Biological control of torch cactus in South Africa: finding a suitable agent for a non-native weed with an unknown indigenous distribution
- Authors: Griffith, Tamzin Camilla
- Date: 2024-10-11
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/466781 , vital:76778 , DOI https://doi.org/10.21504/10962/466781
- Description: Trichocereus spachianus is an invasive cactus species in South Africa and poses challenges as a target for biological control due to confusion around its taxonomy and origin. Adapted to arid environments, this cactus is of particular concern in dry savannah and Karoo biomes, where its invasion of rangelands reduces grazing capacity for both indigenous wildlife and livestock. While previous records indicate that T. spachianus is indigenous to Argentina, recent field surveys have failed to verify its presence. Determining the origin of the target weed was important in developing a biological control programme since it enables the collection of potential agents directly from native populations of the target plant. Successful biological control programmes against cactus species in South Africa have often involved utilising both new associations and oligophagous insects, made possible because of the lack of indigenous and valued cacti in the region. Lack of T. spachianus locations in the native distribution, meant direct collection of insects from the target weed was not possible. Efforts to find biological control agents were focused on new associations between closely related cacti and their oligophagous herbivores. Suitability of multiple Hypogeococcus (mealybug) entities and a cochineal species, Dactylopius confertus, were investigated for their efficacy on various South African weedy cactus species, including T. spachianus. Findings revealed that none of the Hypogeococcus entities (species or lineages) were effective biological control agents, but D. confertus demonstrated potential as a biological control agent with a relatively high fecundity and survival rate on T. spachianus. Efficacy trials indicated that D. confertus could reach population densities sufficient to cause mortality of T. spachianus plants. Limited host specificity trials revealed that D. confertus was suitably host specific for release in South Africa, provided host specificity testing was conducted on additional plants. Approval and release of D. confertus has the potential to reduce the invasiveness of this damaging cactus in a sustainable and environmentally friendly manner. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Griffith, Tamzin Camilla
- Date: 2024-10-11
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/466781 , vital:76778 , DOI https://doi.org/10.21504/10962/466781
- Description: Trichocereus spachianus is an invasive cactus species in South Africa and poses challenges as a target for biological control due to confusion around its taxonomy and origin. Adapted to arid environments, this cactus is of particular concern in dry savannah and Karoo biomes, where its invasion of rangelands reduces grazing capacity for both indigenous wildlife and livestock. While previous records indicate that T. spachianus is indigenous to Argentina, recent field surveys have failed to verify its presence. Determining the origin of the target weed was important in developing a biological control programme since it enables the collection of potential agents directly from native populations of the target plant. Successful biological control programmes against cactus species in South Africa have often involved utilising both new associations and oligophagous insects, made possible because of the lack of indigenous and valued cacti in the region. Lack of T. spachianus locations in the native distribution, meant direct collection of insects from the target weed was not possible. Efforts to find biological control agents were focused on new associations between closely related cacti and their oligophagous herbivores. Suitability of multiple Hypogeococcus (mealybug) entities and a cochineal species, Dactylopius confertus, were investigated for their efficacy on various South African weedy cactus species, including T. spachianus. Findings revealed that none of the Hypogeococcus entities (species or lineages) were effective biological control agents, but D. confertus demonstrated potential as a biological control agent with a relatively high fecundity and survival rate on T. spachianus. Efficacy trials indicated that D. confertus could reach population densities sufficient to cause mortality of T. spachianus plants. Limited host specificity trials revealed that D. confertus was suitably host specific for release in South Africa, provided host specificity testing was conducted on additional plants. Approval and release of D. confertus has the potential to reduce the invasiveness of this damaging cactus in a sustainable and environmentally friendly manner. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2024
- Full Text:
- Date Issued: 2024-10-11
Investigating thermal physiology as a tool to improve the release efficacy of insect biological control agents
- Authors: Griffith, Tamzin Camilla
- Date: 2018
- Subjects: Aquatic weeds -- Biological control , Water hyacinth -- Biological control , Insects -- Physiology , Miridae -- Effect of low temperatures on , Cold adaptation , Insects as biological pest control agents , Eccritotarsus catarinensis
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/63301 , vital:28391
- Description: Biological control is commonly used for the control of invasive aquatic weeds, which often involves the release of multiple host-specific agents. Releasing multiple agents has inherent safety concerns as the introduction of each new agent is associated with risks, but is often required to improve control where establishment is limited. Climatic incompatibility between the agent’s thermal physiology and its introduced range often causes agents to fail to establish. However, it has been suggested that the thermal physiology of insects is plastic. Therefore, the potential to manipulate their thermal physiologies before releasing them into the field needs to be explored; reducing the need to release additional agents, thereby ensuring the safety of biological control. This thesis therefore aimed to firstly, determine whether season and locality influenced the thermal physiology of two field populations of a water hyacinth (Eichhornia crassipes) control agent, the mirid Eccritotarsus catarinensis; one collected from the hottest establishment site, and one collected from the coldest establishment site in South Africa. Their thermal physiology was significantly influenced by season and not by the sites’ climate, suggesting their thermal physiology is plastic under field conditions. Secondly, the classical method of determining the lower critical thermal limit (CTmin), and a new respirometry method of determining this limit, compared the thermal physiology of two Eccritotarsus species reared in quarantine. Eccritotarsus catarinensis was significantly more cold tolerant than the more recently released Eccritotarsus eichhorniae, despite similar maintenance conditions, and as such, was used to establish whether cold hardening under laboratory conditions was possible. Successfully cold hardened E. catarinensis had a significantly lower CTmin compared to the field cold acclimated population, suggesting that cold hardening of agents could be conducted before release to improve their cold tolerance and increase their chances of establishment, allowing for further adaptation to colder climates in the field to occur. Increasing establishment of the most effective agents will decrease the number of agents needed in a biological control programme, thus encouraging a more parsimonious approach to biological control.
- Full Text:
- Date Issued: 2018
- Authors: Griffith, Tamzin Camilla
- Date: 2018
- Subjects: Aquatic weeds -- Biological control , Water hyacinth -- Biological control , Insects -- Physiology , Miridae -- Effect of low temperatures on , Cold adaptation , Insects as biological pest control agents , Eccritotarsus catarinensis
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/63301 , vital:28391
- Description: Biological control is commonly used for the control of invasive aquatic weeds, which often involves the release of multiple host-specific agents. Releasing multiple agents has inherent safety concerns as the introduction of each new agent is associated with risks, but is often required to improve control where establishment is limited. Climatic incompatibility between the agent’s thermal physiology and its introduced range often causes agents to fail to establish. However, it has been suggested that the thermal physiology of insects is plastic. Therefore, the potential to manipulate their thermal physiologies before releasing them into the field needs to be explored; reducing the need to release additional agents, thereby ensuring the safety of biological control. This thesis therefore aimed to firstly, determine whether season and locality influenced the thermal physiology of two field populations of a water hyacinth (Eichhornia crassipes) control agent, the mirid Eccritotarsus catarinensis; one collected from the hottest establishment site, and one collected from the coldest establishment site in South Africa. Their thermal physiology was significantly influenced by season and not by the sites’ climate, suggesting their thermal physiology is plastic under field conditions. Secondly, the classical method of determining the lower critical thermal limit (CTmin), and a new respirometry method of determining this limit, compared the thermal physiology of two Eccritotarsus species reared in quarantine. Eccritotarsus catarinensis was significantly more cold tolerant than the more recently released Eccritotarsus eichhorniae, despite similar maintenance conditions, and as such, was used to establish whether cold hardening under laboratory conditions was possible. Successfully cold hardened E. catarinensis had a significantly lower CTmin compared to the field cold acclimated population, suggesting that cold hardening of agents could be conducted before release to improve their cold tolerance and increase their chances of establishment, allowing for further adaptation to colder climates in the field to occur. Increasing establishment of the most effective agents will decrease the number of agents needed in a biological control programme, thus encouraging a more parsimonious approach to biological control.
- Full Text:
- Date Issued: 2018
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