Mistletoes intimately connect to their host trees with a haustorium that allows them to access nutrients and water. Mistletoes in South Africa vary greatly in their degree of host specificity. Most species occur on a wide range of host families, while others are restricted to a single host family or—at the extreme—to a single host species. Mistletoes that are host generalists at a larger spatial scale may become host-specific at a local scale. One of the challenges in mistletoe biology is determining the factors that maintain local host specificity. Birds potentially reinforce the mistletoe–host interactions by direct dispersal. However, many mistletoe species coexist while parasitising different co-occurring host species. This suggests that host trees may impose more selection pressure than birds in determining host specificity. Thus, my thesis examines the role of host trees as ecological and physiological filters that influence the infection patterns and determine host specificity of mistletoes in South Africa. The second chapter of this thesis synthesises the literature on host specificity in mistletoes. I then present the results of four field and laboratory experiments that were used examine the features affecting host specificity in representatives of two families of mistletoes (Loranthaceae and Viscaceae) in South Africa. My main research objectives focus on host abundance and morphology, host compatibility, host water and nutrient content, abiotic influences on mistletoe seedling survival and growth and mistletoe–host stomatal morphology in relation to water potential that affect nutrient acquisition by mistletoes from their host trees. The geographic mosaic approach was explored as a potential explanation for the mistletoe–host interactions that direct host specificity in mistletoes. I synthesised the available literature on the mechanisms and factors that direct mistletoe host specificity. This was supported by data analysed from South African herbarium collections, books describing the South African flora and field observations in South Africa. I suggest that host abundance (host availability through time and space) and host compatibility (as determined by genetic, morphological, physiological and chemical factors) play a primary role in determining host specificity in South African mistletoes, while differential bird dispersal strengthens or weakens mistletoe–host interactions. Analysis of the network structure of mistletoe–host interactions at different levels (e.g., at the level of population, species and genus) followed by genetic and reciprocal germination experiments may reveal the patterns and mechanisms of host specificity in mistletoes. I quantified the mistletoe–host composition, height of potential host trees and nutrient and water content of mistletoes and their hosts at Pniel Estates. Surveys of the study site revealed a single mistletoe species, Viscum rotundifolium, parasitising only Ziziphus mucronata and Ehretia rigida. Both parasitised host species were not the most abundant trees, were not the tallest trees and did not have the highest water or nutrient content of trees in the area, although these factors have been found to be good predictors for mistletoe parasitism in other studies. Subsequently, I tested mistletoe–host compatibility by conducting a germination experiment in the greenhouse by inculcating seeds of V. rotundifolium on freshly cut branches of nine available potential host trees. I found that mistletoe seeds had a greater chance of attachment and subsequent survival on branches of E. rigida and Z. mucronata as compared with seeds on co-occurring Acacia and other potential host species. This suggests that host compatibility plays a role in directing the host specificity of V. rotundifolium at Pniel Estates. I found that individuals of V. rotundifolium had more negative water potentials than their host trees and, by doing so, they passively maintain the flow of nutrients. In addition, I found evidence that the mistletoe uses active uptake to access nutrients from host phloem because the leaf tissue of a mistletoe had a nitrogen-to-calcium ratio (N:Ca) >1. Conventionally, a high N:Ca ratio (>1) in the leaf tissue of a mistletoe is taken as evidence of active uptake from host phloem because N is highly phloem-mobile while Ca is a large molecule and is phloem-immobile. This method has shortcomings discussed at greater length in the chapter but my findings suggest that the mistletoe V. rotundifolium uses a combination of passive and active nutrient uptake. I quantified the mistletoe–host community composition and host physical features (height and diameter at breast height) in two sites in KwaZulu-Natal, South Africa—Highover and Mtontwane. The mistletoe Agelanthus natalitius (Loranthaceae) is common at both sites, parasitising the most abundant host species—Acacia karroo—and the second most abundant host tree—Acacia caffra. Prevalence of mistletoe infection (percentage of trees parasitised) was positively correlated with tree size (height and diameter at breast height). The two host species did not differ significantly in height. At Highover the host species A. caffra and A. karroo had a similar prevalence of mistletoe infection but at Mtontwane a significantly higher percentage of A. caffra trees was parasitised in comparison with A. karroo. However, the intensity of mistletoe infection (mean number of mistletoes per tree) was lower for A. caffra (Highover: 0.66 ± 0.01, Mtontwane: 0.89 ± 0.04) than for A. karroo (Highover: 0.73 ± 0.04, Mtontwane: 1.03 ± 0.64). There were two highly infected big trees in Highover and one in Mtontwane where many mistletoe-dispersing birds were nesting which inflated the numbers for intensity of mistletoe infection in A. caffra, however. I tested mistletoe–host compatibility by conducting a reciprocal transplant experiment in the two study sites. I applied a paired design, using one local and one non-local mistletoe seed in each pair, with seed pairs placed on the two main host species at the different sites. Except in Highover where an unidentified pathogen retarded growth and survival, mistletoe seeds placed on the same substrate and in the same site as their source host grew a longer hypocotyl and had greater survival. Regardless of source, mistletoes placed on A. karroo had longer hypocotyls and greater survival than mistletoes on A. caffra. These results suggest that there may be adaptation of the mistletoe Agelanthus natalitius to the most frequently encountered host species, Acacia karroo. To simulate the conditions encountered by mistletoes during the dry and cold South African winter, mistletoe seedlings were monitored at different levels of microclimate (light, temperature and moisture) in a growth chamber. I found that higher light availability (20% and 40% shade versus 80% shade), cool temperatures (15°C and 20°C versus 25°C) and continuous moisture availability improved seedling development and subsequent survival of two mistletoe species (Viscum rotundifolium and Agelanthus natalitius). I studied the leaf stomata of two host–mistletoe pairs (Acacia karroo–Agelanthus natalitius and Vitex obovata–Erianthemum dregei) using a scanning electron microscope to investigate some of the underlying mechanisms that enable mistletoes to maintain more negative water potentials than their host trees and at the same time control water loss. In addition, I examined the response of mistletoes to the application of abscisic acid (ABA), a plant growth regulator that controls stomatal closure. I found that the mistletoes had a higher density of stomata and had larger stomata than their host trees. In addition, both mistletoe and host leaves closed their stomata during midday and in response to exogenous ABA. The ability of mistletoes to control water loss in this way may be one reason why mistletoes rarely kill their host trees, which would be maladaptive. The mistletoes used in my studies are known to be host generalists at a larger spatial scale but I found that they were host specific at a local scale. The results of my research suggest that host abundance and compatibility play a role in directing host specificity, while host nutrient and water status have little effect on host specificity at this local scale. The interactions between the generalist mistletoes used in my studies and their hosts are likely to vary over the geographic ranges of the mistletoe and alternate among different hosts. This may create multiple locally host–specific mistletoe populations and produce a complex geographic mosaic of mistletoe–host combinations across space and time. I suggest that mistletoe populations in South Africa may comprise numerous lineages incapable of parasitising the full range of host species, which could potentially lead to the formation of distinct host races over time. In the future, it would be interesting to document the infection patterns of these generalist mistletoe species across their entire geographic ranges in southern Africa, with particular focus on the patterns of mistletoe infection in places where the host abundance changes among sites. Host preferences may vary with changes in host frequency and host community composition. This could be paired with reciprocal transplant germination experiments in several sites to ascertain whether the mistletoe species have higher fitness on the most locally abundant hosts. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Please read the abstract in the section 07chapter7. / Dissertation (MSc (Genetics))--University of Pretoria, 2002. / Genetics / unrestricted

Solanum mauritianum (bugweed, woolly nightshade) is a perennial tree native to South America that has invaded many countries including South Africa and New Zealand. In South Africa, after 143 years of naturalization, the plant is ranked as the country‟s sixth worst weed and has invaded 1.76 million ha. Invaded areas include agricultural lands, forest plantations, water courses and conservation areas, especially in the eastern higher rainfall regions. The success of the spread of this weed is due to its production of very high numbers of bird-dispersed seeds. Since conventional control methods are unsustainable in the long term, the weed has been targeted for classical biological control since 1984. Following exploration work in its native range, biological control experts recommended that agents that are able to limit the weed‟s reproductive potential would help to manage the spread and invasiveness of this weed. Anthonomous santacruzi, a flower-feeding weevil found throughout the native range of the weed, was imported and tested between 1998 and 2002. Following approval for its release in South Africa in 2007, a new colony was imported and propagated at the University of KwaZulu-Natal Pietermaritzburg. This study was initiated to investigate aspects that could influence the release and establishment of this agent. Three aspects were investigated namely: (1) reassessing the weevil's host range to confirm that the new colony is not different from the colony tested originally and to assess the risks associated with the release of the weevil in New Zealand; (2) surveying the arthropods associated with S. mauritianum in the field to identify groups of predators that could interfere with the establishment of the weevils as well as to investigate, through laboratory-based trials using spiders as surrogate, the impact of these predators on the survival and proliferation of the weevils; and (3) propagation and release of the weevil and monitoring of its establishment. Host-specificity tests revealed that the host range of new colony is not different from that of the originally tested culture. In no-choice trials, the weevils fed and reproduced on some non-target Solanaceae species but reverted back to S. mauritianum in the choice tests. Although the risks for releasing the weevils in New Zealand were calculated to be very low, additional evidence is needed to demonstrate this conclusively. Future research to provide this evidence includes open-field trials complemented with a chemical ecology study, to resolve the case of two species, a New Zealand native and South African native, which have shown higher risks in comparison to the other tested species. For arthropods associated with S. mauritianum in the field, Araneae (especially Thomisidae), Thysanoptera, Hemiptera (especially Miridae) and Hymenoptera (especially Formicidae) were identified as generalist predators that could interfere with the establishment of A. santacruzi. However, their numbers in the field appear to be too low to provide a major threat. Also, laboratory trials using spiders as a surrogate suggested that A. santacruzi populations can survive and reproduce in the presence of such predators. The weevils were released at four sites in KwaZulu-Natal and monitoring of three of these has confirmed establishment at the warmest site along the South Coast but not at the coldest site in the Midlands. Further releases in the province are intended to complement these promising results, while additional studies are intended to facilitate the weevil's release in New Zealand. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.

The Russian wheat aphid (RWA) (Diuraphis noxia Kurdjumov) is one of the important insect pests of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and other grasses. To date, there are four RWA biotypes identified in South Africa. The virulent biotypes emerged, partly due to climate change and new genetic variations within populations of RWA; hence there is a need to improve host-plant resistance, as an effective control measure. Bread wheat (Triticum aestivum L.) accession Cereal Introduction (CItr) 2401 is known to be resistant to all RWA biotypes worldwide. The goal of this study was to use a backcrossed near-isogenic line (NIL) BC5F5 mapping population, developed from a cross between CItr 2401 and susceptible Kavkaz, to identify and validate single nucleotide polymorphism (SNP) markers linked to the resistance phenotype in CItr 2401. This was achieved by (i) conducting a preliminary study that evaluated the suitability of simple sequence repeat (SSR) markers previously reported in literature for discriminating stacked RWA resistance genes and, (ii) employing SNP markers for the first time in a RWA resistance study as a future alternative to the widely used SSR markers. None of the tested SSR markers showed potential use in marker-assisted selection (MAS). The mapping population was phenotypically evaluated for RWA resistance using the four South African biotypes, viz. RWASA1, RWASA2, RWASA3 and RWASA4. Analysis of variance (ANOVA) showed significant (P<0.001) differences of genotypes after confirming the normality of residuals and homogeneity of variance. The Illumina iSelect 9,000 wheat SNP platform was used to genotype the two crossing parents and a selection of 24 NIL genotypes from the mapping population. Eight SNP markers found to be linked to the phenotype were converted to breeder-friendly and high-throughput Kompetitive allele-specific polymerase chain reaction (KASP) markers. The designed KASP markers were validated on the two crossing parents, the 24 NIL sent for SNP genotyping, on the mapping population and on the preliminary study genotypes for their effectiveness. The KASP assays developed in this study will be useful for stacking the RWA resistance from CItr 2401 with other Dn genes effective against the RWA. / Life and Consumer Sciences / M. Sc. (Life Sciences)

Predatory Ground beetles (Coleoptera: Carabidae) and Lady beetles (Coleoptera: Coccinellidae) are two of the most diverse groups found in wheat agroecosystems, globally. These groups are important from both an economic and ecological perspective due to their natural services provision. The effect of wheat agroecosystem management on species diversity, abundance, biomass and composition in South Africa is not yet documented, and there is no existing data indicating which predatory carabid and coccinellid species provides essential ecosystem services and bioindicator roles. Therefore, we examined the effects of organic, conventional and intercropped agroecosystems on ground beetle and lady beetle abundance, dried weight (biomass), composition and diversity. Sampling of wheat agroecosystems was conducted in three systems i.e. organic, conventional and organic intercropped. Post-hoc Tukey test indicated a statistically significant difference between species diversity, biomass and abundance in organic and intercropped systems compared to the conventional systems. Regression analysis indicated significant positive correlation between aphid’s density and predatory carabid and coccinellid beetles in the intercropped systems. Amongst the weather factors temperature influenced aphid density and carabid and coccinellid beetles’ abundance. PCA (Principal Component Analysis) revealed significant positive correlation between individual biomass and cropping system. Conventional system showed a negative correlations with carabid and coccinellid individual biomass. We found that some carabid and coccinellid species can be used to measure the quality of agroecosystems. This study provides a fundamental basis for identification and monitoring of carabid and coccinellid species and their role as bioindicators of ecological disturbance. The identified bioindicator species in this study can assist in developing conservation and biomonitoring strategies within agroecosystems. / Agriculture and  Animal Health / M. Sc. (Agriculture)

Capsicum baccatum var. pendulum was first grown in the Makana District in 2005. Extremely little was known about best practices for cultivation or the insects and diseases associated with the crop in this area. The study was conducted during the second year of production, November 2005 and November 2006, in an attempt to identify the composition and phenology of insects occurring on C. baccatum. In the more rural parts of the Eastern Cape, and more particularly in Grahamstown, there are very few industries. With the advent of this new agricultural venture, a processing factory has been opened in Grahamstown creating more than 600 seasonal jobs in the factory and 1000 seasonal jobs on farms for local people. This business enterprise has not only brought about the creation of jobs, but also training and skills development and empowerment, generating much-needed income in this area. An extensive literature review yielded limited information on insect pests associated with Capsicum. Data from a pilot sampling trial undertaken were statistically analyzed to establish the number of plants to be scouted per site and the most effective scouting techniques to use. Based on the data available and insects collected during the pilot sampling trial, a surveillance programme was designed. Five different types of monitoring traps were placed in each of the eight study sites. Collection of trap catches and scouting of fifteen individual plants per site was undertaken on a weekly basis over the 52-week study period. The most commonly occurring potential insect pests were African Bollworm Helicoverpa armigera (Hübner), False Codling Moth Thaumatotibia leucotreta (= Cryptophlebia leucotreta) (Meyrick), Mediterranean Fruit Fly Ceratitis capitata (Wiedemann) and several species of thrips. Population densities of these pests and their phenology on Capsicum were determined. Statistical analyses established the efficacy of the monitoring traps for each pest, tested for differences among and between study sites, calculated an estimate of the number of pods damaged and a measure of plant damage.The results show that the majority of damage caused to the Capsicum baccatum cropping system was due to Mediterranean Fruit Fly populations. It was established that, although African Bollworm and False Codling Moth were present during the study period, their numbers were negligible and only nominal damage was caused by these pests. Damage caused by thrips species was apparent but not quantifiable. Intervention strategies using an Integrated Pest Management approach, are discussed.