Browsing by Author "Viljoen, Nadia"

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    Alternative pest control methods for false codling moth, Thaumatotibia leucotreta (Lepidoptera: Tortricidae): exploring juvenile hormone analogs and entomopathogenic nematodes for integrated pest management
    (Stellenbosch : Stellenbosch University, 2023-03) Viljoen, Nadia; Karsten, Minette; Terblanche, John; Stokwe, Nomakholwa; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: False codling moth (FCM) (Thaumatotibia leucotreta (Meyrick, 1913), Lepidoptera: Tortricidae) is a major phytosanitary pest of diverse crops in southern Africa, especially deciduous fruit and citrus. Here, I provide an overview of the significance of FCM in a South African context, a brief overview of current pre- and post-harvest control methods, and the factors contributing to the increased pressure towards alternative, pesticide-limited control. False codling moth control is currently reliant on integrated pest management (IPM) that employs multiple strategies in combination. To assess the potential inclusion of juvenile hormone analogs (JHA), fenoxycarb and methoprene, into current IPM programmes and/ or Sterile Insect Technique (SIT) programmes I investigated their effect on the development, cold stress resistance and flight performance in false codling moth under different constant temperatures in two experiments. In a third experiment, the concept of adding additional stressors to the system was investigated, by assessing the interaction of methoprene and fenoxycarb with a heat wave exposure on the susceptibility of FCM to the entomopathogenic nematode, Steinernema yirgalemense Nguyen, Tesfamariam, Gozel, Gaugler and Adams (Rhabditida: Steinernematidae). In the first experiment, I exposed fourth to fifth instar larvae to four concentrations of fenoxycarb and methoprene at 25 °C: 0.1 µg/µL, 1 µg/µL, 5 µg/µL and 10 µg/µL and compared these to a handling control. Larval and pupal development time, number of pupae formed, number of adults eclosed, and longevity were recorded daily. In adults that emerged, I also tested chill coma recovery time and spontaneous behaviour as a measure of low temperature stress resistance. At 25 °C (Experiment 1), both fenoxycarb and methoprene-exposed larvae had significantly lower pupation and emergence numbers compared to the control group, and overall mixed or weak effects on development, and no effect on longevity. Low temperature performance was not affected by either methoprene or fenoxycarb at 25 °C. Next (Experiment 2), to explore the impact of temperature on JHA effects, I exposed fourth to fifth instar larvae to either 1 µg/µL methoprene or 0.1 µg/µL fenoxycarb and maintained larvae under either 20 °C, 25 °C or 30 °C while recording the same parameters as described previously (Experiment 1), with the addition of flight performance tests in adults. Cooler temperatures significantly slowed development time from larvae to adults in both treated and control groups, and reduced the number of pupae in the fenoxycarb treated group. Adult emergence was reduced by methoprene at 30 °C, and by fenoxycarb at all temperatures. Temperature afffected chill coma recovery time and sponateous behaviour in fenoxycarb-treated individuals, with larvae kept at 20 °C having a faster response time than fenoxycarb-treated FCM at 25 °C and 30°C, as well as the control group at 30 °C. Temperature also increased response time for spontaneous behaviour at 30 °C compared to 20 °C and 25 °C. Temperature, but not JHA treatments, affected the flight ability of moths, with larvae kept at 30 °C producing significantly more dispersers compared to larvae kept at 25 °C. For the third experiment, fourth to fifth instar FCM larvae were exposed to a heat wave (38 °C) for two hours, allowed to recover at 25 °C for six hours, treated with either 1 µg/µL methoprene or 0.1 µg/µL fenoxycarb, and inoculated individually with 50 IJs/50 µL of in-vitro cultured entomopathogenic nematode (EPN) S. yirgalemense. Mortality was determined after 48 hours and dissections were carried out to verify infection performed after another 24 hours. Heat wave exposure did not significantly affect EPN susceptibility for either of the JHA treated groups. Infection was much lower than expected, based on the virulence of S. yirgalemense on FCM, and additional work is needed to form a better understanding of all the contributing factors. Overall, JHA had largely predictable and pronounced negative effects on life history traits and performance in false codling moth, making it a potential addition to integrated pest management programmes as a larvicide, but would not be beneficial to SIT mass-rearing programmes. Further research into the practicality of field application of JHA is essential, as well as the role of multiple simultaneous stressors (e.g., temperature and JHA) on the efficacy of biological control agents in the field.
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    A logistics barometer for South Africa : towards sustainable freight mobility
    (AOSIS Publishing, 2016) Havenga, Jan H.; De Bod, Anneke; Simpson, Zane P.; Viljoen, Nadia; King, David
    ENGLISH SUMMARY : Background: South Africa has a disproportionately high freight transport demand owing to industrial development far from ports, low domestic beneficiation and improper modal use. Historical freight transport policy supported primary economic development, failing to preempt the changing economic structure and the resulting freight transport needs, resulting in excessive transport costs and externalities. Objectives: To share the macroeconomic freight transport challenges revealed by South Africa’s Logistics Barometer, and to identify key interventions to address these. Method: Freight flows are modelled by disaggregating the national input–output model into 83 commodity groupings and 372 geographical areas, culminating in a 30-year forecast at 5-year intervals for three scenarios, followed by distance-decay gravity modelling to determine freight flows. Logistics costs are calculated by relating these flows to the costs of fulfilling associated logistic functions. Results: Long-distance transport remains the largest general freight typology and is, due to inefficient macro logistics design, extremely costly, both in terms of intrinsic and extrinsic costs. Conclusion: South Africa’s freight task will grow 2.5-fold by 2043. Logistics and externality costs are already untenable at current levels. The development of domestic intermodal solutions will support the drive towards sustainable freight mobility.