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A survey of Sternorryncha on tomatoes and potatoes across South Africa with implications for disease vectoring
(Rhodes University, 2026-03-27) Zongo, Ayandisa; Hill, M P (https://orcid.org/0000-0003-0579-5298 ); Maseko, Zolile (https://orcid.org/0000-0001-6372-8443)
Pests significantly decrease both the yield and quality of many agricultural crops, ultimately leading to significant economic losses for farmers. Many hemipterans feed on plant sap and affect a wide range of economically important crops. Among these, the most significant phytophagous species are found within the suborder Sternorrhyncha which comprises aphids, psyllids, scales and whiteflies. This group of insects is effective at vectoring plant diseases that lead to production losses. This group of insects is problematic to some of the most important vegetable crops in South Africa, specifically potatoes and tomatoes. Aphids are some of the most important pests of potatoes which can cause damage and yield through direct feeding, but their main impact is due to their ability to transmit viruses such as Potato Leaf Roll Virus (PLRV) and Potato Virus Y (PVY). Whiteflies on the other hand not only suck plant sap but also transmit plant viruses such as Tomato Yellow Leaf Curl Virus (TYLCV), while psyllids spread bacteria such as Candidatus Liberibacter solanacearum. In order to reduce the negative impact of these insect pests, sound pest management strategies have to be implemented. Insect monitoring is a crucial component of integrated pest management (IPM), intended to ensure that pest populations remain below the economic injury level (EIL). Pest surveillance is also essential for assessing the presence and economic impact of pests in agriculture and is carried out to establish the pest status in a given area, enhance the chances of early detection, and track pest prevalence. Therefore, this study aimed to map the prevalence of important aphid species in two different potato producing areas and the impact of environmental factors on aphid species diversity and populations. It also aimed to map the potential outbreaks using a continuous monitoring approach. The study further sought to conduct a survey for incursion by the invasive Bactericera cockerelli Šulc (Hemiptera: Triozidae) as well as the prevalence of other Sternorryncha on Solanaceae plants in South Africa, establishing whether B. cockerelli or other non-native psyllids are present in the surveyed regions. This surveillance is intended to inform future biosecurity measures and contribute to South Africa’s preparedness against high-risk agricultural pests. A continuous monitoring approach across multiple growing seasons was carried out in two potato growing regions of South Africa, Christiana, North-West and Douglas, Northern Cape to collect aphid populations using suction traps. A field national survey using sticky traps, bucket traps and physical sampling was also carried to monitor Sternorryncha of interest on solanaceae in South Africa over 2 years. Collected insects were preserved in alcohol and identified to species or genus level using dichotomous identification keys. Furthermore, DNA extractions and sequencing were done on the aphid specimens to confirm the morphological identifications. For the continuous monitoring survey, 39 species were collected with, Acyrthosiphon kondoi Shinji (Hemiptera: Aphididae), Aphis spp Linnaeus (Hemiptera: Aphididae), Metopolophium dirhodum Walker (Hemiptera: Aphididae), Pemphigus spp Hartig (Hemiptera: Aphidoidea, Eriosomatidae), Rhopalosiphum maidis Fitch (Hemiptera: Aphididae), Rhopalosiphum padi Linnaeus (Hemiptera: Aphididae) and Therioaphis trifolii Monell (Hemiptera: Aphididae) recorded as the most abundant species that potentially vector the important potato viruses, PVY and PLRV. Rhopalosiphum padi and Rhopalosiphum maidis were the most abundant potential PVY vectors while Myzus persicae Sulzer (Hemiptera: Aphididae) was the most abundant potential vector for PLRV. The aphid populations were mostly influenced by temperature (both monthly averages of daily maximum and minimum), windspeed and precipitation where an increase in temperature led to an increase in aphid populations. Temperature data were recorded daily and averaged over each month for the analysis. The diversity, evenness and abundance of aphid species collected was similar across the sampling locations. For the national survey, with targeted surveillance for the invasive B. cockerelli, a total of 49 aphid species, 37 psyllid species and 1 whitefly species and other non-sternorryncha (Diptera, Hymenoptera, Thysanoptera, Nueroptera, Mantidae, Coleoptera, Lepidoptera and Hemiptera) were collected. Out of the three collection methods used to collect insects, sticky traps captured a lot more psyllid species while bucket traps captured more aphid species and outperformed the other methods used. The most abundant aphid species were, R. maidis, Aphis spp and R. padi; while the tipu psyllid, Platycorypha nigrivirga Burckhardt (Hemiptera: Psylloidea) was the most dominant psyllid species. The whitefly, Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) was most abundant on the tomato crops compared to the other crops sampled. There were no psyllid species associated with solanaceous crops collected in the field and all the sampled plants did not host B. cockerelli, however, its congeneric species Bactericera capensis Hollis, 1984 was collected during sampling hosted by non-crop solanaceae. The diversity, evenness and abundance of the Sternorryncha insects collected was also similar across the sampling locations. The findings indicate that there was a high diversity of Sternorryncha that feed on solanaceous crops, potentially spreading viruses with temperature influencing these populations. Furthermore, agricultural crops may harbour insects not associated with them. Additionally, B. cockerelli was not encountered during the field work even though B. capensis was collected, indicating that South Africa is still safe from the invasive pest. Thus, constant monitoring should remain active in order to detect its incursion into the country. Further studies into the biology and distribution of B. capensis need to be conducted in order to compare it with B. cockerelli. This will help researchers understand whether these two species would have a synergistic effect to each other should there be an incursion into the country. Furthermore, it will be worthwhile to test B. capensis and its wild hosts for any Liberibacter that might potentially be spread by B. cockerelli should there be incursion.
Feedbacks between Hard Infrastructure and Cultural Ecosystem Services in South Africa’s National Parks
(Rhodes University, 2026-03-27) Parker, Tess Catherine; De Vos, Alta (https://orcid.org/0000-0002-9085-4012 ); Cumming, Graeme S (https://orcid.org/0000-0002-3678-1326 )
While protected areas are critical for safeguarding nature, the role of hard (i.e., human-made) infrastructure in mediating their ecological and social outcomes remains poorly understood. This thesis examines hard infrastructure investment and condition across South Africa’s national park system and explores the relationship between patterns of infrastructure investment and visitor preferences for cultural ecosystem services (CES). The study draws on an existing registry of above-ground infrastructure assets (including roads, buildings, and fences) across 19 of South Africa’s 20 National Parks, comprising several thousand individual assets. Infrastructure condition and quality were assessed using indicators of age, condition, and retained economic value, while investment intensity was standardised at the park level (unit cost per square kilometre). K-means clustering of parks based on variables of infrastructure investment intensity identified three distinct infrastructure archetypes: (i) remote parks with low overall infrastructure investment, (ii) high-access urban or peri-urban parks with high investment but no fence infrastructure, and (iii) well-funded parks with integrated infrastructure and relatively balanced investment across asset types. Visitor preferences for CES were analysed using an existing national visitor survey dataset (n = 3,131 respondents) collected across South African National Parks by Ament et al. (2017). The survey used Likert-scale statements capturing visitors’ motivations and valued experiences. Factor analysis identified five CES bundles (Natural History, Recreation, Sense of Place, Safari Experience, and Outdoor Living), and k-means clustering of park-level mean factor scores revealed four CES-based park clusters: (i) Nature-Learning & Heritage, (ii) Safari & Charismatic Wildlife, (iii) Place-Attachment & Immersive Experiences, and (iv) Urban Recreation. Relationships between infrastructure archetypes and CES clusters were explored using MANOVA and Kruskal-Wallis tests with post-hoc comparisons. Results showed partial alignment between infrastructure and CES patterns: Recreation-oriented CES were associated with high-access, infrastructure-dense parks, while Natural History values were more prominent in infrastructure-light parks. In contrast, Sense of Place and Outdoor Living did not differ significantly across infrastructure archetypes. Overall, the findings support the hypothesis that institutional priorities for infrastructure investment in South Africa’s national park system are shaped by visitor demand-driven feedback loops, and an emerging differentiation in park roles. Where misalignments between CES and infrastructure were observed, these may reflect weak or lagged feedbacks between visitor values and governance responses. This study contributes to a better understanding of protected area dynamics and the importance of infrastructure within conservation contexts, particularly in mediating the delivery of cultural ecosystem services. Future research could extend this work through longitudinal monitoring of infrastructure-CES feedbacks, deeper investigation of links between infrastructure and ecological outcomes, more explicit examination of governance processes, “soft infrastructure,” and justice and equity in protected area systems.
A comparison of life distributions in Bayesian reliability theory
(Rhodes University, 2026-03-27) Ntanjana, Sinoyolo Mzukhona (https://orcid.org/0000-0001-5559-1141); Izally, Sharkay (https://orcid.org/0000-0002-7686-138X ); Raubenheimer, Lizanne (https://orcid.org/0000-0003-0756-7459 )
Reliability analysis plays a fundamental role in assessing the lifetime behaviour of components, systems, and materials. In the Bayesian approach, the uncertainty about model parameters can be measured using the posterior distribution. This study presents a comparative analysis of two life distributions, the Weibull and Birnbaum-Saunders distributions under Bayesian reliability theory. The study focuses on the derivation of posterior distributions using a range of objective priors, including the Jeffreys prior, divergence prior, reference prior, and the probability matching prior, for both complete and type I right censoring cases. These priors are derived from the Fisher information matrix for both models, and the properness of the resulting posterior distributions is examined both graphically and analytically. Markov Chain Monte Carlo techniques, including the Metropolis-Hastings sampler, Gibbs sampler, and the Metropolis-within-Gibbs algorithm, are employed to simulate from the posterior distributions of the model parameters. Convergence of the posterior samples is assessed using standard diagnostics such as the trace plots, the Gelman-Rubin convergence diagnostic, and the Geweke diagnostic. Simulation studies are conducted to assess model performance across different sample sizes and priors, with evaluation based on coverage rates and mean interval lengths. Predictive reliability analyses are performed to analyse the ability of both distributions to predict future lifetimes. Applications include fitting and evaluating two fatigue lifetime datasets using both the Weibull and the Birnbaum-Saunders distributions. Bayesian estimation is carried out, and posterior summaries are analysed to assess parameter behaviour, credible intervals, and overall model fit. Model comparison using the deviance information criterion (DIC) is performed to determine which distribution provides a better fit and more stable parameter estimates.
Multicomponent solid-state forms of antitubercular and antiretroviral active pharmaceutical ingredients
(Rhodes University, 2026-03-27) Manilal, Binita (https://orcid.org/0000-0001-6739-2115); Smith, Vincent J (https://orcid.org/0000-0003-3546-3884 ); Walker, Roderick B (https://orcid.org/0000-0003-2781-4154)
Embargo applied. Embargo period to expire in March of 2028.
Variability analysis of a sample of southern calibrators observed using MeerKAT
(Rhodes University, 2026-03-27) Kamau, Elizabeth Wanjiru (https://orcid.org/0000-0001-9633-5398); Smirnov, Oleg M (https://orcid.org/0000-0003-1680-7936); Oozeer, Nadeem (https://orcid.org/0000-0002-0155-6321 )
MeerKAT is a state-of-the-art radio telescope array in South Africa and a precursor to the revolutionary Square Kilometre Array (SKA). Given the critical role of calibration in ensuring that radio interferometers provide accurate scientific products, this thesis investigates the flux stability of a sample of 33 southern sky calibrators observed by MeerKAT. Our work builds upon a previous study of potential southern calibrators observed by the Karoo Array Telescope (KAT-7), a precursor to MeerKAT. The MeerKAT sample was first characterized by creating polynomial spectral models (which included archival data: 80MHz - 4.8GHz) using the SPECFIT package. We found that 26% of the calibrators had a peak, while 15% had a trough in their spectra, and 59% were fit with a linear power law. The flux variability analysis involved Long-Term Variability (LTV), corresponding to approximately a 2 year timescale, and Short-Term Variability (STV), corresponding to an order of hours timescale, with a threshold for potential variability set at variability metrics exceeding 3σ. Our LTV analysis across the three MeerKAT bands (U-, L-, and S- band) concluded that the majority of sources were stable below 1.283GHz. However, PKS J0854+2006, a well-known blazar, exhibited variability at frequencies > 1.283GHz. For the STV component, we performed a detailed statistical comparison on PKS J2152-2828, utilizing two distinct variability indices: VIAller, which provides a measure of the maximum fractional variation of the flux density and VIdebias, which takes into account the measurement uncertainties and is normalized by the mean flux. We specifically analysed two annular regions (within 0.5◦, and between 0.5◦ - 1.2◦ from the phase center) to ensure any variability observed was intrinsic to the source. We concluded that PKS J2152-2828 exhibits no significant STV, confirming its reliability as a phase calibrator for similar duration observations.