An initial investigation into biological control options for Schinus terebinthifolia in South Africa
- Authors: Magengelele, Nwabisa Laurencia
- Date: 2020
- Subjects: Anacardiaceae -- Biological control -- South Africa , Plants, Ornamental -- South Africa , Invasive plants -- Biological control -- South Africa , Insects as biological pest control agents -- South Africa , Brazilian pepper tree -- Biological control -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/103835 , vital:32306
- Description: Schinus terebinthifolia Raddi (Anacardiaceae) (Brazilian pepper tree) is a native tree to subtropical South America that was introduced into South Africa as an ornamental plant. Globally, it is regarded as one of the world’s worst invasive trees. In South Africa, this aggressive pioneer species is becoming increasingly problematic and is being considered as a target for biological control. In South Africa the tree has acquired a native seed-feeding wasp, Megastigmus transvaalensis Hussey (Hymenoptera: Torymidae). The wasp’s native hosts are indigenous Rhus species (Anacardiaceae), but it has expanded its host range to form a new association with both S. terebinthifolia and its close relative S. molle L. (Anacardiaceae). In order to quantify the seed predation by M. transvaalensis on S. terebinthifolia seeds, tree populations were surveyed across the Eastern Cape and KwaZulu-Natal provinces. The wasp was present at 99% of the S. terebinthifolia populations with an average of 22% of the seeds being destroyed. In the Eastern Cape Province, the highest seed damage occurred at the start of the winter months, when about 35% of seeds were damaged. This fell to less than 12% in spring and summer when the plants were flowering. Megastigmus transvaalensis may have slowed the rate of spread of the plant, but it is unlikely to reduce population sizes of S. terebinthifolia in South Africa in the long-term. Biological control efforts can be assisted by knowing the origin and invasion history of the target species. Genetic analyses are often the only way to elucidate the invasion history of invasive alien plants because it is rare to find detailed records of plant introductions. Both microsatellite and chloroplast DNA analysis were conducted on S. terebinthifolia trees from the plant’s introduced distribution in South Africa and both Florida and Hawaii, USA. These samples were compared to plants from the native distribution of South America. The analysis indicated that the S. terebinthifolia in South Africa was most likely sourced from the state of Rio de Janeiro in Brazil, which is the same source of the invasive populations in Florida and Hawaii. Importantly, the South African populations were all found to be “haplotype A”. Plants samples collected from Hawaii USA were the closest match to the South African plants. Biological control agents known to damage haplotype A which have been considered for use in Hawaii and Florida should therefore be prioritised for South Africa. Schinus terebinthifolia has a broad distribution in South Africa; however, the majority of the current distribution is limited to the coastal regions along the eastern coast in KwaZulu-Natal Province. This suggests that the species may be climatically limited. Species distribution models in MaxEnt were used to predict the suitable ecological niche of the species. Using occurrence localities from both the native and invaded range to calibrate the models resulted in 56% of the modelled areas being considered suitable for the growth of S. terebinthifolia in South Africa. This included areas in the Eastern Cape, Western Cape and Limpopo provinces. When the models were calibrated using just the native range data, or just the invaded range data, predicted distributions were more restricted and limited to the coastal areas of the Eastern Cape and KwaZulu-Natal provinces. The coastal areas between Florianopolis and Santos in Brazil were highlighted as the most climatically similar to the invasive populations of S. terebinthifolia in South Africa. These areas should be prioritised if native range surveys for potential biological control agents are conducted in South America. Although the native seed-feeding wasp is damaging to S. terebinthifolia in South Africa, the tree is still not under suitable levels of biological control and is likely to spread and increase in density. New biological control agents are therefore required. Genetic and climatic matching has determined where the most appropriate region to collect new potential biological control agents is. The genetic matching data has also indicated that biological control agents that have been released, or are being considered for release, in Hawaii and Florida, are likely to be suitable for the South African plants because they have been shown to be damaging to ‘haplotype A’. These agents should therefore be the first to be considered for release in South Africa.
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Initiating biological control for Nymphaea mexicana zuccarini (Nymphaeaceae) in South Africa
- Authors: Reid, Megan Kim
- Date: 2020
- Subjects: Nymphaea mexicana zuccarini -- Biological control -- South Africa , Nymphaeaceae -- Biological control -- South Africa , Invasive plants -- Biological control -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/144510 , vital:38352
- Description: Nymphaea mexicana Zuccarini (Nymphaeaceae) is an aquatic plant originating from south-eastern USA that is becoming increasingly invasive in South Africa as other invasive aquatic plants are being managed successfully through biological control. Mechanical and chemical control of aquatic weeds is expensive, damaging to the environment, and only effective in the short term, so biological control is more desirable as a management strategy for N. mexicana. The biological control of invasive alien plants requires that agents are host specific so that non-target risks are mitigated. For success to be achieved, it is important to ensure that the genetic structure of invasive populations is clarified so that agents can be collected from populations in the native range that match genetically to populations in the invasive range. This is especially important in cases where the morphology of invasive alien plants does not reflect genetic differences between populations. A previous study of the genetic structure of the invasive populations of N. mexicana in South Africa suggests the presence of hybrid forms of the plant in South Africa, with only one of these populations matching with samples from the native range. However, the study only used samples from two sites in the native range using amplified fragment length polymorphisms (AFLPs), so it was necessary to conduct further genetic analyses using samples from more sites in the native range. Hence, the first aim of this study was to develop a better understanding of the genetic structure of N. mexicana populations in the native and invaded range. Genetic samples were collected from sites in the native range during field surveys for potential biological control agents, and inter-simple sequence repeats (ISSRs) were used to compare the genetic structure of invasive and native populations of N. mexicana in South Africa. The results from these analyses suggest that seven of the 14 investigated invasive populations of N. mexicana in South Africa are genetically similar to populations in the native range, while the remaining seven populations are likely to be hybrid forms of the plant. This knowledge will be useful to target populations for biological control and highlights the need for further genetic analyses to determine the parentage of these hybrids so that biological control efforts are more likely to be successful. The initiation of a biological control programme requires that a series of steps are taken in order to maximise the likelihood that this form of intervention will be successful. The first few steps include: identification of the target weed and its genetic structure; exploration in the native range for potential biological control agents; and prioritisation of these agents based on factors such as climatic and genetic compatibility, feeding damage, abundance, and likely host range. Hence, the second aim of this study was to conduct surveys for potential biological control agents in the native range of N. mexicana, and to prioritise these agents. Field surveys were conducted between August and October in 2018 at 17 sites in Florida, Louisiana, and Texas, USA. Sites were selected based on climatic similarity of native sites compared to invasive sites by use of MaxEnt modelling. Native N. mexicana plants were searched for natural enemies, and these were prioritised based on feeding damage, abundance, incidence, and observations of field host range. Two species were prioritised: Bagous americanus LeConte (Coleoptera: Curculionidae) and Megamelus toddi Beamer (Hemiptera: Delphacidae). These species will be imported into quarantine facilities at Rhodes University for host specificity tests to be conducted. Understanding the factors that contribute to the successful establishment of biological control agents is important to improve the efficiency and reduce the costs incurred during the initiation of biological control programmes. Acquiring knowledge of the factors that predict the efficacy of biological control agents is similarly important, and these factors are discussed in the last chapter of this study. The challenges of the biological control of hybrids are also considered, and recommendations are made for the control of N. mexicana and other plants in South Africa.
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Managing the invasive aquatic plant Sagittaria platyphylla (Engelm.) J.G. Sm(Alismataceae): problems and prospects
- Authors: Ndlovu, Mpilonhle Sinothando
- Date: 2020
- Subjects: Aquatic weeds -- Biological control -- South Africa , Sagittaria latifolia -- Biological control -- South Africa , Noxious weeds -- Biological control -- South Africa , Invasive plants -- Biological control -- South Africa , Listronotus , Insects as biological pest control agents
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/167121 , vital:41439
- Description: Sagittaria platyphylla (Engelm.) J.G.Sm. (Alismataceae), commonly known as Delta arrowhead, is an invasive aquatic macrophyte native to southern United States of America (USA) that has become a serious weed in freshwater systems in South Africa, New Zealand, Australia, and recently China. In South Africa, the plant was first detected in Krantzkloof Nature Reserve, KwaZulu-Natal Province in 2008, and due to its known impact in other countries, it was listed as a Category 1a invader species under the National Environmental Management: Biodiversity Act 2004 (NEM: BA). This listing required mechanical and chemical control methods to be implemented by the South African National Biodiversity Institute’s (SANBI), Invasive Species Programme (ISP), with the aim of eradicating the weed. Despite the eradication efforts, by 2016, the weed was recognized as one of the country’s top 10 worst and fastest spreading invasive alien plants. Since its introduction in 2008, the plant has spread both within and between sites in South Africa, increasing from one site in 2008 to 72 sites by 2019. Once introduced into lotic systems, the plant spread rapidly downstream, in some cases up to 120km within six years, with an average of 10 km per year. Extirpation over the last ten years was only possible at a limited number of sites. Under the current management approach, the invasion is foreseen to spread to new sites within a 5 km radius of the current populations. Due to the failure of conventional control mechanisms, biological control is currently being considered as a potential control option. Four potential biological control agents are under investigation, but none have been released. Amongst them is the fruit and flower feeding weevil Listronotus appendiculatus Bohm. (Coleoptera: Curculionidae) which showed most potential as a suitable biological control agent. This study demonstrated that L. appendiculatus herbivory negatively influenced the overall fitness of S. platyphylla by reducing the plant’s growth rate and above ground biomass. Listronotus appendiculatus herbivory also reduced the plant’s size and the potential to kill adult plants. Most importantly, L. appendiculatus larval feeding damage significantly reduce viable-germinating seeds, the weed’s primary dispersal mechanism. Therefore, a biological control programme is advised to be integrated within the current management plan.
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Quantifying ecosystem restoration recovery and restoration practice following the biological control of invasive alien macrophytes in Southern Africa
- Authors: Motitsoe, Samuel Nkopane
- Date: 2020
- Subjects: Salvinia molesta , Ceratophyllum demersum , Nymphaea mexicana , Invasive plants -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Restoration monitoring (Ecology) -- South Africa , Biolotical invasions -- Environmental aspects
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167519 , vital:41488
- Description: Invasive alien aquatic plants (IAAP) species are known to have deleterious effects on the freshwater ecosystems they invade. This includes both socio-economic and ecologically important ecosystem goods and services. Thus, IAAP species are declared a serious threat, second only to habitat modification for causing a loss of aquatic biodiversity. Three control methods have been widely applied to control IAAP species invasion globally; mechanical, chemical and biological control. Both mechanical and chemical control methods are considered short-term and expensive, whereas biological control methods are regarded an effective and long-term solution for IAAP species control at the landscape level. But, little is known of the ecological recovery following the biological control of IAAP species, with mechanical control known to have had mixed success and chemical control to have non-targeted effects on aquatic ecosystems, causing harm to wildlife and human well-being. Biological control practitioners measure the success of biological control based on: (1) the biological control agents’ establishment and the negative impacts they impose on the targeted weed; and (2) the weeds biomass reduction and an increase in native macrophytes species. Arguably, measures of biological control success have been subjective and variable across the globe. Although some field studies have demonstrated biological control success to have positive socio-economic returns, there is little literature on ecological benefits. Furthermore, there is limited understanding on ecosystem recovery and possible restoration efforts following the biological control IAAP species, as compared to alien weeds in terrestrial and riparian ecosystems. Thus, this thesis aimed to quantify the ecological recovery i.e. aquatic biodiversity, ecosystem processes and trophic interactions following the management of Salvinia molesta in freshwater ecosystems. The research employed a suite of Before-After Control-Impact mesocosm and field studies to investigate the response of aquatic microalgae, macroinvertebrates and their interactions (food web structure and function) during S. molesta infestation and after mechanical and biological control. The mesocosm experiment (Before invasion, During invasion & After control) showed that both aquatic microalgae and macroinvertebrate diversity indices were reliable biological indicators of S. molesta ecological impacts and recovery following control. The restored treatment (100% S. molesta cover + biological control agents), demonstrated complete aquatic microalgae and macroinvertebrate recovery following biological control, similar to the control treatment (open water), where the degraded/impacted treatment (100% S. molesta cover with no biological control agents) showed a drastic decline in aquatic biodiversity and a complete shift in aquatic biota assemblage structure. Thus, the biological control effort by Cyrtobagous salviniae, the biological control agent for S. molesta, assisted in the recovery of aquatic biota following successful biological control. The field study (four field sites, two sites controlled mechanically and two biologically) investigated water quality, aquatic biodiversity and community trophic interactions (aquatic food web) “before and after” S. molesta control. The study showed a drastic decline in aquatic biodiversity (with three sites showing no record of aquatic macroinvertebrates, thus no biotic interactions during infestation) and poor water quality due to the shade-effect (light barrier due to floating S. molesta mats on the water surface) during the “before” S. molesta control phase. However, following both mechanical and biological control (“after” S. molesta control phase), there was a significant shift in abiotic and biotic ecosystem characteristics as compared to the “before” S. molesta control phase. Thus, rapid ecosystem recovery was apparent as a result of aquatic microalgae and macroinvertebrates recolonisation. Sites showed a normal functioning ecosystem where improved water quality, increased biodiversity, productivity and trophic interactions, was indicative of the return of biologically and functionally important species which were lost during the “before” S. molesta phase. Although the clear water state showed positive outcomes at Westlake River, these were short lived when the system was dominated by a cosmopolitan submerged Ceratophyllum demersum, and later replaced by a floating-leaved emergent IAAP Nymphaea mexicana. Each state was responsible for a significant shift in both biotic and abiotic characteristics, affirming macrophyte abilities to influence aquatic environments structure and functions. Furthermore, this event showed a clear example of a secondary invasion. Thus, a holistic IAAP species management strategy is necessary to restore previously invaded ecosystems and prevent subsequent secondary invasion and ecosystem degradation. In conclusion, the S. molesta shade-effect like any other free-floating IAAP species, was identified as the main degrading factor and responsible for water quality reduction, loss of aquatic diversity and shift in aquatic biota assemblage structure. Following S. molesta removal (or shade-effect elimination), there was a positive response to aquatic ecosystem species abundance, richness, diversity and community structure. Therefore, in combination, aquatic biota recolonisation rate and increases in biological and functional diversity were instrumental in the recovery of ecosystem structure and functions, following the control of S. molesta. Echoing existing literature, this thesis recommends: (1) IAAP species management programmes (mechanical and/or biological control) should not only aim to control the weed but also focus on ecosystems recovery and possible restoration goals; (2) biological control should be used where appropriate to combat free-floating IAAP species in freshwater ecosystems, followed by active introduction of native macrophyte propagules since they are limited by anthropogenic activities; and (3) more freshwater case studies are needed to add to our understanding of IAAP species management and restoration effort incorporating long-term monitoring.
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