Follow the yellow flag road : advancing the biological control of a global wetland invader, and a case study on ecosystem recovery
- Authors: Minuti, Gianmarco
- Date: 2025-04-03
- Subjects: Aquatic plants , Invasive plants Biological control , Iris pseudacorus , Azolla filiculoides , Aphthona nonstriata , Host specificity
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/479908 , vital:78378 , DOI 10.21504/10962/479908
- Description: Global change is accelerating habitat degradation and extinction rates, disproportionately affecting freshwater ecosystems. Biological invasions, a core component of global change, have been recognized as a major driver of this phenomenon. Invasive alien aquatic plants (IAAPs) are a prime example, disrupting ecological dynamics and hindering the economic and social activities that depend upon freshwater ecosystems. The best approach to managing IAAPs is preventing their introduction and taking swift action when infestations are detected. Unfortunately, most IAAPs are only recognized after they have spread, making eradication difficult and requiring expensive, ongoing management through manual, mechanical, or chemical means, which become less effective as infestations grow. In this regard, a sustainable alternative against IAAPs is provided by classical biological control. This method lays on the assumption that invasive species have been introduced in an environment which is free from their natural enemies, and that such release allowed them to redirect their resources towards growth and reproduction. Accordingly, weed biocontrol involves introducing one such enemy - a so-called biocontrol agent - to regulate the population of the plant, ideally reducing its density below a critical threshold, minimizing its impacts. Biocontrol programs must follow stringent protocols in order to a) ensure the selection of the most appropriate agents; b) guarantee their establishment and effectiveness in controlling their target; c) minimize the risks involved with their release; and d) monitor their impacts after they have been released. This thesis is composed of two parts, focusing on pre-release studies (Part I, Chapters 1-4) and post-release evaluations (Part II, Chapter 5), respectively. Part I explores the biological control of Iris pseudacorus L. (Iridaceae) – yellow flag – a Eurasian plant that has invaded temperate wetlands worldwide. The first step consisted of conducting field surveys across the native range of the weed, which led to the selection of three candidate biocontrol agents. As a second step, bioclimatic niche modelling was used to forecast the current and future suitability of the weed and its prospective agents. These models helped to identify areas most at risk of invasion and predict how climate change might affect biocontrol efficacy, showing differing impacts in the Northern and Southern Hemispheres. Further, a scoring system was developed to prioritize non-target plants for host-specificity testing, focusing on species within the family Iridaceae. Most taxa prioritized are native to the Americas, South Africa and eastern Asia, highlighting the importance of collaboration across regions when dealing with global invaders with complex phylogeographic distributions. Finally, a germination study confirmed that I. pseudacorus seeds from Argentina germinate more than Belgian seeds, especially in absence of cold stratification, suggesting increased performances of invasive populations and an adaptation to warmer temperatures. Nonetheless, impact assessments conducted on one of the three biocontrol agents demonstrated that it could cause juvenile mortality, suggesting it may effectively reduce seedling recruitment, though further research on agents targeting reproductive structures is recommended. Part II of the thesis addresses an often understudied aspect of post-release evaluations: ecosystem recovery following biological control. To do so, a case study is presented, investigating the biological control of water fern, Azolla filiculoides Lamb. (Salviniaceae), in Belgium. The results of this study are used to discuss the shortcomings of different experimental designs, the lack of baseline data in invasion ecology, and the intricacies of defining and quantifying ecosystem recovery in light of legacy effects and context-dependency. , Wereldwijde verandering versnelt de degradatie van habitats en de uitstervingscijfers, met een onevenredige impact op zoetwaterecosystemen. Biologische invasies, een kernonderdeel van klimaatverandering, worden erkend als een belangrijke drijvende kracht achter dit fenomeen. Invasieve uitheemse waterplanten (IAAPs) zijn een duidelijk voorbeeld, omdat ze ecologische dynamiek verstoren en economische en sociale activiteiten belemmeren die afhankelijk zijn van zoetwaterecosystemen. De beste aanpak voor het beheer van IAAPs is het voorkomen van hun introductie en het nemen van snelle maatregelen bij het detecteren van besmettingen. Helaas worden de meeste IAAPs pas herkend nadat ze zich hebben verspreid, wat uitroeiing moeilijk maakt en dure, voortdurende beheersing vereist door handmatige, mechanische of chemische middelen, die minder effectief worden naarmate besmettingen toenemen. In dit opzicht biedt klassieke biologische bestrijding een duurzaam alternatief tegen IAAPs. Deze methode berust op de veronderstelling dat invasieve soorten zijn geïntroduceerd in een omgeving die vrij is van hun natuurlijke vijanden, waardoor ze hun middelen konden richten op groei en reproductie. Bijgevolg omvat onkruidbestrijding het introduceren van een dergelijke vijand - een zogenaamde biocontrole-agent - om de populatie van de plant te reguleren, idealiter door de dichtheid ervan onder een kritische drempel te verlagen en zo de impact ervan te minimaliseren. Biocontroleprogramma's moeten strikte protocollen volgen om a) de selectie van de meest geschikte agens te waarborgen; b) hun vestiging en effectiviteit bij het beheersen van hun doel te garanderen; c) de risico's van hun vrijlating te minimaliseren; en d) hun impact na vrijlating te monitoren. Dit proefschrift bestaat uit twee delen, gericht op respectievelijk pre-release studies (Deel I, Hoofdstukken 1-4) en post-release evaluaties (Deel II, Hoofdstuk 5). Deel I onderzoekt de biologische bestrijding van Iris pseudacorus L. (Iridaceae) - gele lis - een Euraziatische plant die gematigde moeraslanden wereldwijd heeft gekoloniseerd. De eerste stap bestond uit het uitvoeren van veldonderzoeken in het inheemse verspreidingsgebied van het onkruid, wat leidde tot de selectie van drie kandidaat-biocontroleagenten. Als tweede stap werd bioklimatisch nis-modelleren gebruikt om de huidige en toekomstige geschiktheid van het onkruid en zijn potentiële agens te voorspellen. Deze modellen hielpen gebieden met het grootste risico op invasie te identificeren en voorspelden hoe klimaatverandering de effectiviteit van biocontrole zou kunnen beïnvloeden, met verschillende effecten op het noordelijk en zuidelijk halfrond. Daarnaast werd een scoresysteem ontwikkeld om niet-doelplanten te prioriteren voor host-specifieke tests, met de nadruk op soorten binnen de familie Iridaceae. De prioritaire taxa zijn inheems in Amerika, Zuid-Afrika en Oost-Azië, wat het belang van samenwerking tussen regio’s onderstreept bij het omgaan met mondiale invasieve soorten met complexe fylogeografische verspreiding. Ten slotte bevestigde een kiemingsstudie dat I. pseudacorus zaden uit Argentinië meer kiemen dan Belgische zaden, vooral bij afwezigheid van koude stratificatie, wat wijst op verhoogde prestaties van invasieve populaties en aanpassing aan warmere temperaturen. Nochtans toonden effectbeoordelingen van een van de drie biocontroleagens aan dat deze de mortaliteit van juvenielen kon veroorzaken, wat suggereert dat het effectief kan zijn bij het verminderen van de zaailingrekrutering. Toch wordt verder onderzoek naar agenten die zich richten op reproductieve structuren aanbevolen. Deel II van het proefschrift behandelt een vaak onderbelicht aspect van post-release evaluaties: ecosysteemherstel na biologische bestrijding. Om dit te realiseren wordt een casestudy aangeboden over de biologische bestrijding van watervaren, Azolla filiculoides Lamb. (Salviniaceae), in België. De resultaten van deze studie worden gebruikt voor de bespreking van de tekortkomingen van verschillende experimentele ontwerpen, het gebrek aan basisgegevens in invasie-ecologie en de complexiteit van het definiëren en kwantificeren van ecosysteemherstel in het licht van nalatenschapseffecten en contextafhankelijkheid. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2025
- Full Text:
- Authors: Minuti, Gianmarco
- Date: 2025-04-03
- Subjects: Aquatic plants , Invasive plants Biological control , Iris pseudacorus , Azolla filiculoides , Aphthona nonstriata , Host specificity
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/479908 , vital:78378 , DOI 10.21504/10962/479908
- Description: Global change is accelerating habitat degradation and extinction rates, disproportionately affecting freshwater ecosystems. Biological invasions, a core component of global change, have been recognized as a major driver of this phenomenon. Invasive alien aquatic plants (IAAPs) are a prime example, disrupting ecological dynamics and hindering the economic and social activities that depend upon freshwater ecosystems. The best approach to managing IAAPs is preventing their introduction and taking swift action when infestations are detected. Unfortunately, most IAAPs are only recognized after they have spread, making eradication difficult and requiring expensive, ongoing management through manual, mechanical, or chemical means, which become less effective as infestations grow. In this regard, a sustainable alternative against IAAPs is provided by classical biological control. This method lays on the assumption that invasive species have been introduced in an environment which is free from their natural enemies, and that such release allowed them to redirect their resources towards growth and reproduction. Accordingly, weed biocontrol involves introducing one such enemy - a so-called biocontrol agent - to regulate the population of the plant, ideally reducing its density below a critical threshold, minimizing its impacts. Biocontrol programs must follow stringent protocols in order to a) ensure the selection of the most appropriate agents; b) guarantee their establishment and effectiveness in controlling their target; c) minimize the risks involved with their release; and d) monitor their impacts after they have been released. This thesis is composed of two parts, focusing on pre-release studies (Part I, Chapters 1-4) and post-release evaluations (Part II, Chapter 5), respectively. Part I explores the biological control of Iris pseudacorus L. (Iridaceae) – yellow flag – a Eurasian plant that has invaded temperate wetlands worldwide. The first step consisted of conducting field surveys across the native range of the weed, which led to the selection of three candidate biocontrol agents. As a second step, bioclimatic niche modelling was used to forecast the current and future suitability of the weed and its prospective agents. These models helped to identify areas most at risk of invasion and predict how climate change might affect biocontrol efficacy, showing differing impacts in the Northern and Southern Hemispheres. Further, a scoring system was developed to prioritize non-target plants for host-specificity testing, focusing on species within the family Iridaceae. Most taxa prioritized are native to the Americas, South Africa and eastern Asia, highlighting the importance of collaboration across regions when dealing with global invaders with complex phylogeographic distributions. Finally, a germination study confirmed that I. pseudacorus seeds from Argentina germinate more than Belgian seeds, especially in absence of cold stratification, suggesting increased performances of invasive populations and an adaptation to warmer temperatures. Nonetheless, impact assessments conducted on one of the three biocontrol agents demonstrated that it could cause juvenile mortality, suggesting it may effectively reduce seedling recruitment, though further research on agents targeting reproductive structures is recommended. Part II of the thesis addresses an often understudied aspect of post-release evaluations: ecosystem recovery following biological control. To do so, a case study is presented, investigating the biological control of water fern, Azolla filiculoides Lamb. (Salviniaceae), in Belgium. The results of this study are used to discuss the shortcomings of different experimental designs, the lack of baseline data in invasion ecology, and the intricacies of defining and quantifying ecosystem recovery in light of legacy effects and context-dependency. , Wereldwijde verandering versnelt de degradatie van habitats en de uitstervingscijfers, met een onevenredige impact op zoetwaterecosystemen. Biologische invasies, een kernonderdeel van klimaatverandering, worden erkend als een belangrijke drijvende kracht achter dit fenomeen. Invasieve uitheemse waterplanten (IAAPs) zijn een duidelijk voorbeeld, omdat ze ecologische dynamiek verstoren en economische en sociale activiteiten belemmeren die afhankelijk zijn van zoetwaterecosystemen. De beste aanpak voor het beheer van IAAPs is het voorkomen van hun introductie en het nemen van snelle maatregelen bij het detecteren van besmettingen. Helaas worden de meeste IAAPs pas herkend nadat ze zich hebben verspreid, wat uitroeiing moeilijk maakt en dure, voortdurende beheersing vereist door handmatige, mechanische of chemische middelen, die minder effectief worden naarmate besmettingen toenemen. In dit opzicht biedt klassieke biologische bestrijding een duurzaam alternatief tegen IAAPs. Deze methode berust op de veronderstelling dat invasieve soorten zijn geïntroduceerd in een omgeving die vrij is van hun natuurlijke vijanden, waardoor ze hun middelen konden richten op groei en reproductie. Bijgevolg omvat onkruidbestrijding het introduceren van een dergelijke vijand - een zogenaamde biocontrole-agent - om de populatie van de plant te reguleren, idealiter door de dichtheid ervan onder een kritische drempel te verlagen en zo de impact ervan te minimaliseren. Biocontroleprogramma's moeten strikte protocollen volgen om a) de selectie van de meest geschikte agens te waarborgen; b) hun vestiging en effectiviteit bij het beheersen van hun doel te garanderen; c) de risico's van hun vrijlating te minimaliseren; en d) hun impact na vrijlating te monitoren. Dit proefschrift bestaat uit twee delen, gericht op respectievelijk pre-release studies (Deel I, Hoofdstukken 1-4) en post-release evaluaties (Deel II, Hoofdstuk 5). Deel I onderzoekt de biologische bestrijding van Iris pseudacorus L. (Iridaceae) - gele lis - een Euraziatische plant die gematigde moeraslanden wereldwijd heeft gekoloniseerd. De eerste stap bestond uit het uitvoeren van veldonderzoeken in het inheemse verspreidingsgebied van het onkruid, wat leidde tot de selectie van drie kandidaat-biocontroleagenten. Als tweede stap werd bioklimatisch nis-modelleren gebruikt om de huidige en toekomstige geschiktheid van het onkruid en zijn potentiële agens te voorspellen. Deze modellen hielpen gebieden met het grootste risico op invasie te identificeren en voorspelden hoe klimaatverandering de effectiviteit van biocontrole zou kunnen beïnvloeden, met verschillende effecten op het noordelijk en zuidelijk halfrond. Daarnaast werd een scoresysteem ontwikkeld om niet-doelplanten te prioriteren voor host-specifieke tests, met de nadruk op soorten binnen de familie Iridaceae. De prioritaire taxa zijn inheems in Amerika, Zuid-Afrika en Oost-Azië, wat het belang van samenwerking tussen regio’s onderstreept bij het omgaan met mondiale invasieve soorten met complexe fylogeografische verspreiding. Ten slotte bevestigde een kiemingsstudie dat I. pseudacorus zaden uit Argentinië meer kiemen dan Belgische zaden, vooral bij afwezigheid van koude stratificatie, wat wijst op verhoogde prestaties van invasieve populaties en aanpassing aan warmere temperaturen. Nochtans toonden effectbeoordelingen van een van de drie biocontroleagens aan dat deze de mortaliteit van juvenielen kon veroorzaken, wat suggereert dat het effectief kan zijn bij het verminderen van de zaailingrekrutering. Toch wordt verder onderzoek naar agenten die zich richten op reproductieve structuren aanbevolen. Deel II van het proefschrift behandelt een vaak onderbelicht aspect van post-release evaluaties: ecosysteemherstel na biologische bestrijding. Om dit te realiseren wordt een casestudy aangeboden over de biologische bestrijding van watervaren, Azolla filiculoides Lamb. (Salviniaceae), in België. De resultaten van deze studie worden gebruikt voor de bespreking van de tekortkomingen van verschillende experimentele ontwerpen, het gebrek aan basisgegevens in invasie-ecologie en de complexiteit van het definiëren en kwantificeren van ecosysteemherstel in het licht van nalatenschapseffecten en contextafhankelijkheid. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2025
- Full Text:
A native weevil and an exotic planthopper: investigating potential biological control agents for nymphaea mexicana zuccarini (nymphaeaceae) and its hybrids in South Africa
- Authors: Reid, Megan Kim
- Date: 2023-03-31
- Subjects: Nymphaeaceae South Africa , Water lilies Biological control South Africa , Host specificity , Genetic variation , Bagous longulus
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/422554 , vital:71957 , DOI 10.21504/10962/422554
- Description: Nymphaea mexicana Zuccarini (Nymphaeaceae) is an invasive plant originating from southern USA and Mexico that has become problematic in South Africa, invading several water bodies around the country. Manual removal of this plant is very labour intensive and is not cost efficient or effective for long term control, while the use of herbicides is damaging to the environment and expensive. Consequently, this plant is a desirable candidate for biological control, which takes advantage of enemy release of the target weed and aims to re-establish population suppression induced by host specific natural enemies. Initiating biological control requires that several steps are followed to maximise the success of the programme, and the first few of these, including overseas surveys in the native range of the plant, have already been completed. This thesis aimed to continue biological control research for this species to take further steps at effectively managing the plant. Firstly, pre-release surveys in the invaded range are necessary to: determine what factors (including enemy release) contribute to the invasiveness of the target weed; establish a baseline of information to allow for comparison after biological control agents have been released; and identify any insect herbivores that may already be present in the country. The pre-release surveys conducted in this study revealed useful information about N. mexicana invasions in South Africa and provided evidence that enemy release is applicable to this case. However, these studies determined that a native weevil, Bagous longulus Gyllenhal (Coleoptera: Curculionidae), has expanded its host range to include the exotic N. mexicana at three sites, and may thus have potential for management of the species through augmentative releases. The invasion of N. mexicana in South Africa is further complicated by the presence of several Nymphaea hybrids originating from a complex history of horticultural trade. Although previous research has shown that several hybrid groups are present in South Africa, their parentage is not known. As biological control requires the use of host specific insects adapted to overcome the unique chemical and morphological defences utilised by plant species, hybrids are notoriously difficult to manage because they possess intermediate characters inherited from parent species to which natural enemies may not have adapted. Although biological control of hybrids is challenging, other case studies have demonstrated that it is possible to find suitable agents, but the chances of success are increased if putative parents of the hybrids are known so that they can be surveyed for natural enemies. Further molecular studies including possible parents of the Nymphaea hybrids in South Africa were thus carried out in this thesis to focus future surveying efforts. Two main hybrid groups were identified with genetic similarity to two tested putative Nymphaea parents, and this will allow further investigations of these species to improve the chances of successfully managing these hybrid groups. Some of the tested hybrids showed genetic contributions from multiple groups, some of which were unidentified, so it is necessary to prioritise the most problematic hybrids for biological control. With more insight into the genetic makeup of the Nymphaea hybrids in South Africa, investigations into the host specificity of potential biological control agents can be conducted. The ideal biological control agent should have a broad enough host range to impact and survive on both N. mexicana and its hybrids, but without a host range so broad that it would pose risk to native South African species. Host specificity trials are thus necessary to determine the suitability of potential agents. The identification of B. longulus feeding on N. mexicana during pre-release surveys motivated further investigations to determine the natural distribution, field host range, and host specificity of B. longulus in experimentally controlled conditions. Further surveys were therefore conducted at native Nymphaea sites around South Africa in addition to host specificity trials using the native Nymphaea nouchali Burm. f. (Nymphaeaceae), two populations of N. mexicana, and a cultivated hybrid. Results from the surveys and host specificity tests suggest that B. longulus is widely distributed across South Africa, is specific to Nymphaea with no observed preference between N. mexicana and the native N. nouchali, and does not perform well on Nymphaea hybrids. Hence, B. longulus is promising for use in new association biological control through augmentative releases but is not suitable for management of hybrids. In addition to the potential use of the South African B. longulus, it is necessary to conduct host specificity trials for natural enemies from the native range of N. mexicana that were prioritised in previous studies. Megamelus toddi Beamer (Hemiptera: Delphacidae) is one such species that was imported into quarantined laboratory conditions from Florida, USA. Host specificity trials were conducted using the same test plants as described for the studies on B. longulus, in addition to multigeneration trials to determine how long M. toddi could survive on non-target host plants. As with the B. longulus studies, no statistically significant differences in preference were observed between N. mexicana and N. nouchali, but M. toddi could not complete development on the test hybrid, indicating that this species is also unsuitable for the management of Nymphaea hybrids. Despite suboptimal plant health, M. toddi completed development for three generations on the native N. nouchali. This lack of host specificity deems M. toddi unsafe for release in South Africa but highlights the importance of following predefined steps to develop a biological control programme. The concluding chapter of this thesis discusses the aforementioned findings in a broader context by considering the driving forces of plant invasions in general and specifically for N. mexicana in South Africa. Case studies are also consulted to provide insight into how to proceed with managing Nymphaea hybrids in South Africa, while the factors governing host specificity and host range expansion are also discussed and considered in the context of B. longulus and M. toddi. Finally, after a consideration of the limitations of these studies, recommendations are made to continue the development of biological control for N. mexicana in South Africa. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
- Authors: Reid, Megan Kim
- Date: 2023-03-31
- Subjects: Nymphaeaceae South Africa , Water lilies Biological control South Africa , Host specificity , Genetic variation , Bagous longulus
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/422554 , vital:71957 , DOI 10.21504/10962/422554
- Description: Nymphaea mexicana Zuccarini (Nymphaeaceae) is an invasive plant originating from southern USA and Mexico that has become problematic in South Africa, invading several water bodies around the country. Manual removal of this plant is very labour intensive and is not cost efficient or effective for long term control, while the use of herbicides is damaging to the environment and expensive. Consequently, this plant is a desirable candidate for biological control, which takes advantage of enemy release of the target weed and aims to re-establish population suppression induced by host specific natural enemies. Initiating biological control requires that several steps are followed to maximise the success of the programme, and the first few of these, including overseas surveys in the native range of the plant, have already been completed. This thesis aimed to continue biological control research for this species to take further steps at effectively managing the plant. Firstly, pre-release surveys in the invaded range are necessary to: determine what factors (including enemy release) contribute to the invasiveness of the target weed; establish a baseline of information to allow for comparison after biological control agents have been released; and identify any insect herbivores that may already be present in the country. The pre-release surveys conducted in this study revealed useful information about N. mexicana invasions in South Africa and provided evidence that enemy release is applicable to this case. However, these studies determined that a native weevil, Bagous longulus Gyllenhal (Coleoptera: Curculionidae), has expanded its host range to include the exotic N. mexicana at three sites, and may thus have potential for management of the species through augmentative releases. The invasion of N. mexicana in South Africa is further complicated by the presence of several Nymphaea hybrids originating from a complex history of horticultural trade. Although previous research has shown that several hybrid groups are present in South Africa, their parentage is not known. As biological control requires the use of host specific insects adapted to overcome the unique chemical and morphological defences utilised by plant species, hybrids are notoriously difficult to manage because they possess intermediate characters inherited from parent species to which natural enemies may not have adapted. Although biological control of hybrids is challenging, other case studies have demonstrated that it is possible to find suitable agents, but the chances of success are increased if putative parents of the hybrids are known so that they can be surveyed for natural enemies. Further molecular studies including possible parents of the Nymphaea hybrids in South Africa were thus carried out in this thesis to focus future surveying efforts. Two main hybrid groups were identified with genetic similarity to two tested putative Nymphaea parents, and this will allow further investigations of these species to improve the chances of successfully managing these hybrid groups. Some of the tested hybrids showed genetic contributions from multiple groups, some of which were unidentified, so it is necessary to prioritise the most problematic hybrids for biological control. With more insight into the genetic makeup of the Nymphaea hybrids in South Africa, investigations into the host specificity of potential biological control agents can be conducted. The ideal biological control agent should have a broad enough host range to impact and survive on both N. mexicana and its hybrids, but without a host range so broad that it would pose risk to native South African species. Host specificity trials are thus necessary to determine the suitability of potential agents. The identification of B. longulus feeding on N. mexicana during pre-release surveys motivated further investigations to determine the natural distribution, field host range, and host specificity of B. longulus in experimentally controlled conditions. Further surveys were therefore conducted at native Nymphaea sites around South Africa in addition to host specificity trials using the native Nymphaea nouchali Burm. f. (Nymphaeaceae), two populations of N. mexicana, and a cultivated hybrid. Results from the surveys and host specificity tests suggest that B. longulus is widely distributed across South Africa, is specific to Nymphaea with no observed preference between N. mexicana and the native N. nouchali, and does not perform well on Nymphaea hybrids. Hence, B. longulus is promising for use in new association biological control through augmentative releases but is not suitable for management of hybrids. In addition to the potential use of the South African B. longulus, it is necessary to conduct host specificity trials for natural enemies from the native range of N. mexicana that were prioritised in previous studies. Megamelus toddi Beamer (Hemiptera: Delphacidae) is one such species that was imported into quarantined laboratory conditions from Florida, USA. Host specificity trials were conducted using the same test plants as described for the studies on B. longulus, in addition to multigeneration trials to determine how long M. toddi could survive on non-target host plants. As with the B. longulus studies, no statistically significant differences in preference were observed between N. mexicana and N. nouchali, but M. toddi could not complete development on the test hybrid, indicating that this species is also unsuitable for the management of Nymphaea hybrids. Despite suboptimal plant health, M. toddi completed development for three generations on the native N. nouchali. This lack of host specificity deems M. toddi unsafe for release in South Africa but highlights the importance of following predefined steps to develop a biological control programme. The concluding chapter of this thesis discusses the aforementioned findings in a broader context by considering the driving forces of plant invasions in general and specifically for N. mexicana in South Africa. Case studies are also consulted to provide insight into how to proceed with managing Nymphaea hybrids in South Africa, while the factors governing host specificity and host range expansion are also discussed and considered in the context of B. longulus and M. toddi. Finally, after a consideration of the limitations of these studies, recommendations are made to continue the development of biological control for N. mexicana in South Africa. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
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