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The effects of intraspecific and interspecific competition, host plant, and prior host plant experience on fecundity, rate of oviposition and mortality of adults and larval survival of Delia radicum Bouche (Cabbage Maggot (CM)), D. antiqua Meigen (Onion Maggot (OM)), and D. platura Meigen (Seed Corn Maggot (SCM)), were studied on cabbage, onion and bean plants. / An optimum density for maximum fecundity per female was observed when the four experimental densities were compared. This optimum density was higher on host than on non-host plants. CM females were host specific and did not oviposit on non-host plants. Rates of oviposition and mortality over a 30-day period were calculated for each density. The rate of oviposition was slower at higher densities on host plants for CM, OM and SCM. The rate of mortality increased at the highest density for CM (cabbage), OM (bean) and SCM (cabbage), but remained unaffected for OM on onion and cabbage and for SCM on onion and bean, when densities were compared. For OM, a delay in the rate of oviposition and mortality on cabbage (non-host plant) when compared with onion (host plant), suggests that cabbage was not as readily accepted as an oviposition site. Interspecific competition experiments at six density ratio's of SCM:OM indicated increased fecundity, or an increase in the rate of oviposition for OM, at the lower densities when single and mixed species were compared. For SCM no effects on fecundity were recorded, but the rate of oviposition was slower and rate of mortality faster at the lowest density in the presence of OM. Similar studies with SCM and CM showed no such effects of competition. / Host plant exposure of SCM females during the pre-oviposition period resulted in a delay in initial acceptance of subsequent host plants as oviposition sites. This happened only when females were exposed to a secondary host during the pre-oviposition period. Once oviposition began, host discrimination ceased and a switch in oviposition sites to the preferred host did not alter the rate of oviposition. In CM, the rate of larval development increased at density 6 (optimum density). Above this density a decrease in the rate of development and a significant reduction in pupal weight was observed. Time required for fly emergence was not affected by increasing larval densities.

Please read the abstract in the section 00front of this document / Dissertation (MSc (Genetics))--University of Pretoria, 2002. / Genetics / unrestricted

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To make use of the information on infestation estimates of forage insect pests obtained by the conventional methods of sampling, it is important to estimate the actual losses taking place in association with definite numbers of the insect. Six month old red clover, alfalfa, and birdsfoot trefoil plants were exposed for a month to spittlebug infestations of 0, 50, 100, and 150 adults. Although the spittlebug mortality was lowest when reared on alfalfa and highest on birdsfoot trefoil, yellowing appeared on alfalfa and birdsfoot trefoil. On red clover there was a consistent increase in protein and nitrogen free extract and a reduction in crude fiber and ash content indicating slight reduction in growth. None of these differences were significant. On alfalfa the loss of xylem sap due to feeding caused a consistent general reduction of proteins, fat and ash content and an increase in dry matter and nitrogen free extract. Of these constituents only the increase in dry matter was significant with an infestation of 150 spittlebugs per cage. No differences in dry matter, proteins and fat were significant. Another test on 4 month old alfalfa with infestation levels of 0, 100, 200 and 300 showed similar consistent patterns but the difference in dry matter was significant only when the infestation was 200 spittlebugs or more. On birdsfoot trefoil there was a consistent increase of dry matter, proteins, and ash and reduction of fat and crude fiber. Only the difference of dry matter and fats was consistent showing that an infestation of 100 or more spittlebugs per sq ft will affect the general growth of the plants. The survival of 1st instar nymphs of the pea aphid (a phloem feeder) was very poor on birdsfoot trefoil. There was a fair amount of survival on red clover and those on alfalfa survived fully. There was no visible symptoms on birdsfoot trefoil and red clover but the alfalfa showed retarded growth. On red clover there was a consistent increase of dry matter, fat, crude fiber and nitrogen free extract but a reduction of proteins. The difference in crude fiber was significant when the infestation was 1000 pea aphid nymphs or more. A consistent reduction of proteins and crude fiber was also recorded on alfalfa but these differences were not significant. On birdsfoot trefoil, there was a decrease in dry matter and an increase in fat and ash content. The difference in ash content was significant when the infestation of pea aphid nymphs was 1000 or more. Potato leafhopper (phloem feeder) injury was very characteristic on birdsfoot trefoil. The terminal 1 to 2 inches of the sterns became wilted and bent down. On alfalfa there was gradual yellowing and the retarded growth was evident. The red clover did not show any characteristic symptoms. Infestations of 20 or more caused a consistent increase in protein, fat, crude fiber and a reduction of nitrogen free extract. On alfalfa there was consistent decrease in dry matter. On birdsfoot trefoil, there was consistent increase in dry matter, fat, and nitrogen free extract but a reduction of crude fiber showing that feeding on birdsfoot trefoil probably retards growth and condensation of carbohydrates to form crude fibers. The increase in fat was significant when the infestation was 40 potato leafhoppers or more. Alfalfa weevil adult feeding caused characteristic stripping of leaves, girdling of stem and drying of the entire plant. Feeding of 10 or more weevils for 18 days on a two month growth of alfalfa after the 1st cutting, cause a consistent reduction of proteins, fat and crude fiber and an increase of dry matter, ash, and nitrogen free extract. Increase of percent dry matter was significant when the infestation of alfalfa weevil adults was 20 or more. In another test, the dry matter, fat and ash gave similar consistent results, when 40, 60, and 80 adults were fed on 4 month old alfalfa for 18 days. Reduction of percent fat was significant when the infestation was 40 or more alfalfa weevil adults. Alfalfa weevil larval feeding showed stripping of the leaves and buds. Infestation of 30 or less, 2nd to 4th instar larvae for 12 days caused a consistent reduction of fat and ash and an increase of dry matter, proteins and crude fiber. The infestation was not high enough to show a significant change in plant constituents. / Ph. D.

Thesis (MscAgric (Horticulture))--University of Stellenbosch, 2005. / The occurrence of both pre- and postharvest leaf blackening in certain Protea species and cultivars is a problem that severely limits their marketability, vase life and transport options. This research focuses on : (I) The distribution of carbohydrates in inflorescence bearing stems of certain Protea cultivars from harvest, following pulsing with a 10 g.L-1 glucose solution until four weeks postharvest. Stems were held under a variety of postharvest conditions, and (II) The suppression of Protea postharvest leaf blackening with specific focus on the cultivar ‘Sylvia’ (P. eximia x P. susannae).

Studies presented in this dissertation highlight the importance of fungal pathogens in forestry nurseries in South Africa. Both Acacia meamsii seedlings and Eucalyptus hybrid cuttings are shown to be affected by important nursery pathogens. Chapter one presents an evaluation of the potential importance of pathogens to Eucalyptus hedge plants maintained in hydroponics. Hydroponics is a new technology being used in South African forest nurseries, which allows for the rapid establishment of Eucalyptus hedge plants. However, no information is available on pathogens affecting Eucalyptus in hydroponics. By applying information on pathogens of other hydroponic crops, several potentially important pathogens were identified and these reside in the genera Phytophthora, Pythium and Fusarium. Possible disease symptoms in Eucalyptus caused by these pathogens include wilting, stem cankers and root rots. Implementation of appropriate control measures that include cultural, biological and chemical practices could prevent and/or reduce disease impact in hydroponics. Chapter two presents the results of a survey of the roots of Eucalyptus hedge plants grown in an ebb and flow hydroponic system. An interesting result of the survey was the discovery of Cylindrocladium pauciramosum in the hydroponic system. This is the first report of the pathogen in a hydroponic system. Other important pathogens in the genera Phytophthora and Pythium were also isolated. Two Pythium species, namely P. dissotocum and P. helicoids, found in the roots and nutrient solution are new to Eucalyptus. Several Fusarium species were also isolated of which two, namely F. nygamai and F. lateritium, are also new to Eucalyptus. Chapter three of this dissertation presents the results of a survey of Eucalyptus cuttings conducted at four forestry nurseries in KwaZulu-Natal, South Africa. Several well-known Eucalyptus nursery pathogens were isolated. Cylindrocladium pauciramosum was identified as the dominant pathogen on Eucalyptus cuttings. This was confirmed based on morphological characteristics and DNA sequence comparisons. Pathogenicity tests conducted using a spore suspension of C. pauciramosum indicated that this pathogen is capable of infecting most commercial Eucalyptus clones used in South Africa. Chapter four considers a serious disease of Acacia mearnsii seedlings caused by an unidentified species of Cylindrocladium. Cylindrocladium pauciramosum was isolated from A. mearnsii seedlings showing girdling and stem canker symptoms. The pathogen was identified based on morphological characteristics and DNA sequence comparisons. Pathogenicity tests with Acacia seedlings confirmed the susceptibility of this tree to C. pauciramosum infection. This dissertation clearly indicates that Cylindrocladium pauciramosum is an important nursery pathogen in South African forestry nurseries. This pathogen has already been shown to be limiting during production of planting stock. I hope to have highlighted the importance of C. pauciramosum and other nursery pathogens in forestry nurseries in South Africa. This study will also hopefully provide information to forestry nursery managers and help them improve production. / Dissertation (MSc)--University of Pretoria, 2004. / Microbiology and Plant Pathology / Unrestricted