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In the Swaziland lowlands, biological control of the citrus pest red scale, Aonidiella aurantii (Mask), has been consistently unsuccessful although it is successful in middle altitude regions. However, recent cases of resistance to organophosphate insecticides elsewhere in southern Africa, make it important to reconsider biological control of red scale in the lowlands. Should similar resistance be evolved in this region, the pest could be controlled only by natural means together with oils. This account attempts to answer questions about the natural control of a red scale population in the lowlands, by examining data collected between 1972 and 1975. The population was continuously breeding with overlapped generations. It consisted of three sections, on leaves, twigs and fruit, having different rates of reproduction, development and mortality. These sections are considered separately but that on leaves, being relatively unimportant, is largely ignored. The generation mortality is estimated in the sections on twigs and fruit and related to climatic and biotic indices. The causes of seasonal and between-years variation in the sections on twigs and fruit are elucidated. The effectiveness of each kind of natural enemy is examined and the factors affecting natural enemy numbers are investigated. The question of population regulation is discussed. Differences in mean infestation levels in middle altitude and lowland regions are explained in terms of different rates of reproduction and mortality.

Please read the abstract in the section 00front of this document / Dissertation (MSc)--University of Pretoria, 2005. / Microbiology and Plant Pathology / Unrestricted

Simulium chutteri is considered a major pest in South Africa and it has been estimated that it can potentially cause stock losses amounting to more than R88 million per annum. Although a larval control programme has been launched to control the pest, major outbreaks still occur, since major fitness traits such as longevity are ignored in the planning of control actions. To improve the control programme, the aim of this study was to study the longevity of S. chutteri females under various conditions, and also to relate longevity to factors such as size, mass and metabolic reserves as these traits show variation that can be linked to changes in developmental temperature. It was conclusively demonstrated in this study that the body size and mass of S. chutteri increases with a decrease in temperature and therefore both seasonal and geographical variations occur. A consequence of larger body size is that these individuals carry proportionally more lipid reserves than smaller ones, although these relationships were not found for glycogen. It was shown that major seasonal variation occurred in the survival of S. chutteri, although these could not be attributed solely to variations in size, mass and metabolic reserves. For example, despite the large body size and mass and metabolic reserves of the winter population, it showed comparatively lower survival than all the other populations. It is argued that this is due to the interaction between fecundity and other fitness traits such as desiccation and starvation resistance. Adaptive explanations are, however, also proposed. These results are used to explain the seasonal variation found in the annoyance levels of S. chutteri along the lower Orange River. Moreover, recommendations are given for the improvement of the current blackfly control programme. These include the need to control the summer population (when certain criteria are met) and showing the importance of effectively controlling the winter population. / Dissertation (MSc (Entomology))--University of Pretoria, 2006. / Zoology and Entomology / unrestricted

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Studies on the seasonal changes in the moisture content of western hemlock Tsuga heterophylla (Raf.) Sargent, demonstrated two moisture minima, one in the spring and one in the fall, and two maxima, one in the spring and the second in the winter. Significant differences were noted for both bark and wood moistures between good and poor sites, between seasons of the year and for the interaction between site and seasons of the year. The relative turgidity of bark from the good site was below the 80 per cent level for 95 days through an entire year. On the poor site it remained under this critical value for 123 days of the 8 months observation period. Dwarf mistletoe did not appear to affect the water economy of the portion of branches that had not been invaded by the endophytic system. However the parasite produced moisture stress in the bark at the central area of infections. An investigation of the fungi that were associated with cankered areas of dwarf mistletoe infections revealed the occurrence of twelve different species of fungi. Nine of these were Ascomycetes and three were Fungi Imperfecti. There were two undescribed species, and one species, Mytilidion decipiens Karst. had not been reported previously for North America. Preliminary tests of the parasitism of seven species indicated that one, a member of the Fungi Imperfecti, was mildly parasitic. / Science, Faculty of / Botany, Department of / Zoology, Department of / Graduate

The behaviour of the orchard mites Bryobia pretiosa (Koch) and Metatetranychus ulmi (Koch) and four species of predators was investigated both on unsprayed and on sprayed apple trees. On unsprayed trees the host mites were found to be most abundant on the east sides; no consistent directional preference was found for the predacious species. Small apple leaves supported the same number of M. ulmi per unit area as large leaves, but more mites of the genus Tetranychus. On unsprayed trees a negative correlation between the total number of predators and the number of host mites was recorded. In abandoned orchards the predators were capable of controlling the host mites and maintaining them at a level below that of economic significance. Each of the insecticides DDT, Parathion, and lead arsenate, was found to be very toxic to all species of predators recorded, and thus hindered or entirely prevented natural control of the host mites. In every instance lead arsenate and DDT produced an increase in the population of B. pretiosa, this effect being partially chemical and partially due to a lack of predators on the sprayed trees. Lead arsenate caused an increase in the population of M. ulmi whereas DDT reduced it to a slight extent. Under arid conditions Parathion caused an increase in the population of B.pretiosa; under less arid conditions the mite was eliminated. Parathion had no effect on M. ulmi in a commercial orchard while, in the same orchard, it caused a great increase in Tetranychus spp. The latter species were reduced by DDT in the commercial orchard. / Science, Faculty of / Zoology, Department of / Graduate

Rhabdogloeum pseudotsugae Sydow has been suspected of being the imperfect stage of Rhabdocline pseudotsugae Sydow since their original description by Sydow in 1922. Because Rhabdogloeum has been reported so seldom in North America, and never in Europe, this possibility has been considered slight by most investigators. Investigation of the problem of Rhabdocline pseudotsugae in British Columbia revealed several new aspects of the association between the two leaf-cast diseases. Inspection of Rhabdocline and Rhabdogloeum collections from the interior of British Columbia indicated that the association of Rhabdogloeum pseudotsugae and Rhabdocline pseudotsugae is more prevalent than was formerly believed. Collections from the coast, and on Vancouver Island in particular, indicated that the association is probably less prevalent there than in the interior. In relation to the frequent and widespread occurrence of Rhabdocline throughout the Douglas fir range however, Rhabdogloeum is seldom found. If Rhabdogloeum pseudotsugae is the imperfect stage of Rhabdocline pseudotsugae, then it appears that frequently the Rhabdogloeum stage and occasionally the Rhabdocline stage is completely suppressed, as both stages have been found dissociated from each other. Frequent observation of a group of tagged Rhabdocline-infected trees on Vancouver Island over a period of two years did not reveal the presence of a Rhabdogloeum stage. Tissue cultures of lesions from these tagged trees, however, resulted in the growth of a fungus quite similar in appearance to that produced in tissue cultures by lesions typical of those produced by Rhabdogloeum pseudotsugae. This supports the view that Rhabdogloeum pseudotsugae is the imperfect stage of Rhabdocline pseudotsugae. Observation of these trees indicated that Rhabdocline pseudotsugae is capable of vegetating two years before producing apothecia. Apparently it is also capable of vegetating more than two years or of infecting leaves other than those of the current year. / Science, Faculty of / Botany, Department of / Graduate

Poria weirii Murr. isolates cultured from various hosts may be divided into two groups on the basis of their ability to parasitize Thuja plicata Donn and to grow on media containing water-soluble T. plicata heartwood extractives. The role of these extractives in determining whether a Poria weirii isolate can parasitize Thuja plicata was examined. Measurement of linear growth along wood micro-sections confirmed that substances inhibitory to one isolate group are hot-water soluble T. plicata heartwood extractives. The tolerance of isolates from T. plicata (Thuja) and Pseudotsuga menziesii (Mirb.) Franco (non-Thuja) to a hot-water extract of Thuja plicata heartwood and its hexane soluble (principally thujaplicins) and insoluble (thujaplicin-free) fractions was studied in detail. Dry weight of mycelium produced in liquid glucose-asparagine medium was measured. Thuja isolates were more tolerant than non-Thuja of the hot-water extract and its fractions; thujaplicins caused the greatest response differences. Thuja isolates showed greater stimulation than non-Thuja at subinhibitory thujaplicin concentrations. Higher thujaplicin concentrations were required to produce fungistatic and -toxic conditions in Thuja isolates than in non-Thuja. There were highly significant differences between isolate groups in response to 7.5 ppm thujaplicin. Tolerance was not related to date of isolation or geographical location. With sufficient inoculum potential, a non-Thuja isolate may overcome T. plicata heartwood toxicity. The resulting infection is usually localized. Unlike Thuja isolates, non-Thuja were unable to increase thujaplicin tolerance during three serial generations on liquid medium containing thujaplicins. Long exposure of non-Thuja isolates to T. plicata heartwood under field conditions did not increase their thujaplicin tolerance. Thuja isolates may be able to adapt the enzyme systems blocked by thujaplicin or to detoxify thujaplicins. These results have significance for control of Poria weirii. Planting Thuja plicata in and around infection centers caused by non-Thuja, isolates could limit their size by reducing the number of root contacts between susceptible species. / Science, Faculty of / Botany, Department of / Graduate

Nutritional mutants of Ustilago hordel were used to demonstrate that parasexual recombination occurs within the host plant (Hordeum vulgare) prior to the production of teliospores. The nutritional mutants were also used to show that resistance of the newly-germinated seedling of H. vulgare to U. hordei and of subsequently formed tillers to infection was not correlated, and was probably not controlled by the same gene or genes. The application of gibberellic acid to H. vulgare was found to cause an increase in the overall tiller height of healthy plants by increasing the elongation of Internodal regions 0-1, 1-2, 2-3, and 3-4. There was no increased elongation of internodal regions 4-5 and 5-6. The healthy tillers of diseased plants showed no Internodal elongation when gibberellic acid was applied. Diseased tillers, which are usually shorter than healthy tillers, were also unaffected by the presence of gibberellic acid. The injection of both mating types of U. hordei into the young developing spike of a normally resistant strain of H. vulgare resulted in the production of diseased spikes. This would indicate that blockage to normal infection occurs at the time of seedling penetration, at the level of tiller primordia development, or at the time of spike primordia development. / Science, Faculty of / Botany, Department of / Graduate