Paul D. Pratt

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.

Herbivory by Introduced Insects Reduces Growth and Survival of Melaleuca quinquenervia Seedlings

Abstract We studied the influence of herbivory by two introduced insect herbivores on the survival and performance of seedlings of Melaleuca quinquenervia (Cav.) Blake (Myrtaceae), an invasive tree that threatens the Florida Everglades ecosystem. Boreioglycaspis melaleucae (Moore) (Homoptera: Psyllidae) nymphs and Oxyops vitiosa (Pascoe) (Coleoptera: Curculionidae) larvae were transferred onto Melaleuca seedlings within replicated 0.25-m2 caged plots in Palm Beach County, FL. The treatments included three densities of Boreioglycaspis first instars at 1, 15, and 50 nymphs per seedling, one treatment of a single first Oxyops larval instar per seedling, another treatment of both one Oxyops larva and one Boreioglycaspis nymph per seedling, and caged and uncaged controls. Herbivory by Oxyops did not affect Melaleuca seedling height, leaf number, or survival. Feeding by Boreioglycaspis decreased survival, height, and leaf number, with these measures of plant performance ≈50% lower in the high and medium densities compared with controls. In a field plot adjacent to the experimental area, we measured growth, survival, and naturally occurring insect density and damage on 1,100 seedlings. Although insect densities were on average lower in the field plot than in the experiment, mortality and growth rates of the seedlings were comparable with those in the experiment. The results indicate that, above a threshold density, Boreioglycaspis herbivory may be effective in reducing growth and survival of Melaleuca at the potentially critical seedling life stage. It also seemed that effects of the insects were independent rather than antagonistic or synergistic.

Predation by phytoseiid mites on Tetranychus lintearius (Acari: Tetranychidae), an established weed biological control agent of gorse (Ulex europaeus)

Development of trophic relationships among introduced biological control agents and native (or introduced) parasitoids, predators, or pathogens can interfere with successful establishment, spread, and ecological impact on the target pest. For the introduced weed biological control agent Tetranychus lintearius (Dufor), we assessed predator acquisition, the ability of these predators to survive and reproduce when held with the weed biological control agent, and their impacts on agent populations. Surveys of T. lintearius (spider mite) colonies demonstrated that this biological control agent of gorse, Ulex europaeus L., has acquired generalist and specialist feeding predatory mites in its adventive range but association alone is insufficient evidence to conclude that these mites are negatively affecting the weed biological control agent. Predation bioassays demonstrated that all predatory mites tested can feed, survive, and reproduce when held with T. lintearius lending evidence to the interference hypothesis. The number of spider mite colonies and volume of gorse foliage colonized were higher when the introduced predatory mite Phtyoseiulus persimilis Athias– Henriot was excluded from plots, indicating these mites are suppressing T. lintearius populations. We report the predation of T. lintearius by a complex of predatory mites that are routinely used for biological control of spider mites in agricultural systems. Published by Elsevier Science (USA).

No evolution of increased competitive ability or decreased allocation to defense in Melaleuca quinquenervia since release from natural enemies

If invasive plants are released from natural enemies in their introduced range, they may evolve decreased allocation to defense and increased growth, as predicted by the evolution of increased competitive ability (EICA) hypothesis. A field experiment using the invasive tree Melaleuca quinquenervia was conducted to test this hypothesis. Seeds were collected from 120 maternal trees: 60 in Florida (introduced range) and 60 in Australia (home range). Plants grown from these seeds were either subjected to herbivory by two insects from Australia that have recently been released as biological control agents or protected from herbivores using insecticides. Genotypes from the introduced range were initially more attractive to herbivores than genotypes from the home range, supporting EICA. However, genotypes from the introduced and home range did not differ in resistance to insects or in competitive ability, which does not support EICA. Plants from the introduced range had a lower leaf hair density, lower leaf: stem mass ratio, and a higher ratio of nerolidol: viridifloral chemotypes compared to plants from the native range. Plants with an intermediate density of leaf hairs and with high specific leaf area were more susceptible to herbivory damage, but there were no effects of leaf toughness or chemotype on presence of and damage by insects. Herbivory had a negative impact on performance of Melaleuca. Other than an initial preference by insects for introduced genotypes, there was no evidence for the evolution of decreased defense or increased competitive ability, as predicted by the EICA hypothesis. It does not appear from this study that the EICA hypothesis explains patterns of recent trait evolution in Melaleuca.

Geographic Distribution and Dispersal Rate of Oxyops vitiosa (Coleoptera: Curculionidae), a Biological Control Agent of the Invasive Tree Melaleuca quinquenervia in South Florida

Abstract We assess the geographic distribution and rate of spread of Oxyops vitiosa (Pascoe), a classical biological control agent of the invasive Australian tree Melaleuca quinquenervia (Cav.) S.T. Blake. This weevil has been released at 135 locations in south Florida, where it now occurs in 9 of 19 infested counties. When averaging dispersal distances among four representative sites, O. vitiosa spread at a rate of 0.99 (±0.28) km/yr, ranging from 0.10 to 2.78 km/yr. The rate of spread by O. vitiosa across melaleuca-dominated habitats was influenced by both ecological- and human-mediated parameters, including M. quinquenervia stand fragmentation (spatial separation among host plants), the number of weevils released, and time since release. The rate of spread was positively correlated with stand fragmentation level: high = 2.04, medium = 1.07, and low = 0.30 km/yr. By incorporating the dispersal rate from the highest fragmentation level into a simulation model we predicted that 138 months (June 2008) would be required for 50% of the habitat currently invaded by melaleuca to become infested at an economic weevil density (0.5 individuals per branch tip). At medium and low fragmentations, the model predicts 182 (February 2012) and 191 (November 2012) months, respectively. After examining the output from this basic model, we identified 16 possible redistribution sites that may accelerate the spread of the weevil.

Herbivory by Boreioglycaspis melaleucae (Hemiptera: Psyllidae) Accelerates Foliar Senescence and Abscission in the Invasive Tree Melaleuca quinquenervia

Abstract We quantified the density-dependent effects of herbivory by the psyllid Boreioglycaspis melaleucae Moore on the senescence of expanding and fully expanded leaves from two chemical variants (chemotypes) of the invasive tree Melaleuca quinquenervia (Cav.) S. T. Blake. Foliar chlorophyll content (OD) and percent nitrogen were not influenced by leaf age classes and chemotypes. In contrast, increases in the level of herbivory resulted in concomitant decreases in chlorophyll compared with undamaged leaves, with medium and high levels of herbivory reducing chlorophyll content by 64 and 72%, respectively. Likewise, low, medium, and high levels of herbivory resulted in 20, 53, and 60% reductions in percent nitrogen, respectively. Color analysis showed that increased herbivory also increased the amount of damaged tissue per leaf across both age classes, but younger leaves showed less susceptibility to herbivory than older leaves. Leaves sustaining moderate to high levels of herbivory progressed from dark green to yellow and finally to light tan as they deteriorated. These changes in color, particularly the yellowing aspect, were often more pronounced along the main leaf veins and vascular tissues. Feeding by B. melaleucae increased the likelihood of leaf abscission by 4.7-fold compared with leaves not subjected to herbivory and was not influenced by leaf age or chemotype. Implications for biological control of M. quinquenervia are discussed.

Selection on herbivory resistance and growth rate in an invasive plant.

The evolution of increased competitive ability (EICA) hypothesis proposes that invasive species evolve decreased defense and increased competitive ability following natural enemy release. Previous tests of EICA examined the result of evolution by comparing individuals from home and introduced ranges, but no previous study of this hypothesis has examined the process of evolution by analyzing patterns of selection. On the basis of EICA, there should be selection for competitive ability without herbivores and selection for defense with herbivores. Selection on competitive ability should be stronger for genotypes accustomed to herbivores (home range genotypes), and selection on defense should be stronger for genotypes unaccustomed to herbivores (introduced range genotypes). Using a field experiment, we tested these hypotheses for the invasive plant Melaleuca quinquenervia. There was a negative genetic correlation between resistance and growth, indicating a trade‐off. However, selection for stem elongation (an indicator of competitive ability) was always positive, and selection on resistance was always negative and did not depend on genotype source or the presence of herbivores. The patterns of selection found in this study contrast with predictions from EICA and accurately predict the lack of evolutionary change in growth and resistance following the introduction of this species from Australia to Florida.

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.

Hybrid Vigor for the Invasive Exotic Brazilian Peppertree (Schinus terebinthifolius Raddi., Anacardiaceae) in Florida

How can successful invaders overcome reduced genetic variation via small founder population sizes to persist, thrive, and successfully adapt to a new set of environmental conditions? An expanding body of literature posits hybridization, both inter- and intraspecific, as a driver of the evolution of invasiveness via genetic processes. We studied Brazilian peppertree (Schinus terebinthifolius), a tree species native to South America that is a successful invader throughout Florida. The tree was introduced separately to the east and west coasts of Florida more than 100 years ago from genetically distinct source populations. We conducted a common garden experiment to compare the early life-stage performance of hybrids versus their progenitors. We hypothesized that hybrids would outperform their progenitors due to the positive genetic effects of intraspecific hybridization (i.e., hybrid vigor). Hybrid seeds germinated at higher rates than eastern seeds. Over the 8-mo experiment, a greater proportion of hybrid seedlings survived than did western seedlings, and hybrids attained greater biomass than the western types. The cumulative hybrid advantage of both seed germination and seedling survival led to the establishment of nearly 45% more hybrid seedlings versus either progenitor. Documenting fitness advantages for hybrids over their progenitors is a requisite finding to consider hybridization as a factor in the success of invasive species.

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.