Browsing by Author "Thomas, Saskia"

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    Development of integrated pest management approaches for yellow sugarcane aphid Sipha flava in Zambia
    (Stellenbosch : Stellenbosch University, 2019-12) Thomas, Saskia; Terblanche, J. S.; Conlong, D. E.; Barton, Madeleine; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: The yellow sugarcane aphid Sipha flava (Forbes) (Homoptera: Aphididae) has recently emerged as a pest of concern for sugarcane (Saccharum spp.) farmers in the Mazabuka region of Zambia, and been argued to have contributed to recent declines in productivity and crop yields. Limited options have been available in effective monitoring methodologies of this pest and other more sustainable control options. One of the key factors in the implementation of effective integrated pest management programs is having appropriate methodologies to accurately monitor and detect pest infestation levels, in addition, promoting natural enemies can aid pest management through non-chemical means. Consequently, there is a pressing need to develop monitoring methods and survey strategies to better understand the spatial- temporal dynamics of S. flava population outbreaks. In Chapter 2, smaller sections (± 6ha) of larger sugarcane fields (14- 50ha) were systematically and consistently surveyed for a 7-wk period, weekly during the summer (October-November) and fortnightly for a 12-wk period during the winter season (May- August) for the presence of S. flava. Sampling effort was proven to be reduced from 31 (effort employed in this study) to 17 or 14 stalks/ha, depending on the ‘presence’ or ‘infestation percentage’ method, to produce equally reliable estimates of the infestation levels in the sugarcane field. Most aphids were found on the bottom 4 green leaves for all sugarcane varieties in the different seasons. No systematic spatial pattern of invasion was observed for this aphid’s infestation in the fields during the winter season, but instead were found to have random field distributions with little or weak spatial autocorrelation. However, further studies are required to identify the economic threshold for the proposed monitoring methodologies before they can be adopted more broadly. Little is known about the existence of biological control agents that could be naturally existing within the agroecosystem and assisting in the control of S. flava in Zambia. In Chapter 3, three sites each consisting of 2 subsites in sugarcane fields and 1 subsite in the bordering vegetation areas, were sampled fortnightly over a 10-wk period (May- August) to collect potential predators of S. flava. This was to determine the likelihood and extent of aphidophagous arthropods to be naturally feeding on S. flava, based on ∆13C and ∆15N enrichment, as specialist or generalist predators. The diversity of the aphidophagous predators found within each site and collectively within this agroecosystem were dominated by arachnids, coccinellids and syrphids. Syritta sp. (Syrphidae: Diptera) and Scymnus sp. (Coccinellidae: Coleoptera) were found to be the most common predators, with many other predators identified found to be important specialist and generalist predators too, but their predator behaviour on S. flava appeared to differ between sampling sites. This suite of arthropods can now be used to select which predators to encourage for increased levels of biological control to allow for more sustainable management of this pest. The results of both chapters are discussed in the context of further development of an integrated pest management programme for S. flava on sugarcane in Zambia.
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    The effect of oxygen limitation on a xylophagous insect's heat tolerance is influenced by life-stage through variation in aerobic scope and respiratory anatomy
    (Frontiers, 2019-11-20) Javal, Marion; Thomas, Saskia; Lehmann, Philipp; Barton, Madeleine G.; Conlong, Desmond E.; Du Plessis, Anton; Terblanche, John S.
    ENGLISH ABSTRAT: Temperature has a profound impact on insect fitness and performance via metabolic, enzymatic or chemical reaction rate effects. However, oxygen availability can interact with these thermal responses in complex and often poorly understood ways, especially in hypoxia-adapted species. Here we test the hypothesis that thermal limits are reduced under low oxygen availability - such as might happen when key life-stages reside within plants - but also extend this test to attempt to explain that the magnitude of the effect of hypoxia depends on variation in key respiration-related parameters such as aerobic scope and respiratory morphology. Using two life-stages of a xylophagous cerambycid beetle, Cacosceles (Zelogenes) newmannii we assessed oxygen-limitation effects on metabolic performance and thermal limits. We complement these physiological assessments with high-resolution 3D (micro-computed tomography scan) morphometry in both life-stages. Results showed that although larvae and adults have similar critical thermal maxima (CTmax) under normoxia, hypoxia reduces metabolic rate in adults to a greater extent than it does in larvae, thus reducing aerobic scope in the former far more markedly. In separate experiments, we also show that adults defend a tracheal oxygen (critical) setpoint more consistently than do larvae, indicated by switching between discontinuous gas exchange cycles (DGC) and continuous respiratory patterns under experimentally manipulated oxygen levels. These effects can be explained by the fact that the volume of respiratory anatomy is positively correlated with body mass in adults but is apparently size-invariant in larvae. Thus, the two life-stages of C. newmannii display key differences in respiratory structure and function that can explain the magnitude of the effect of hypoxia on upper thermal limits.