong compensatory responses.

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.

Melaleuca quinquenervia dominated forests in Florida: analyses of natural-enemy impacts on stand dynamics

Melaleuca quinquenervia (melaleuca) is a native of Australia but has become an invasive plant in Florida, USA. We conducted a long-term demographic study of melaleuca in three sections (central, transitional, and peripheral) of monoculture stands located in Florida, and quantified absolute density, diameter at breast height and basal area of trees by section at three sites. Additionally, we monitored the impacts of natural enemy (insects and fungi) on melaleuca populations which became apparent after 2001. Both absolute density and basal area, from before (1997–2001) and after noticeable natural-enemy impact (2002–2005), were compared. Prior to the natural-enemy impact, absolute density of melaleuca trees declined primarily due to self-thinning and associated losses of small trees, but diameter at breast height increased, as did the basal area. Later during the period when natural enemies prevailed, absolute density declined at a significantly greater rate across all sections but was highest at the periphery. The decrease in mean absolute density and basal area/ha of melaleuca during the natural-enemy impacted period coincided with the increased incidence of the populations of plant-feeding insects and fungi. The mean diameter at breast height continued to increase in all sections of the stands throughout the study period. An increasing trend in basal area prior to natural-enemy impact was reversed after increase in natural-enemy abundance and noticeable impact in all three sections of the stands. These findings lend support to a growing body of literature that implicates natural enemies as increasingly important density-independent regulators of M. quinquenervia populations.

Host range validation, molecular identification and release and establishment of a Chinese biotype of the Asian leaf beetle Lilioceris cheni (Coleoptera: Chrysomelidae: Criocerinae) for control of Dioscorea bulbifera L. in the southern United States

Abstract Dioscorea bulbifera, an Asian vine, is invasive in the southeastern USA. It rarely flowers but propagates from potato-like bulbils formed in leaf axils, which persist into the subsequent growing season. Lilioceris cheni Gressitt and Kimoto, a foliage-feeding beetle (Coleoptera: Chrysomelidae: Criocerinae) from Nepal, had been tested, proven to be a specialist and approved for release as a biological control agent. Regulatory delays, however, resulted in the demise of quarantine-held colonies, and acquisition of new Nepalese stock proved untenable. Searches then undertaken in southern China resulted in the collection of over 300 similar beetles. Two Chinese Lilioceris species were identified: one confirmed to be L. cheni and the other identified as Lilioceris egena (Weise). Mitochondrial analysis revealed an exact DNA match between some Chinese and one of the two Nepalese c oxidase subunit I haplotypes and all Chinese L. cheni haplotypes clustered as a single species but the comingling of the two species aroused concerns over possible hybridisation. These concerns were allayed by nuclear D2 analysis showing the absence of dual parental sequences. Nonetheless, diligence was exercised to ensure that the Chinese strains were safe to release. Abridged host testing using critical test species verified specificity. Caged releases during autumn 2011 documented the ability of adult beetles to overwinter in south Florida despite a prolonged lack of foliage. Open releases the following year produced vigorous populations that caused extensive defoliation. Preliminary observations indicate that L. cheni now contributes to the control of D. bulbifera and the bulbil-feeding L. egena should complement these effects if its host range proves appropriate.

Establishment, Population Increase, Spread, and Ecological Host Range of Lophodiplosis trifida (Diptera: Cecidomyiidae), a Biological Control Agent of the Invasive Tree Melaleuca quinquenervia (Myrtales: Myrtaceae)

ABSTRACT The Australian tree Melaleuca quinquenervia (Cavanilles) Blake is an invasive weed in wetland systems of Florida. A biological control program targeting M. quinquenervia has resulted in the release of the gall forming midge Lophodiplosis trifida Gagné (Diptera: Cecidomyiidae). Populations of the introduced herbivore readily established at all 24 release sites across the weeds range in Florida, and there was no evidence that founding colony size (100,2,000, or 6,000 adults) influenced herbivore establishment or local population growth rates. Landscape level spread of L. trifida from release sites averaged nearly 6 km/yr, ranging as high as 14.4 km/yr. Prerelease host range testing predicted that L. trifida oviposits indiscriminately on test plant species but does not complete development on any of the test species, including congeners present in Florida. To test the predictability of these host range tests, L. trifida was released in a common garden consisting of 18 test plant species that were interplan ted with M. quinquenervia. Plant species postulated to be at risk experienced no gall development by L. trifida while intermingled M. quinquenervia trees supported 704.8 (± galls per plant. Historically, many introduced Cecidomyiidae have limited effect on plant performance of target weeds because of recruitment of native parasitoids that disrupt biological control efficacy. In contrast to this trend, there has been no evidence to date that parasitoids are exploiting L. trifida in Florida.

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.

Development Rate, Consumption, and Host Specificity of Carea varipes (Lepidoptera: Nolidae)

Abstract The Asian shrub Rhodomyrtus tomentosa is an invasive weed in Florida and Hawaii, USA. Surveys for natural enemies of this exotic shrub in Hong Kong, China, resulted in the development of a laboratory colony and initial host range testing of Carea varipes Walker as a potential biological control agent of R. tomentosa. Twelve critical test plant species were presented to C. varipes larvae, but complete development was limited to R. tomentosa, the ornamental Myrtus communis, and the Florida native Morella cerifera. Total development time was >10 d faster when held with R. tomentosa and M. communis versus M. cerifera, with the latter species requiring an extra larval instar to reach the pupal stage. Total consumption levels over an individuals larval stage were similar among R. tomentosa and M. cerifera but, when the longer development time associated with the latter host is accounted for, consumption rates indicate that larvae consumed >2 times more leaf material when held with the Florida native. Despite an apparent preference for R. tomentosa, it is clear that the physiological host range of C. varipes includes the Florida native M. cerifera and the ornamentally important M. communis. The generalized feeding patterns exhibited in this research indicates that additional resources dedicated to the development of this species as a biological control agent are unwarranted.

Within-plant distribution of the Melaleuca quinquenervia biological control agent Lophodiplosis trifida

The gall-producing midge Lophodiplosis trifida Gagné (Diptera: Cecidomyiidae) is an established biological control agent of the exotic tree Melaleuca quinquenervia (Cav.) Blake, which invades wetland systems of Florida (USA). Host use patterns within the tree canopy were investigated. Plant height affected within-plant distribution of galls as L. trifida attack rates were greater in lower versus higher portions of the M. quinquenervia canopy. Nonetheless, galls occurred even at tree-top levels of 13 m.