Browsing by Author "Steyn, Vernon Murray"
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- ItemCauses and consequences of dispersal in the Mediterranean fruit fly, Ceratitis capitata(Stellenbosch : Stellenbosch University, 2015-12) Steyn, Vernon Murray; Mitchell, Katherine A.; Terblanche, J. S.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.ENGLISH SUMMARY: The ability of insects to perform under challenging environmental conditions is paramount to their survival, population growth and evolutionary fitness. Understanding why some organisms persist in certain habitats but not others is the first step to comprehending present, past and future species distributions, of particular importance under future global climate change. Key traits that may assist a species to continue to perform under poor conditions and may potentially assist in surviving future variable and warming conditions include enhanced dispersal capabilities, a wide performance breadth and plastic responses. Repeated mark-release-recapture (MRR) experiments were conducted to measure the performance (dispersal) and plastic responses of the Mediterranean fruit fly, Ceratitis capitata (a prolific global invader). Dispersive and philopatric individuals were morphometrically assessed (including wing size and shape, body mass, abdomen mass, thorax mass and various ratios thereof) to identify phenotypic traits associated with enhanced dispersal. Thereafter, focussed laboratory experiments were undertaken to determine which aspects of flight performance are enhanced, or associated with, potential dispersal traits. Performance was then compared under various thermal limits (chill coma recovery, CCRT; heat knockdown time, HKDT; critical thermal minimum and maximum, CTmin and CTmax, respectively) to examine the influence of different thermal acclimation regimes and determine the responses of phenotypic plasticity in C. capitata. Subsequently, the costs and benefits of dispersal and its plasticity were measured under semi-field (greenhouse) and field conditions to determine how close laboratory predictions are to the real world. These experiments allowed the discovery of the phenotypic trait associated with dispersal (larger thorax mass: body mass). However, contrary to a widely-held expectation, it did not result in enhanced whole-animal flight performance, but was rather related to willingness to disperse (i.e. dispersal propensity). Furthermore, the integration of the three operational environments (laboratory, semi-field and field) illustrated that C. capitata’s performance is influenced by thermal conditions and highlighted the best acclimation treatment (20°C acclimation, especially in warmer conditions) for enhanced performance. A challenge for invasion biology is the development of a predictive understanding of species invasion ability. Clarity on the species dispersal potential and the factors that influence it is an integral part of the problem. From this study, it is shown that dispersal is condition dependent (e.g. phenotypic traits and behaviour) as well as context dependent (e.g. thermal history and environmental temperature). This may benefit predictions of the future invasion risk of C. capitata and potentially improve current management strategies.
- ItemIntegrated management of false codling moth, Thaumatotibia leucotreta, on stone fruit and table grapes(Stellenbosch : Stellenbosch University, 2019-03) Steyn, Vernon Murray; Addison, Pia; Malan, Antoinette P.; Stenekamp, Daleen; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.ENGLISH ABSTRACT: Integrated pest management (IPM) strives to control insect pests with a multifaceted approach that mirrors that of the ecology and endeavours to restore balance to the agricultural environment. To ensure the success of an IPM programme it is vital to understand the biology and the life cycle of the target insect. False codling moth (FCM), Thaumatotibia leucotreta is a tortricid moth with an extensive host range of more than 70 plants and is a key pest on citrus, stone fruit and table grapes. Insecticides are still relied upon for the control of FCM however, due to stricter regulations and their cascading environmental effects, there is a drive to reduce the chemical load in our agricultural environments. This encourages the need for a more sustainable approach such as IPM, which targets every stage in the life cycle of FCM. In this study, entomopathogenic nematodes (EPNs) and entomopathogenic fungi (EPF) were isolated from agricultural soils. The susceptibility of eggs and pupae to EPNs was investigated, and both entomopathogen types were screened against larvae. The EPNs proved to be the more potent pathogen and the larval stage was the least resilient against the pathogen. The EPN species that showed the most promise in laboratory trials were used as an in vitro cultured product and were tested in the field against FCM. The EPNs caused mortality of FCM larvae within 48 h and remained effective four weeks after application. Mating disruption is a nonchemical control technique that interferes with the mate-finding ability of insects in order to reduce mating events and is therefore an ideal candidate for the control of FCM. However, the effect of mating disruption has not been quantified against FCM in stone fruit and table grapes. Using multiple mark-release-recapture experiments, this is the first study to reliably quantify mating disruption in stone fruit and table grapes. Mating disruption proved to be a highly successful technique causing up to 99% disorientation of male FCM, but how the method alters FCM’s mate-finding ability was unclear. To determine FCM’s behavioural response to the addition of the female pheromone, quantitative tools and dosage-response profiles were used. The study proved that at low dosages the success of mating disruption is dependent on FCM density, however at high dosages there is a shift to non-competitive disruption causing mating disruption to remain effective even at high pest densities with few pheromone point sources. The integration of the techniques investigated here will ensure the effective control of FCM at each stage of the life cycle, ensuring low population growth that may lead to the successful suppression of this problematic pest.