Browsing by Author "Kleynhans, Elizabeth"

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    Environmental physiology of Eldana saccharina (Lepidoptera:Pyralidae) in South Africa : implications for pest management
    (Stellenbosch : Stellenbosch University, 2014-12) Kleynhans, Elizabeth; Conlong, D. E.; Terblanche, J. S.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Eldana saccharina Walker (Lepidoptera: Pyralidae) is a stem borer and food crop pest of economic importance. Temperature and moisture availability possibly influence E. saccharina distribution and abundance, however, the thermal biology and desiccation physiology of E. saccharina are not fully understood. Furthermore, physiological adaptation probably facilitates the invasion success of E. saccharina into novel environments and this too remains unstudied. Here, the thermal- and desiccation-trait variation of E. saccharina were studied and population responses were modelled. The results of this work provided insights into novel physiological outcomes of E. saccharina that is coupled with its environmental climatic stress resistance, overwintering ability and population fitness in general. In determining thermal limits to activity and survival of E. saccharina results showed that chill coma onset temperature (CTmin) and critical maximum temperature (CTmax) of E. saccharina moths collected from sugarcane (Saccharum spp. hybrids) were significantly lower than those from Cyperus papyrus L. (CTmin = 2.8 ± 0.4 vs. 3.9 ± 0.4 °C; CTmax = 44.6 ± 0.1 vs. 44.9 ± 0.2 °C, P < 0.0001 in both cases). These results holds important implications for habitat management (or ‘push-pull’) strategies in the sense that host plant may strongly mediate lower critical thermal limits. Results for pronounced variation in adult CTmin (± 4 °C) across the geographic range of E. saccharina in South Africa was found and it was significantly positively correlated with the climatic mean minimum temperature. Slower developmental time in the most low-temperature tolerant population suggests lower CTmin adaptation has come at a cost to fitness, but allows greater survival and activity in that environment. There are a significant reduction of phenotypic plasticity in the laboratory population and a strong genetic component to CTmin trait variation. Physiological acclimation within a single generation, during immature life stages, resulted in altered adult water balance physiology to enhance fitness. Results from a biophysical population model showed that over-wintering life stage and climate significantly affected the number of E. saccharina generations, predicted stress, relative moth fitness and relative adult abundance. Larval overwintering led to less generations and more frequent cold- and heat stress at a cold field site compared to a warm one. This in turn reflected on the relative adult fitness and –abundance. Larval presence predictions overlapped well with positive scout records averaged across a matrix of sugarcane ages and cultivars. The results from this work are important on which to base integrated pest management strategies and are applicable to a large audience across agricultural landscapes and in the sugarcane industry of South Africa.
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    Impacts of climate change on tsetse (Diptera: Glossinidae) : water balance physiology and mechanistic modelling
    (Stellenbosch : Stellenbosch University, 2011-12) Kleynhans, Elizabeth; Terblanche, J. S.; Porter, Warren P.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Climate change will alter both temperature and moisture availability in the future and therefore will likely affect vector borne disease prevalence. Organisms faced with changes in weather can respond in a variety of ways and this complicates any predictions and inferences for these organisms with climate change. Cause-and-effect links between climate change, insect vector responses, and changes in risk of disease transmission are poorly established for most vector borne diseases. Tsetse (Diptera, Glossinidae) are important vectors of trypanosome parasites posing a major threat to human health and socio-economic welfare in Africa. Water balance plays an important role in determining activity patterns, energy budgets, survival and population dynamics and, hence, geographic distribution and abundance of insects. Glossina species occupy a wide range of habitats in Africa and are notable for their desiccation resistance in xeric environments. Yet, whether or not the different species, subgroups or ecotype groups differ in susceptibility to changes in weather remain undetermined. The first main focus of my thesis was to test the effects of climate change on water balance traits (water loss rate, body water content and body lipid content) of adult tsetse flies. Four species from xeric and mesic habitats were exposed to a range of temperature (20 – 30 °C) and relative humidity (0 – 99 %) combinations. Water loss rates were significantly affected by measurement treatments, while body water content, body lipid content and mass were less affected and less variable across treatment combinations. The results provide support for mass-independent inter- and intra-specific variation in water loss rate and survival times. Therefore, water balance responses to variation in temperature and relative humidity are complex in Glossina, and this response varies within and among species, sub-groups and ecotypes in terms of magnitude and the direction of effect change. Secondly, I apply a mechanistic distribution model for G. pallidipes to predict potential population responses to climate change. I validate the mechanistic model (NicheMapperTM) results spatially and temporally using two methods. Both tests of the model showed that NicheMapper‟s predicted resting metabolic rate has great potential to capture various aspects of population dynamics and biogeography in G. pallidipes. Furthermore, I simulate the effect of phenotypic plasticity under different climate change scenarios and solve for the basic reproductive number of the trypanosomiasis disease (R0) under a future climate scenario. This integrated thesis provides strong evidence for a general decrease in optimal habitat for G. pallidipes under future climate change scenarios. However, it also provides strong support for a 1.85 fold increase in R0 based on changes in biting frequency as a result of higher predicted metabolic rates in the future. This might suggest that the reduction in optimal habitat could be outweighed by the increase in R0. The results demonstrate that an understanding of the physiological mechanism(s) influencing vectors of disease with climate change can provide insight into forecasting variation in vector abundance and disease risk.