Masters Degrees (Conservation Ecology and Entomology)
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- ItemInvestigations into the ecology and management of the polyphagous shot hole borer beetle (PSHB, Euwallacea fornicatus) in South Africa(Stellenbosch : Stellenbosch University, 2023-12) Roberts, Elise; Roets, Francois; Paap, Trudy; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: The polyphagous shot hole borer beetle (PSHB) (Euwallacea fornicatus) (Coleoptera: Curculionidae: Scolytinae) and its primary fungal symbiont, Fusarium euwallaceae, is a pest-disease complex that has killed a wide range of tree species in multiple countries on six continents, including South Africa. As with any invasive pest, monitoring programmes reveal the drivers behind species population dynamics in invaded regions, which helps to mitigate potential impacts and inform management strategies. This study involved a 26- month monitoring programme in an urban-agricultural fringe setting of the Western Cape province, South Africa. Baited traps were used to determine activity patterns in comparison with temperature, as well as infestation dynamics over time in comparison with climatic and biological factors. Results suggest that activity is strongly temperature dependent, and numbers peak in late summer. The number of surrounding infested reproductive hosts was an important explanatory variable for increasing beetle abundance and infestation levels, while focal tree infestation “saturation”, tree stress and tree size may also play a role. Infested host tree removal may be important for agricultural areas where crops are surrounded by reproductive hosts either deliberately planted as windbreaks or growing naturally. Two prominent management strategies for invasive forest pests were trialled: tree removal and therapeutic chemical treatment. Results suggest that tree removal and appropriate disposal of infested wood can decrease local propagule pressure, especially if conducted in colder months. Furthermore, some therapeutic chemical treatments reduced fungal growth, number of new PSHB attacks, and beetle colony success, but control was never completely successful. These results indicate that a combination of physical and chemical control options may help to reduce propagule pressure, but is unlikely to prevent infestations for long periods.
- ItemEvaluating the potential biological control of Margarodes prieskaensis using South African entomopathogenic fungi, and/or entomopathogenic nematodes(Stellenbosch : Stellenbosch University, 2023-04) Erasmus, Talitha; Stokwe, Nomakholwa F.; Allsopp, Elleunorah; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Margarodes prieskaensis (Jakubski) (Homoptera: Coccoidea: Margarodidae) occurs naturally in the Northern Cape, Limpopo and Mpumalanga in South Africa, where it is a serious pest on table and raisin grapes (Vitis vinifera L.) The larvae of M. prieskaensis feed on grapevine roots, causing poor growth and reduced vigor which eventually result in the death of the infested plant. Currently, there are no chemicals registered for the control of M. prieskaensis in South Africa, and no resistant rootstocks or natural enemies of M. prieskaensis have been identified to date. The aim of the study was to investigate the potential of local entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN) to control M. prieskaensis females in table and raisin grapes. Six local EPF isolates, obtained from the Stellenbosch University collection, were screened for their pathogenicity against M. prieskaensis females under laboratory and semi-field conditions: Beauveria bassiana, Metarhizium robertsii, M. pinghaense, M. brunneum, M. majus and M. anisopliae performed the best and achieved high percentages of infection and mortality and were selected for subsequent trials. The other local EPF isolates did not perform adequately, even though infection did occur and thus warrant further investigation. Metarhizium pinghaense outcompeted M. majus in the concentration trials, especially in terms of overall infection success, however both EPF had a high infection success rate. In semi-field trials where M. majus and M. pinghaense were evaluated under optimal conditions, M. majus was outperformed by M. pinghaense, producing higher levels of mycosisi n insect cadavers. The EPF treatments showed high infection rates, while there was no infection in the untreated control. However, the efficacy of the EPF in inducing M. prieskaensis mortality could not be determined accurately due to high levels of insect mortality in the control. Field trials were conducted in the Northern Cape and Limpopo to test the efficacy of M. pinghaense to control females of M. prieskaensis. The infection rate was significantly lower than in the laboratory and semi-field trials. Moisture, temperature ranges and other environmental factors can affect the efficiency of EPF in the soil. Limpopo experienced more optimal temperatures during the trial, with an average infection rate of 28,98%. The trial site in the Northern Cape experienced harsher environmental conditions with extremely cold temperatures during the trial period, resulting in a lower infection rate of 24,61%. Solar radiation also possibly contributed to the overall lower infection rate during the field trials. Suitable formulations of EPF could possibly reduce the dire impact of environmental factors like extreme temperatures, low humidity and solar radiation. The field trials in the Northern Cape also assessed the efficacy of a local EPN species, Steinernema yirgalemense to control females of M. prieskaensis. Overall, little to no infection was achieved. Margarodes prieskaensis presents challenges to field applications of EPN and S. yirgalemense was unable to reproduce within the M. prieskaensis females, indicating that it is possible that M. prieskaensis either inhibit the symbiotic bacteria secreted by the EPN or that the females secrete repellent volatiles that preventinfestation. Before attempts are made to test other EPN species against this pest, the possibility that M. prieskaensis females can inhibit EPN infestation warrants further investigation. A combined application of S. yirgalemense and M. pinghaense was also included in the field trials. The infection rate for both the EPN (3.7%) and EPF (11.1%) was low in the combination application, with indications of antagonism between the EPF and EPN. This might explain why the infection rate for M. pinghaense was lower in the combination treatment than when applied on its own. This should be resolved before further studies with combined applications are done. This study is the first to show that females of M. prieskaensis are susceptible to infection by EPF species and that their use as biocontrol agents warrants further investigation. Male pre-pupae of M. prieskaensis spend between 30 and 50 days just underneath the soil surface before developing into pupae and should also be investigated as targets for biocontrol. It is therefore recommended that the use of EPF as a soil drench application against the pre- pupae should be investigated. This study provided crucial baseline information on the efficacy of local EPF and EPN against M. prieskaensis females. For future studies on EPF to control M. prieskaensis, it would be beneficial to resolve the problems identified in this study, including the method of collecting and handling M. prieskaensis females, and adequate formulation of EPF for protection against environmental factors.
- ItemExploring the social-ecological drivers and impacts of invasive alien plant induced regime shifts in the grasslands of the Eastern Cape, South Africa(Stellenbosch : Stellenbosch University, 2023-03) Mandlana, Qhamani; Selomane, Odirilwe; Biggs, Reinette; Luvuno, Linda; Malgas, Rhoda; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Invasive alien plant species (IAPs) are currently invading South Africa’s grasslands. In this study, I explore the drivers and social-ecological impacts of invasive alien plant induced-regime shifts on the grasslands of the Eastern Cape Province in South Africa. This thesis consists of two data chapters. The first chapter (Chapter 2) synthesizes literature of the social-ecological drivers and processes underlying the invasive alien plant-induced regime shift based on the Regime Shift Database framework (RSDB). In the second data chapter (Chapter 3), I used semi-structured interviews to understand how the regime shift is perceived to affect the ecosystem services provided by grasslands and the well-being of the people who live around the Upper Tsitsana Catchment, Eastern Cape. Chapter 2 identified drivers that promote the invasion of IAPs that potentially push grassland across a critical threshold to cause a shift from a grass-dominated regime (grassland) into a woody invasive alien plant-dominated regime. These drivers included (1) the introduction of certain alien plants for various agricultural and aesthetic purposes that are now problematic; which reproduce, naturalize, become invasive, and disrupt grassland structures and functions, (2) land and soil disturbance caused by non-sustainable land uses and inappropriate land management practices, timber extraction and overgrazing, which facilitate the expansion of IAPs, (3) increased carbon dioxide (CO2) levels that promote invasive plant growth due to their leaf-level traits associated with carbon capture, (4) land abandonment that creates less competitive conditions for native grasses, and thus more opportunities for IAPs to invade and dominate, (5) high rainfall, which increases water content in the surface layer, and allows IAPs to absorb water better than native vegetation due to their high infiltration rates and superficial root system which favours their invasion success, and (6) clearing of invasive alien plants which can promote their growth due to re-sprouting seedlings. In this chapter, I have also highlighted the key feedback mechanisms that maintain the grassland regime (i.e., fire) and invasive alien plant-dominated regime (i.e., high rate of germination and seed production) using a causal loop diagram (CLD). Understanding the interaction between the key drivers and feedback dynamics can inform integrated planning processes to monitor, reduce, and prevent the ability of IAPs to cause regime shifts. Chapter 3 reports interview results amongst the respondents (n=50) of the Upper Tsitsana Catchment that are dependent on the grasslands for their everyday life for farming, grazing and cultivation in the Eastern Cape Province. More than 90% of the respondents stated that invasive alien plants, especially Acacia dealbata (silver wattle) and Acacia mearnsii (black wattle) have caused substantive changes and now dominate the grasslands of Upper Tsitsana Catchment. On the one hand, respondents stated that the invasion by these woody IAPs have many negative impacts such as reduced water supply, poor grazing areas, crop yields and providing refuge for criminals, and would like to see a decrease in Acacia mearnsii and Acacia dealbata dense stands. The respondents mentioned benefits provided A. mearnsii and A. dealbata such as fuelwood, building material, medicine, and livestock feed. The benefits of IAPs create conflicts and debates on whether these species should be controlled to provide the mentioned benefits. Policymakers and researchers should engage with land-users to understand their perceptions towards IAPs to address these conflicts. This chapter also highlighted that the fight against IAPs is currently spearheaded by the Working for Water (WfW) program through a tree debarking and chemicals approach. Despite the current control measures, these plants are re-establishing, forming dense stands, and invading larger areas, which makes it increasingly difficult to restore the native grassland. Rapid strategic management options are required in addition to the control methods by WfW to prevent further shifts that will drive the grassland ecosystem and human livelihoods into even more vulnerable conditions.
- ItemEarthworms as bioindicators of soil health in a conservation agriculture setting in the Western Cape(Stellenbosch : Stellenbosch University, 2023-03) Steyl, An-Mari; Roets, Francois; Strauss, Johann; Nxele, Thembeka; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Adopting conservation agriculture (CA) principles aims to increase crop production while improving overall soil health. One benefit of adopting these principles is increased biological activity, such as earthworms. Earthworms are essential soil ecosystem engineers and sensitive to management practices, making them suitable as potential soil health indicators. Since conservation agriculture could promote the numbers and activity of earthworms, they can most likely be used as soil health indicators in these systems. However, this has received very little attention in the Mediterranean climatic region of the Western Cape province. This study assessed whether conservation agricultural practices increase earthworm populations and whether earthworms have the potential to be used as indicators of soil health in these systems. The study was based on two experimental farms in the Western Cape province, each with a mixture of different CA crop rotations, varying levels of legume incorporation, and two conventional tillage sites used as controls. Earthworm samples were collected using three different sampling techniques: hand-sorting, mustard extraction and a combination of the two methods during four sampling stages based on crop growing season. The influence of the sampling method, sampling stage and farming system on earthworm abundance and generic richness was tested. Earthworm abundance and generic richness were also correlated against different soil variables to investigate whether earthworms can be indicators of soil health parameters. The hand-sorting method proved sufficient for obtaining a good representation of earthworm fauna in these systems. CA practices, even those with increased utilization of legume crops, did not necessarily benefit earthworm populations. However, a strong link between soil moisture, earthworm abundance, and generic richness appears to be the most crucial factor driving earthworm numbers in this dry region. Furthermore, earthworm abundance only correlated with a few soil variables, indicating that earthworms could potentially not be useful as indicators of soil health in this region. The exception was a strong correlation between earthworm abundance and soil organic carbon content, indicating that earthworms could be used to indicate this agriculturally important soil variable. Based on this study's results, practices that conserve water and increase carbon content in the soil will likely increase earthworm populations in this region and improve the services they provide.
- ItemEvaluating the short-term effects of conservation agriculture on soil health and crop production(Stellenbosch : Stellenbosch University, 2023-03) Andrews, Jade; Swanepoel, Pieter Andreas ; Roets, Francois; Crous, Casper; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Conservation agriculture (CA) – broadly defined as the implementation of three management principles – permanent soil cover, diverse crop rotations, and reduced soil disturbance – has increased steadily over the years and now accounts for 12.5% of global croplands. The implementation of these principles has improved soil health and crop yields, reduced production costs, and increased the profitability of agricultural systems. However, the application of CA in vegetable production systems is limited. As vegetable production is constrained by its high nutrient and water requirements, and crops are plagued by pests and diseases, the implementation of sustainable farming practices in these intensive systems is essential for sustained food production. This study aimed to evaluate the short-term effects of CA adoption in an irrigated vegetable production system located in Lutzville, South Africa. This area is known for wine and table grape production; vegetable production is also a major contributor towards the agricultural economy in the region. The effects of three treatments – control (“business-as-usual”), mulch (wheat straw mulch), and cover crop (a mixture of two cereals and a legume) – was determined on (i) soil fertility and biological activity; and (ii) crop yield (cauliflower seed, watermelon, tomato, and sweet melon). Soil fertility indicators included pHKCl, electrical resistance, organic C, and macronutrients, while biological activity indicators, such as enzyme activity, active C, and microbial activity, were measured. These treatments were applied to three crop rotation systems: (1) cauliflower- watermelon-soil cover; (2) soil cover-tomato-cauliflower; and (3) soil cover-sweet melon-soil cover. Clear site and crop effects were observed on soil biochemical properties. While soil biological indicators did not differ between treatments, the effects of treatments on soil fertility varied between crop rotations. Sodium content and pHKCl were higher on mulch plots, resistance lower on control plots, and P lowest on cover crop plots on rotation 2, while no treatment differences were found on rotation 1. However, soil potassium content was higher on mulch plots and phosphorus was lowest on cover crop treatments on crop rotation 3. Treatment effects on summer crop yield were varied between crops species. Mulch treatments increased cauliflower seed yield, but not watermelon yield on rotation 1. This increase in cauliflower yield was likely due to the higher application rate of compost on mulch plots (30 m³ ha⁻¹) in comparison to control and cover crop plots (15 m³ ha⁻¹), rather than soil cover treatment as it was only applied towards the end of the study period. While tomato yield was highest on control plots, no treatment effects were observed for sweet melon yield. The findings of this study suggest that the effects of treatments on both soil biochemical properties and crop yield may differ with rotation systems. Although this was a short-term study – and differences are usually only seen after at least 2 years of CA adoption – these results are promising for CA adoption and provides a good starting point for further studies in irrigated vegetable production systems in semi-arid regions.