No description available.

The potential impact of generalist predators on armyworm mortality was evaluated in the field, through a predator removal study, and in the laboratory, through controlled feeding trials. The most common potential armyworm predators removed from the field included ground beetles (Carabidae), rove beetles (Staphylinidae), ants (Formicidae), and spiders (Araneae). Armyworm damage to corn plants was significantly greater where generalist predator populations were reduced, through the use of pitfall traps and exclusion arenas, than in the control where predator populations were unaltered. The differences in the proportion of damaged plants and the degree of damage between the predator removal treatment and the control were statistically significant. Generalist predator consumption rates of live armyworm larvae in the laboratory were variable, however most predators did feed on the larvae. Large carabid beetles, including Pterostichus chalcites Say, Pterostichus lucublandus Say, and Scarites subterraneus F., exhibited the highest consumption rates. Generalist predators were sampled in four reduced-tillage corn systems which differed in the degree of soil disturbance and quantity and structure of the surface mulch due to tillage and cover crop management practices. The two sampling methods which were used, pitfall trapping and vacuum sampling, showed similar trends in predator abundance. The treatment with the highest degree of mulch ground cover had the highest overall predator abundance while the treatment which was disked and had no surface mulch had the lowest. Although several species tended to prefer the system with the least amount of ground cover, most of the common species preferred the treatment with the most groundcover. Pitfall trap catches over a three-day period indicated that predator activity was significantly higher during the day than night in all treatments. However, a laboratory study provided evidence that night activity in the field may have been reduced due to unusually low temperatures. Catch data from pitfall traps, unbaited and baited with live armyworm larvae, indicated that long-distance chemical detection is not an important cue for generalist predators in finding armyworm as prey. A removal sampling technique was used and evaluated for estimating the absolute densities of ground beetles (Carabidae). Removal sampling is a method of absolute density estimation based on the decline in successive catch numbers as individuals are removed from a population. Field arenas were used to isolate sampling areas in a no-till corn field and barrier pitfall traps were installed within the arenas to remove the carabids. Three three-week sampling trials were conducted in 1991 providing data for estimates on 5 June, 26 June, and 17 July. A single six-week sampling period was conducted in 1992 providing data for estimates on 2 June based on three, four, five, and six weeks of sampling. A linear regression method was used for calculating the estimate from the removal sampling data. The technique's practicality and agreement with the assumptions of removal sampling are discussed. / Master of Science

The two-spotted spider mite (TSM), Tetranychus urticae Koch, is an important pest of many greenhouse and field crops worldwide. The development of resistance in TSM populations to chemical acaricides, allied with public health concerns about pesticide residues, has motivated the search for alternative control measures to suppress the pest. Hyphomycetous fungi are promising agents for mite control and the fungus Beauveria bassiana (Bb) (Balsamo) Vuillemin was investigated in this study as a biocontrol agent. The principal objectives of this study comprised: a) screening Bb strains for their pathogenicity against T. urticae; b) testing the effect of adjuvants on the efficacy of Bb; c) studying the effect of plant type on persistence of Bb and the efficacy of control of Bb against T. urticae; d) evaluating the field efficacy of Bb applications against T. urticae; e) testing the compatibility of Bb with selected fungicides; and f) assessing the synergy between Bb and soluble silicon for T. urticae control. Screening bioassays of sixty-two strains of Bb identified the two most effective strains, PPRI 7315 (R289) and PPRI 7861 (R444), that caused mortality levels of more than 80% of adult mites at 9 d post-inoculation with 2 × 108 conidia ml-1. These strains performed significantly better than the Bb commercial strain PPRI 5339, in laboratory bioassays. The two strains also attacked mite eggs, causing 53.4% and 55.5% reduction in egg hatchability at 2 × 108 conidia ml-1 respectively. However, PPRI 7861 showed relatively higher production of conidia in culture and was, therefore, selected for further trials under greenhouse and field conditions. Greenhouse evaluations of the effects of two adjuvants (Break-thru® and a paraffin oil-based emulsion) on efficacy of Bb demonstrated a higher efficacy of the biocontrol agent (BCA) when it was applied with Break-thru® or the oil solution than with water alone. Moreover, Bb conidia applied in Break-thru® solution resulted in greater control of TSM than conidia applied in the mineral oil. There was also a dose-response effect and the control of TSM by Bb increased when the concentration of conidia was increased. The control of TSM by Bb in beans (Phaseolus vulgaris L), cucumber (Cucumis sativus L.), eggplant (Solanum melongena L.), maize (Zea mays L.) and tomato (Solanum lycopersicum L.) was tested in greenhouse trials. On these crops, the persistence of conidia declined over time. The rate of decline was significantly higher on maize. However, TSM mortality was positively correlated with the amount of conidia deposited on leaves immediately after spraying, rather than their persistence over time. Higher levels of mortality of TSM due to Bb application were observed on beans, cucumber and eggplants, suggesting that the type of crop must be taken into consideration when Bb is applied as a BCA. Field efficacy of Bb against mites was evaluated in two trials on eggplants. Based on assessment of population densities of mites and leaf damage assessments; both trials showed that the strain PPRI 7861 controlled TSM in the field. Two commonly used fungicides, azoxystrobin and flutriafol, were investigated in vitro tests on culture medium and laboratory bioassays on detached bean leaves (Phaseolus vulgaris L.) for their effects on Bb. Azoxystrobin (a strobilurin) was less harmful to Bb while flutriafol was found to be inhibitory. Another important finding of this study was the substantial enhancement of Bb efficacy by soluble silicon. When Bb was combined with soluble Si, the control of TSM was better than when either of the two products was applied alone. Moreover, application of soluble Si as a plant fertilizer in hydroponic water nutrient increased accumulation of peroxidase, polyphenoloxidase and phenylalanine ammonia-lyase enzymes in leaves of plants infested with TSM. Increased activity of these defense enzymes in leaves deters feeding behaviour of mites. We suggested that feeding stress renders them susceptible to Bb infection, which would explain the synergy observed between the two agents. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2009.

Potential vegetative and reproductive increases in yield, as well as the biological efficacy against Fusarium verticillioides and F. proliferatum causing ear and stem rot in maize crops of commercially-formulated micro-organism formulation T-Gro (Trichoderma harzianum isolate DB103 WP) combined with Spartacus (Beauveria bassiana isolate DB 105 WP), T-Gro combined with Armenius (Bacillus subtilis isolate DB 109 WP), T-Gro combined with Maximus (Bacillus subtilis isolate DB 108 WP), T-Gro combined with Shelter (Bacillus subtilis isolate DB 101), T-Gro combined with Bismarck (Microbacterium maritypicum isolate DB 107 WP), as well as individual treatments of T-Gro, Armenius, Bismarck, Maximus and Shelter, were investigated when applied to maize seed and soil under field conditions. All the micro-organism treatments were compared with Thiram 750WP (750g/kg thiram WP) and an untreated control. The micro-organism treatments showed an increase in vegetative as well as reproductive yields when compared to the reference product Thiram 750 WP and the untreated control. There were no observations of adverse effects on the germination of maize seed in all the treatments that were applied. The three isolates B. subtilis, T. harzianum, and M. maritypicum, showed a significant reduction in vascular tissue discolouration of the main and ear stems, indicating a potential to be used in the reduction and control of diseases caused by Fusarium spp. Results also showed poor to very good increases of stem and foliage biomass as well as cob yield per plant produced by the micro-organism treatments when compared to the untreated control. The highest cob yield per plant that differed significantly from the untreated control was produced by T-Gro and Shelter. No phytotoxicity of any kind was observed with the application of the micro-organism formulations and they could therefore be deemed suitable to be used for the treatment of maize seed. The micro-organism formulations containing fungal and bacterial biological control agents have the potential to be used in commercial maize production to increase vegetative and reproductive yields and reduce the severity of ear and stem rot in maize. / Agriculture Animal Health & Human Ecology / M.Sc. (Agriculture)

Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: As overpopulation threatens the world’s ability to feed itself, food has become an invaluable resource. Unfortunately, almost a third of the food produced for human consumption is lost annually. Pests including insects, phytopathogens and weeds are responsible for more than a third of the annual major crop losses suffered around the world. The majority of current post-harvest control strategies employ synthetic agents. These compounds, however, have been found to be detrimental to the environment as well as human health, which has led researchers to investigate alternative strategies. Biocontrol agents are environmentally compatible, have a lower toxicity and are biodegradable, making them an attractive alternative to the synthetic control agents. The lipopeptides produced by Bacillus spp. in particular, have shown great potential as biocontrol agents against various post-harvest phytopathogens. Most biocontrol strategies apply the biocontrol organism, for example Bacillus, directly, whereas this study focused on the use of the lipopeptide itself as an antifungal agent. This is advantageous as the lipopeptides are less sensitive to their surroundings, such as temperature and pH, compared to living organisms, allowing for the production of a standardized product. This study investigated the production of the Bacillus lipopeptides surfactin, fengycin and iturin under controlled batch conditions. Parameters increasing lipopeptide production were quantified, focussing on antifungal lipopeptides (iturin and fengycin), and lipopeptide production was optimized. Experiments were performed in a fully instrumented 1.3 L bench-top bioreactor and lipopeptide analyses were performed via high pressure liquid chromatography (HPLC) and liquid chromatography-mass spectroscopy (LC-MS). After screening four Bacillus spp., Bacillus amyloliquefaciens DSM 23117 was found to be the best antifungal candidate. This was based on it outperforming other candidates in terms of maximum antifungals produced, Yp/x,antifungals (yield per cells), and antifungal productivity. Nitrate, in the form of NH4NO3, was critical for lipopeptide production and an optimum concentration was observed above which the CDW (cell dry weight) no longer increased significantly and both μmax (maximum specific growth rate, h-1) and lipopeptide production decreased. For μmax, the optimum NH3NO4 concentration was 10 g/L and for lipopeptides it was 8 g/L. At these respective NH4NO3 concentrations μmax = 0.58 (h-1), the maximum antifungals (fengycin and iturin) were 285.7 mAU*min and the maximum surfactin concentration was 302 mg/L. The lipopeptides produced by B. amyloliquefaciens, the antifungals (fengycin and iturin) and surfactin, are secondary metabolites, regardless of the optimization treatment, i.e. increased NH4NO3 concentrations. Using 30% enriched air extended the nitrate utilization period, suggesting that when increasing supply concentration, more oxygen was available to act as electron acceptors, allowing nitrate to be used for lipopeptide production. The number of iturin and fengycin homologues generally increased with an increase in nitrate concentration. This suggested that process conditions, such as nitrate concentration, can be used to manipulate homologue ratios, allowing for the possibility to tailor-make biocontrol-agent upstream, during the production process, and possibly increase the efficacy of the biocontrol strategy. The lipopeptides produced by B. amyloliquefaciens showed complete inhibition against Botryotinia fuckeliana and diminished the growth capabilities of Botrytis cinerea. No inhibition was observed against Penicillium digitatum. These results indicate potential of the biocontrol strategy, although scale-up and fed-batch studies are recommended, especially when considering commercial implementation. Studies regarding the lipopeptide application method, i.e. a single application or multiple applications, should also be investigated as this will influence the efficacy of the lipopeptides against the target organisms. / AFRIKKANSE OPSOMMING: Met oorbevolking wat die wêreld se vermoë om die groeiende bevolking te onderhou belemmer, het dit noodsaaklik geword om huidige voedselbronne te beskerm. Daar word beraam dat een derde van die voedsel wat wêreldwyd geproduseer word vir menslike verbruik verlore gaan elke jaar. Verder is insekte, plantpatogene en onkruide verantwoordelik vir meer as ‘n derde van die verliese rakend jaarlikse oeste. Meeste bestaande na-oes beheermetodes maak gebruik van sintetiese stowwe. Ongelukkig kan hierdie verbindings nadelig wees vir die omgewing sowel as menlike gesondheid. Navorsers het hulsef dus toespits daarop om alternatiewe beheermetodes te ondersoek. Bio-beheermetodes is omgewingsvriendelik sowel as bio-afbreekbaar, wat hulle ideale alternatiewe maak vir die sintetiese stowwe. Bacillus spp. lipopeptiede het veral hoë potensiaal getoon as bio-beheermiddels teen verskeie na-oes plantsiektes. Meeste bio-beheermetodes wend die biobeheer organisme, soos Bacillus, direk aan, waar hierdie studie op die gebruik van lipopeptiede as ‘n beheermiddel gefokus het. Die voordeel is dat lipopeptidiede minder sensitief is vir hul omgewings, soos temperatuur en pH, i.v.m. organismes en die moontlikheid bied van ‘n gestandardiseerde produk. Hierdie studie het die produksie van spesifieke Bacillus lipopeptide, naamlik surfactin, fengycin en iturin, onder beheerde lottoestande ondersoek. Parameters wat lipopeptied produksie verhoog is gekwantifiseer, spesifiek antifungiese middels (iturin en fengycin) en lipopeptied produksie is geoptimeer. Eksperimente is uitgevoer in ‘n 1.3 L bioreaktor en lipopeptiedanaliese is met behulp van hoë druk vloeistof chromatografie en vloeistofchromatografie-massa spektroskopie uitgevoer. Van die vier moontlike Bacillus spp., was Bacillus amyloliquefaciens DSM 23117 die mees belowende antifungus-produserende kandidaat. Dit het beter resultate gelewer in terme van maksimale antifungiese produksie, Yp/x,antifungies (opbrengs per sel) asook antifungiese produktiwiteit. Nitraat, in hierdie geval NH4NO3, was noodsaaklik vir lipopeptied produksie en ‘n optimale konsentrasie is waargeneem waarbo die seldigtheid nie meer beduidend toegeneem het nie en beide die μmax (maksimale spesifieke groei tempo, h-1) en lipopeptied produksie afgeneem het. Die optimale NH4NO3 konsentrasie vir μmax was 10 g/L en vir lipopeptiedproduksie was 8 g/L. By 10 g/L NH4NO3 was μmax = 0.58 (h-1) en by 8 g/L was die maksimale antifungiese produksie (fengycin en iturin) 285.7 mAU*min en die maksimale surfactin produksie 302 mg/L onderskeidelik. Die lipopeptide, die antifungiese middels (fengycin en iturin) en surfactin, geproduseer deur B. amyloliquefaciens is sekondêre metaboliete, ongeag van die optimerings-behandelinge wat toegepas word, soos ‘n verhoging in NH4NO3 konsentrasie. Die gebruik van 30% verrykte suurstof het die nitraat verbruikingsperiode verleng, wat voorgestel het dat met die verryking, meer suurstof beskikbaar was om te dien as finale elektron ontvanger en sodoende die nitraat beskikbaar te stel vir lipopeptied produksie. Iturin en fengycin homoloë, oor die algemeen, het toegeneem soos wat die nitraat konsentrasie verhoog is. Hierdie resultate dui daarop dat prosestoestande, soos nitraat konsentrasie, gebruik kan word om die verhouding waarin lipopeptied homoloë geproduseer word te manipuleer. Hierdie resultate dui op die potensiaal vir die stroomop produksie van ‘n unieke bio-beheermiddel, wat die effektiwiteit van die bio-beheermetode moontlik sal verhoog. Die geproduseerde lipopeptiede het totale inhibisie getoon teen Botryotinia fuckeliana en ook fungiese aktiwiteit belemmer met Botrytis cinerea. Geen inhibisie is getoon teen Penicillium digitatum nie. Hierdie resultate toon die potensiaal van die bio-beheermetode, maar ‘n opskalerings-studie asook ‘n voerlot studie word aanbeveel, veral met die oog op moontlike kommersiële implementering van die strategie. Verdere studies met betrekking tot die aanwendingsmetode van die lipopeptiede moet ook verder ondersoek word, m.a.w. enkel teenoor menigte aanwendigs, aangesien dit die effektiwiteit van die lipopeptiede teen die teikenorganismes sal beïnvloed.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Entomopathogenic nematodes have the potential to be outstanding biocontrol agents against agricultural pest insects. Combined with their bacterial symbionts, these biocontrol agents have proven to be very effective against numerous pests. The nematodes belong to the families Steinernematidae and Heterorhabditidae, and are ideal to be used in, and integrated with, pest management systems. There is a dire need for new and innovative methods to control agricultural pests, as numerous pest insects have developed resistance against broad-spectrum insecticides. Together with the environmental impact of these insecticides and the safety aspect regarding humans and animals, the need to develop new technologies, including entomopathogenic nematodes for pest management, is high. In this study, the associated symbiotic bacteria of three entomopathogenic nematodes species were isolated, and the potential of two nematode species to be successfully mass cultured in liquid medium was evaluated. Regarding the symbiotic bacteria, results from the study showed that bacteria species from all three nematode species, Heterorhabditis noenieputensis, Steinernema khoisanae and Heterorhabditis zealandica, were novel. Heterorhabditis noenieputensis was isolated in the Mpumalanga province during a previous survey conducted in citrus orchards. The bacterium isolated from this nematode belongs to the genus Photorhabdus, and bear closest similarity (98.6%) to the type strain of P. luminescens subsp laumondii (TT01T). Photorhabdus luminescens subsp. noenieputensis subsp. nov., derives its name from the area where the nematode was sourced, namely the farm Springbokvlei, near the settlement Noenieput close to the Namibian border. Thus far, 85 Steinernema spp. have been described worldwide, including S. khoisanae which was isolated in the Western Cape province of South Africa. Four S. khoisanae strains, namely SF87, SF80, SF362 and 106-C, were used for characterisating the new bacteria from different localities in South Africa. Using the neighbor-joining method, all the strains were aligned with 97% homology to the 16S rRNA sequences of several Xenorhabdus- type strains, indicating that they belonged to the same genus. The multigene approach was used to distinguish between the Xenorhabdus spp. and partial recA, dnaN, gltX, gyrB and infB gene sequences of the various strains were analysed. The bacterium species was named Xenorhabdus khoisanae sp. nov. after the nematode from which it was isolated. The results showed that the third bacterium species, which was isolated from H. zealandica, was new. The sequence of the bacteria strain clustered with the type strains of P. temperata and P. asymbiotica, indicate that it belonged to the genus Photorhabdus. This is the first study to show that H. zealandica associates with a luminescent Photorhabdus species, rather than with the known non-luminescent P. temperata. The potential of H. zealandica and Steinernema yirgalemense mass culture in liquid was investigated. Results illustrated that H. zealandica and its P. luminescens symbiont can be successfully cultured in liquid. However, two generations occurred during the process time, instead of the desirable one-generation. The growth curve of the symbiotic bacteria during the process time was measured, in order to determine when the stationary phase was reached, with the results showing this to occur after 36 h. Therefore, the optimum amount of time required for inoculating the IJs and for aiding in maximum infective juvenile (IJ) recovery is 36 h for adding the nematodes post pre-culturing of the bacteria. Future research goals should be to increase the percentage recovery in liquid culture, which would increase the number of nematodes produced per ml, which would, therefore, reduce the processing time significantly. The results from mass culturing the second nematode species, S. yirgalemense, indicated an asynchronous nematode development in the first generation. Growth curves were performed with the symbiotic bacteria that showed the exponential phase of Xenorhabdus started after 15 h, and that, after 42 h, the stationary phase was reached, with an average of 51 × 107 cfu·ml-1. Bioassays were performed to compare the virulence between in vitro- and in vivo-produced nematodes, with the results showing that the in vitro-produced nematodes were significantly less virulent than were the nematodes produced in vivo. The success obtained with the production of S. yirgalemense in liquid culture can serve as the first step in the optimising and upscaling of the commercial production of nematodes in industrial fermenters. The last aim of the current study was to determine when Xenorhabdus reached the stationary phase, when it is grown in a 20-L fermenter, as this would be the optimum time at which to add the IJs of S. yirgalemense. Such characteristics as the effect of stationary phase conditions on the bacterial cell density and on the DO2 rate in the fermenter were investigated. The results showed that the stationary phase of Xenorhabdus was reached after 36 h at 30˚C, which took 6 h less than did the same procedures followed with the Xenorhabdus sp. cultured in Erlenmeyer flasks on orbital shakers. This is the first step toward the liquid mass culturing of S. yirgalemense in industrial-size fermenters. Data from this study indicated the optimum amount of time that is required for adding nematodes to the bacterial culture in the fermenter, and for ensuring the optimum recovery of IJs, as well as a subsequent high yield of nematodes within a minimum processing time. This is the first report of its kind to investigate comprehensively the successful liquid culture of two South African entomopathogenic nematode species for the sole purpose of evaluating potential commercialisation. Results emanating from this study could be used as groundwork in future, in combination with similar research such as culturing nematodes intensively in large fermenters. / AFRIKAANSE OPSOMMING: Entomopatogeniese nematodes het die potensiaal om as doeltreffende biologiese beheeragente teen sleutelplaaginsekte gebruik te word. Elke nematood werk interaktief met ‘n spesifieke bakterium. Entomopatogeniese nematodes, behorende tot die families Steinernematidae en Heterorhabditidae, is ideale kandidate vir gebruik in ‘n geïntegreerde plaagbestuurprogram. Tans is daar ʼn behoefte vir nuwe metodes vir die beheer van plaaginsekte, omdat meeste insekte reeds weerstand opgebou het teen bestaande plaagdoders. As gevolg van die negatiewe impak van plaagdoders op die omgewing, asook kommer oor veiligheid vir die mens en diere, is die ontwikkeling en gebruik van alternatiewe plaagbeheermiddels noodsaaklik. In die eerste deel van die studie word drie nuwe bakterie spesies geïsoleer en beskryf. Resultate van hierdie studie het aangetoon dat die bakterië spesies vanuit die nematode spesies, Heterorhabditis noenieputensis, Steinernema khoisanae, en Heterorhabditis zealandica, tot dusver onbeskryf was. Eersgenoemde, H. noenieputensis, is afkomstig van ʼn sitrusboord in die Mpumalanga Provinsie. Die bakterie hieruit geïsoleer behoort tot die genus Photorhabdus en is biologies verwant (98.6%) aan P. luminescens subsp laumondii (TT01T). Die bakterie is benaam as Photorhabdus luminescens subsp. noenieputensis nov. en is na die nematood waaruit dit geïsoleer is vernoem. Tot dusver is wêreldwyd 82 spesies van Steinernema spp. beskryf, insluitende S. khoisanae van die Weskaap provinsie. Vier bakterie isolate is van S. khoisanae, SF87, SF80, SF362 en 106-C geïsoleer. Die buur-koppeling metode was gebruik om te bepaal dat hierdie bakterie isolate tot 97% ooreenstem met verskeie isolate van Xenorhabdus se 16S rRNA DNS volgordebepalings. Om tussen Xenorhabdus spp. te onderskei is ʼn multi-geen benadering gebruik deur gedeeltelike recA, dnaN, gltX, gyrB en infB DNS basispaar volgordebepalings van die verskeie isolate te bepaal. Hierdie bakterie isolaat is soortgelyk ook vernoem as, Xenorhabdus khoisanae sp. nov., na die nematood waaruit dit geïsoleer is. Die derde onbekende bakteriële spesie is uit H. zealandica geïsoleer. Die DNS basispaar volgordebepaling van die 16S geen van SF41 toon aan dat dit in dieselfde groep as P. temperata en P. asymbiotica val en sodoende aan die genus Photorhabdus behoort. Hierdie is die eerste studie met die bevinding dat H. zealandica ook met ʼn ander bakterie spesie geassosieer kan word buiten die normale P. temperata spesie. Die tweede deel van die studie gaan oor die teling van twee nematood spesies, H. zealandica en Steinernema yirgalemense, en hulle is geëvalueer vir hulle potensiaal om geteel te word in ʼn vloeibare medium. Die resultate het gewys dat H. zealandica met sy P. luminescens simbiont suksesvol in vloeistof aangeteel kan word, ten spyte van die feit dat daar twee generasies ontwikkel het, in plaas van die meer ideale enkel generasie. Die groeikurwe van die simbiotiese bakterie was gemonitor om te bepaal wanneer die stasionêre fase bereik word. Die resultate toon dat hierdie fase na 36 uur bereik was. Dus was die infektiewe nematode larwes eers na 36 uur tot die vloeibare medium waarin die bakterie geteel was bygevoeg. Navorsing in die toekoms moet dus gefokus wees om die persentasie herwinning van die infektiewe larwes te verhoog. Dit sal daartoe lei dat meer nematodes per ml geproduseer kan word en ook die prosesseringstyd van die nematodes verminder. ʼn Tweede nematode spesie, S. yirgalemense, was ook in vloeistof geteel. Hier het ʼn asinkroniese ontwikkeling in die eerste generasie plaasgevind wat problematies is. Groeikurwes is bepaal van die bakteriële simbiont en die resultate het gewys dat die groeifase van Xenorhabdus na 15 uur in aanvang geneem het en dat die stasionêre fase bereik was na 42 uur met ʼn gemiddelde van 51 × 107 selle·ml-1. Die virulensie van nematodes wat in vitro geteel is, is vergelyk met die virulensie van nematodes wat in vivo geteel is en die resultate het getoon dat die in vitro geteelde nematodes minder virulent was. Die teling van S. yirgalemense in vloeistof was oor die algemeen meer suksesvol as die teling van H. zealandica in dieselfde medium. Die doelwit van die laaste gedeelte van hierdie studie was om te bepaal wanneer Xenorhabdus die stasionêre fase bereik wanneer dit in ʼn 20-L fermenter gekweek word. Dit bepaal sodoende die optimale tyd wanneer die infektiewe larwes van S. yirgalemense bygevoeg behoort te word. Die uitwerking van die stasionêre fase op die bakteriële selle, asook die DO2-konsentrasie in die fermenter, was geëvalueer. Resultate het gewys dat die stasionêre fase van Xenorhabdus na 36 uur bereik was, wat 6 uur korter is as toe dit gekweek is in Erlenmeyer flesse. Hierdie studie is die eerste stap om die massa teling van S. yirgalemense in industriële fermenters suksesvol te bemeester. Die data wat verkry was, het aangedui wat die ideale tydsduur sal wees om die bakteriegetalle te vermeerder voordat die nematode bygevoeg word. Hierdie is die eerste studie wat die teling van twee Suid-Afrikaanse nematode spesies omvattend in vloeistof evalueer het. Die hoof doelwit is om die potensiaal van hierdie nematode spesies, met die oog op kommersiële gebruik, te meet. Die resultate van hierdie studie kan gekombineer word met toekomstige studies in hierdie spesifieke navorsingsveld.

Thesis (PhD (Agric))--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Biological control studies were conducted with isolates of entomopathogenic fungi Beauveria bassiana and Metarrhizium anisopliae from Ethiopia and South Africa against the spotted stem borer Chilo partellus. The study was conducted from April 2002 to April 2003, at the department of Entomology and Nematology, University of Stellenbosch, South Africa. The objectives were to screen these isolates for pathogenicity and to determine the susceptibility of different larval instars; to study the effect of temperature on fungal development and virulence; to investigate food consumption of fungus treated larvae; to determine compatibility of fungal isolates with insecticides; to study the effect of exposure methods and diets on larval mortality; and to evaluate promising isolates under greenhouse conditions using artificially infested maize plants. Four isolates of B. bassiana and six isolates of M. anisopliae were tested against second instar larvae. Of these isolates, B. bassiana (BB-01) and M. anisopliae (PPRC-4, PPRC-19, PPRC-61 and EE-01) were found to be highly pathogenic inducing 90 to 100 % mortality seven days after treatment. In subsequent assays, the fungal isolates were tested against third, fourth, fifth and sixth instar larvae. Second and sixth instar larvae were more susceptible to these isolates than third, fourth and fifth instar larvae. Conidial germination, radial growth and sporulation of the isolates PPRC-4, PPRC-19, PPRC-61, EE-01 and BB-01 were retarded at 15 and 35 0C. A suitable temperature range for the isolates was from 20 - 30 0C. At 25 and 30 0C the isolates induced 100 % mortality to second instar larvae within four to six days. Second and third instar C. partellus larvae were treated with the isolates PPRC-4 and BB-01, and daily consumption of maize leaf was measured. Treatment with the fungi was associated with a reduction in mean daily food consumption. In in-vitro studies, five concentrations (0.1 ppm, 1 ppm, 5 ppm, 10 ppm, and 100 ppm active ingredients) of the insecticides benfuracarb and endosulfan were tested with the isolates PPRC-4, PPRC-19, PPRC-16, EE-01 and BB-01. Increasing the concentration of the insecticides adversely affected germination, radial growth and sporulation of the isolates. In in-vivo studies combining the fungi, PPRC-4 and BB-01,with low concentrations (1 and 5 ppm a.i.), of the insecticides increased the mortality of third instar larvae from 65 to 100 %. Larvae sprayed directly with conidia, exposed to conidia treated leaves and dipped into conidial suspensions suffered high mortality of 98 to 100 %. Larvae exposed to treated leaves and larvae sprayed directly with conidia produced high numbers mycoses in cadavers. Exposure of larvae to treated leaves yielded high sporulation. At a low conidial concentration (1.25x107 conidia/ml), mycosis and sporulation were high. The optimum temperature was 20 0C for mycosis and 15 0C for sporulation. In greenhouse trails, a conidial suspension of 2 x 108 conidia/ml of the pathogenic isolates was sprayed on 3 to 4 week-old maize plants infested with 20 second instar larvae per plant. This resulted in suppression of foliar damage. Treatment with the fungi also reduced stem tunneling and deadheart. In addition, fungal treatment increased mean plant fresh and dry biomass compared to untreated control plants. In general, results from laboratory and greenhouse studies indicated that there was good potential for the use of these fungal isolates for controlling C. partellus larvae. / AFRIKAANSE OPSOMMING: Biologiese beheerstudies is uitgevoer met isolate van die insekpatogeniese swamme, Beauvaria bassiana en Metarrhizium anisopliae teen die gespikkelde stamboorder, Chilo partellus. Die doelwitte was om hierdie isolate te evalueer vir patogenesiteit; die vatbaarheid van verskillende larvale instars teenoor hulle te bepaal; die invloed van temperatuur op swamontwikkeling en virulensie te bepaal; die voedsel inname van swambehandelde larwes te ondersoek; die verenigbaarheid van die swamisolate met insektedoders te bepaal; die invloed van blootstellingsmetodes en diëte op larvale mortaliteit; en om belowende isolate in glashuisproewe te evalueer met gebruik van kunsmatig besmette mielieplante. Vier isolate van B. bassiana en ses isolate van M. anisopliae is teen tweede instar larwes getoets. Uit dié isolate is B. bassiana (BB-01) en M. anisopliae (PPRC-4, PPRC- 19, PPRC-16 en EE-01) as hoogs patogenies bevind. Hulle het 90 tot 100 % mortaliteit na sewe dae veroorsaak. In daaropvolgende essays, is die swamisolate teen derde, vierde, vyfde en sesde instar larwes getoets. Tweede en sesde instar larwes was gevoeliger vir die isolate as die derde, vierde en vyfde instar larwes. Spoorkeming, radiale groei en sporulasie van die isolate PPRC-4, PPRC-19, PPRC-61, EE-01 en BB-01, is by 15 en 35 0C vertraag. ‘n Aanvaarbare temperatuurreeks vir die isolate is vanaf 20 tot 30 0C. By 25 en 30 0C het die isolate 100 % mortaliteit teen tweede instar larwes binne vier tot ses dae geïndusseer. Tweede en derde instar C. partellus larwes is met die isolate PPRC-4 en BB-01 behandel en die daaglikse inname van mielieblare gemeet. Behandeling met die swamme is met ‘n afname in die gemiddelde voedselinname geassosieer In in-vitro studies is vyf konsentrasies (0.1 dpm, 1 dpm, 5 dpm, 10 dpm en 100 dpm aktiewe bestandele) van die insekdoders, benfuracarb en endosulfan getoets saam met die isolate PPRC-4, PPRC-19, PPRC-16, EE-01 en BB-01. ‘n Toename in die konsentrasie van die insekdoders het ontkieming, radiale groei en sporulasie van die isolate benadeel. In in-vitro studies het die kombinering van die die swamme, PPRC-4 en BB-01, met lae konsentrasies (1 en 5 dpm a.b.) van die insekdoders mortaliteit van derde instar larwes vanaf 65 tot 100 % laat toeneem. Larwes wat direk met spore gespuit is, aan behandelde blare blootgestel is en in spoorsuspensies gedoop is het tot hoë mortaliteit gelei, (98 tot 100 %). Blootstelling aan behandelde blare saam met ‘n lae konidiakonsentrasie, 1.25x106 spore/ml) en ‘n temperatuur van 15 tot 20 0C het tot hoë swammikose en sporulasie in kadawers gelei. In glashuisproewe, is ‘n spoorsuspensies van 2 x 108 spores/ml van die patogeniese isolate op 3 tot 4 weekoud mielieplante wat met 20 tweede instar larwes per plant besmet is gespuit. Dit het blaarskade onderdruk. Behandeling met die swamme het ook stamtonnels en dooiehart verminder. Boonop het swambehandeling die vars- en droë plantbiomassa laat toeneem in vergelyking met die onbehandelde kontrole plante. Oor die algemeen het resultate van laboratorium- en glashuisproewe getoon dat daar goeie potensiaal is vir die gebruik van hierdie swamisolate vir die beheer van C. partellus larwes.

Thesis (MScAgric)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Planococcus citri (Risso) (Hemiptera: Pseudococcidae), the citrus mealybug, is a highly destructive pest of citrus in South Africa. The ability of mealybugs to develop resistance to chemicals, as well as their cryptic nature and protective wax coverings, all individually and combined, impair the ability of insecticides to control them. Furthermore, chemicals deplete natural enemy populations that usually establish control over mealybug populations well before harvest in March to May. The inability of chemicals to control P. citri successfully and the growing public awareness of the detrimental environmental effects and health risks that are associated with pesticides are pressuring citrus growers to find alternative methods to chemical control. Entomopathogenic nematodes of the Rhabditida order, belonging either to the families Heterorhabditidae or Steinernematidae, have proven to be valuable biocontrol agents of a variety of insect pest species. These nematodes are, however, not yet commercially available in South Africa. Various laboratory bioassays were conducted to determine the potential of entomopathogenic nematodes to control P. citri. Adult female P. citri were screened for susceptibility to six indigenous nematode species. Planococcus citri was found to be most susceptible to Steinernema yirgalemense and Heterorhabditis zealandica, causing 97% and 91% mortality, respectively. Both H. zealandica and S. yirgalemense completed their life cycles inside adult female P. citri. Under optimal conditions, H. zealandica and S. yirgalemense, at an application rate of 170 infective juveniles (IJs) / insect (LD90), were able to control P. citri efficiently with a discriminating dosage of 11(LD50). The water activity (aw) bioassay indicated that S. yirgalemense was two times more tolerant to lower levels of free water than H. zealandica, with aw50 = 0.96 and aw90 = 0.99, compared to H. zealandica with aw50 = 0.98 and aw90 = 1.01. Furthermore, S. yirgalemense proved able to locate and infect P. citri at a faster rate than H. zealandica. Nematode activity was not significantly affected when exposed to 15°C, 20°C and 25°C. Results of the exposure trial also showed that the first 2−4 h post-application was the most decisive time for establishing successful infection of mealybugs. Further experiments were conducted to determine the ability of two polymer products, Zeba® and Xanthan gum, and a surfactant, Nu-Film-P®, to improve nematode application suspensions. Despite not being as effective as Xanthan gum, Zeba® still retarded sedimentation significantly. The addition of 0.02% Xanthan gum did not improve the ability of nematodes to control P. citri at 60% and 80% relative humidity containing H. zealandica and 0.03% Zeba® increased mortality by 14% at 60% RH and by 22% at 80% RH. The same polymer formulation was tested with S. yirgalemense, with mortality being found to have increased by 21% at 60% RH and by 27% at 80% RH. The combined addition of Nu-Film-P® and Zeba® was able to retard sedimentation significantly, increasing the average number of nematodes deposited on 2-cm2 leaf discs. To illustrate those factors that should be taken into consideration before applying nematodes in an integrated pest management (IPM) programme for citrus, the compatibility of H. zealandica and S. yirgalemense with biological control agents and agrochemicals to which they are most likely to be exposed was determined. Bioassays showed the coccinellid beetle, Cryptolaemus montrouzieri, to be susceptible to both nematode species. Beetle larvae proved to be highly susceptible, with H. zealandica obtaining 80%, and S. yirgalemense obtaining 92% control. Adult beetles were found to be twice as susceptible to S. yirgalemense, with 64% mortality recorded, as to H. zealandica, with 30% mortality recorded. Tolerance of H. zealandica and S. yirgalemense infective juveniles to the affect of aqueous solutions of an insecticide (Cyperphos 500 E.C.®), two biopesticides (CryptogranTM and HelicovirTM), and two adjuvants (Nu-Film-P® and Zeba®) on the infectivity and survival of H. zealandica and S. yirgalemense was evaluated. Heterorhabditis zealandica proved to be highly compatible with all products tested. Significant increase in mortality of S. yirgalemense was recorded after 12 h exposure to CryptogranTM, HelicovirTM and Cyperphos 500 E.C.®, and after 6 h exposure to Nu-Film-P®. However, there was no decrease in the ability of S. yirgalemense to infect the insect hosts. Towards selecting the appropriate nematode species for field studies, the ability of S. yirgalemense and H. zealandica to control P. citri was first evaluated under less harsh simulated glasshouse conditions. The ability of both the above-mentioned nematode species to control P. citri, with and without the addition of 0.03% Zeba® and 0.06% Nu-Film-P®, was evaluated in a growth chamber at 75 ± 8% relative humidity and 22°C. All treatments resulted in significantly higher mortality of adult female P. citri than did the control. The addition of 0.03% Zeba® and 0.06% Nu-Film-P® to an H. zealandica and an S. yirgalemense suspension increased resultant mortality from 26% to 30%, and from 34% to 45%, respectively. This increase in mortality was, however, not significant. The ability of this formulation to prolong the ability of S. yirgalemense to infect P. citri and to prolong nematode survival was also evaluated under the same conditions and showed the formulation to improve both infectivity and survival for up to 2–3 h post-application. In the semi-field trial, S. yirgalemense was unable to obtain significant control of P. citri without the addition of 0.03% Zeba® to nematode application suspensions. However, with the addition of 0.03% Zeba® S. yirgalemnse was able to obtain up to 53% control. The study established that the polymer product Zeba® improves the ability of S. yirgalemense to infect P. citri by retarding desiccation and by buffering nematodes from suboptimal environmental conditions. / AFRIKAANSE OPSOMMING: Planococcus citri (Risso) (Hemiptera: Pseudococcidae), die sitrus witluis, is ʼn baie skadelike pes van sitrus in Suid Afrika. Die vermoë van witluise om weerstand teen chemiese middels te ontwikkel, hul kriptiese lewenswyse en die beskermende waslaag wat hul liggame omhul inhibeer gesamentlik en individueel die vermoë van insektisiede om witluis bevolkings te beheer. Chemiese middels verminder ook die natuurlike vyande wat gewoonlik witluis bevolkings beheer voor die sitrus oestydperk in Maart tot Mei. Die onvermoë van chemiese middels om P. citri suksesvol te beheer en verhoogde bewustheid van die publiek rond om die vernietigende omgewings impak en gesondheidsrisiko’s verbonde aan chemiese insek beheer, noodsaak sitrus produsente om alternatiewe beheermetodes te ontwikkel. Entomopatogeniese nematodes, van die orde Rhabditida wat aan die families Heterorhabditidae of Steinernematidae behoort, is bekend as effektiewe biologiese beheeragente van ʼn verskeidenheid insek pes spesies. Hierdie nematodes is egter tans nie kommersieel beskikbaar in Suid- Afrika nie. Om ten einde die vermoë van plaaslike nematode spesies te bepaal om P. citri te beheer, is verskeie biotoetse in die laboratorium uitgevoer. ʼn Vinnige siftings proses is uitgevoer om vas te stel watter plaaslike nematode spesies die hoogste persentasie mortaliteit van P. citri wyfies veroorsaak. Daar is bevind dat P. citri die mees vatbaarste is vir Steinernema yirgalemense en Heterorhabditis zealandica wat 97% en 91% mortaliteit respektiewelik veroorsaak het. Die ontwikkeling van beide H. zealandica en S. yirgalemense na die infektering van volwasse P. citri wyfies is gevolg en daar is bevind dat beide nematode spesies hul lewensiklusse kon voltooi in insek kadawers. Onder optimale toestande was H. zealandica en S. yirgalemense in staat om P. citri effektief te beheer, as hul teen ʼn konsentrasie van 170 infektiewe larwes (JIs) per insek (LD90) toegedien word, met ʼn diskriminerende toedienings dosis van 11 (LD50). ʼn Water aktiwiteit biotoets het gewys dat S. yirgalemense twee keer so verdraagsaam is teenoor laer vlakke van vrye water as wat H. zealandica is met aw50 = 0.96 en aw90 = 0.99, in vergelyking met H. zealandica met aw50 = 0.98 en aw90 = 1.01. Resultate van ʼn blootstellings toets het ook gewys dat S. yirgalemense die vermoë het om P. citri vinniger op te spoor en te infekteer as H. zealandica. Die blootstellings toets was uitgevoer teen 15°C, 20°C en 25°C en daar is bevind dat die aktiwiteit van H. zealandica nie beduidend verhoog het met ʼn verhoging in temperatuur nie. Resultate vir die blootstellings toets het ook gewys dat die eerste 2 tot 4 uur na toediening van nematodes die mees kritieke tyd is om suksesvolle infektering van witluise te bevestig. Verdere eksperimente is uitgevoer om te bepaal of die byvoeging van twee polimeer produkte nl. Zeba® en Xanthan gum en ʼn benatter Nu-Film-P®, nematode toedienings suspensies kan verbeter. Deur 0.02% Xanthan gum by nematode toedienings suspensies te voeg is die vermoë van nematodes om P. citri by 60% en 80% relatiewe humiditeit (RH) te beheer nie beduidend verhoog nie, terwyl die byvoeging van 0.03% Zeba® wel ʼn beduidende verhoging in mortaliteit veroorsaak het. ʼn Suspensie van H. zealandica en 0.03% Zeba® het beheer met 14% by 60% RH en met 22% by 80% RH verhoog. Die invloed van dieselfde polimeer formulasie was ook getoets op S. yirgalemense en mortaliteit het verhoog met 21% by 60% RH en met 27% by 80% RH. Ten spyte daarvan dat “Xanthan gum” nie so effektief was om afsakking van nematodes te verhoed nie, het die byvoeginging van 0.03% Zeba® by toedienings suspensies steeds ʼn beduidende invloed gehad. Die gekombineerde byvoeging Nu-Film-P® en Zeba® was in staat om die gemiddelde aantal nematodes gedeponeer op 2-cm2 blaar skyfies te verhoog. Om te wys watter faktore in ag geneem moet word voor EPNs as deel van ʼn geïntegreerde pes beheer program toegedien word, is die verenigbaarheid van H. zealandica en S. yirgalemense vir biologiese beheer agente en landbouchemikalieë, waaraan hulle heel waarskynlik blootgestel gaan word, bepaal. Biotoetse het gewys dat die liewenheersbesie, Cryptolaemus montrouzieri, vatbaar is vir beide nematode spesies. Die larwale fase was hoogs vatbaar met mortaliteit van 80% en 92% verkry deur die toedeining van H. zealandica en S. yirgalemense respektiewelik. Daar is ook bepaal dat volwasse besies twee maal so vatbaar is vir S. yirgalemense wat 64% mortaliteit veroorsaak het, as vir H. zealandica met 30% mortaliteit aangeteken. Verdraagsaamheid van IJs van H. zealandica en S. yirgalemense met oplossings van ʼn insektisied (Cyperphos 500 E.C.®), twee bio-plaagdoders (CryptogranTM en HelicovirTM), en twee byvoegmiddels (Nu-Film-P® en Zeba®), vir infektiwiteit en oorlewing is bepaal. Resultate het gewys dat H. zealandica hoogs verenigbaar is met alle produkte wat getoets is. ʼn Beduidende verhoging in mortaliteit van S. yirgalemense is aangeteken na 12 ure se blootstelling aan CryptogranTM, HelicovirTM en Cyperphos 500 E.C.®, en na 6 ure se blootstelling aan Nu-Film-P®. Resultate het egter ook gewys dat die vermoë van S. yirgalemense om om gashere te infekteer nie deur beinvloed word nie. Om ten einde te bepaal watter een van H. zealandica of S. yirgalemense die beter isolaat sou wees om te gebruik in veldproewe, is hul vermoë om P. citri te beheer eers evalueer onder minder ongunstige gesimuleerde glashuis toestande. Die vermoë van beide nematode spesies om P. citri te beheer is bepaal in ʼn groeikamer by 75 ± 8% (RH) met en sonder die byvoeging van 0.03% Zeba® en 0.06% Nu-Film-P®. Mortaliteit verkry deur alle behandelings was beduidend hoër as die kontrole. Deur 0.03% Zeba® en 0.06% Nu-Film-P® by toedienings suspensies van H. zealandica en S. yirgalemense te voeg is die gevolglike mortaliteit van P. citri verhoog van 26% na 30% en van 34% na 45%, respektiewelik. Hierdie verhoging in mortaliteit was egter nie beduidend nie. Die vermoë van die bo-genoemde formulasie om oorlewing van S. yirgalemense en infeksie tydperk van P. citri te verleng was bepaal onder dieselfde toestande en daar is gevind dat die formulasie beide die infektiwiteit en oorlewing van nematodes met 2 tot 3 uur na toediening verleng het. Tydens die semi-veldproef was S. yirgalemense nie in staat om beduidend hoër mortaliteit van P. citri te verkry, in vergelyking met die kontrole, sonder die byvoeging van 0.03% Zeba® by toedienings suspensies nie. Met die byvoeging van Zeba® is daar tot 53% beheer verkry. Die studie het onweerlegbaar gewys dat die polimeer produk Zeba® die vermoë van S. yirgalemense om P. citri te infekteer verbeter deur die uitdroging van nematodes te vertraag en deur hul te beskerm teen ongunstige omgewings toestande.

Thesis (MSc (Conservation Ecology and Entomology))--University of Stellenbosch, 2011. / Please refer to full text to view abstract.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Two mathematical models are formulated in this dissertation for the population growth of an Eldana saccharina Walker infestation of sugarcane under the influence of partially sterile released insects. The first model describes the population growth of and interaction between normal and sterile E. saccharina moths in a temporally variable, but spatially homogeneous environment. The model consists of a deterministic system of difference equations subject to strictly positive initial data. The primary objective of this model is to determine suitable parameters in terms of which the above population growth and interaction may be quantified and according to which E. saccharina infestation levels and the associated sugarcane damage may be measured. The second model describes this growth and interaction under the influence of partially sterile insects which are released in a temporally variable and spatially heterogeneous environment. The model consists of a discretized reaction-diffusion system with variable diffusion coefficients, subject to strictly positive initial data and zero-flux Neumann boundary conditions on a bounded spatial domain. The primary objectives in this case are to establish a model which may be used within an area-wide integrated pest management programme for E. saccharina in order to investigate the efficiency of different sterile moth release strategies in various scenarios without having to conduct formal field experiments, and to present guidelines by which release ratios, frequencies and distributions may be estimated that are expected to lead to suppression of the pest. In addition to the mathematical models formulated, two practical applications of the models are described. The first application is the development of a user-friendly simulation tool for simulating E. saccharina infestation under the influence of sterile insect releases over differently shaped spatial domains. This tool provides the reader with a deeper understanding as to what is involved in applying mathematical models, such as the two described in this dissertation, to real-life scenarios. In the second application, an optimal diversification of sugarcane habitats is considered as an option for minimising average E. saccharina infestation levels, and as a further consequence, improving the cost-efficiency of sterile insect releases. Although many special cases of the above model classes have been used to model the sterile insect technique in the past, few of these models describe the technique for Lepidopteran species with more than one life stage and where F1-sterility is relevant. In addition, none of these models consider the technique when fully sterile females and partially sterile males are being released. The models formulated in this dissertation are also the first to describe the technique applied specifically to E. saccharina, and to consider the economic viability of applying the technique to this species. Furthermore, very few examples exist of such models which go beyond a theoretical description and analysis towards practical, real-life applications as illustrated in this dissertation. / AFRIKAANSE OPSOMMING: Twee wiskundige modelle word in hierdie proefskrif vir die populasiegroei van ’n Eldana saccha- rina Walker infestasie van suikerriet onder die invloed van gedeeltelik steriele, vrygelate insekte daargestel. Die eerste model beskryf die populasiegroei van en -interaksie tussen normale en steriele E. saccharina motte in ’n dinamiese, maar ruimtelik-homogene omgewing. Die model is ’n stelsel deterministiese verskilvergelykings onderhewig aan streng positiewe aanvangswaardes. Die primˆere doelstelling met hierdie model is om geskikte parameters te bepaal in terme waarvan die bogenoemde groei en interaksie gekwantifiseer kan word, en waarvolgens E. saccharina infestasievlakke en die gepaardgaande suikerrietskade gemeet kan word. Die tweede model beskryf hierdie groei en interaksie onder die invloed van gedeeltelik steriele insekte wat in ’n dinamiese en ruimtelik-heterogene omgewing vrygelaat word. Die model is ’n gediskretiseerde stelsel reaksie-diffusievergelykings met veranderlike diffusieko¨effisi¨ente onderhewig aan streng positiewe aanvangswaardes en zero-vloei Neumann-randwaardes op ’n begrensde ruimtelike gebied. Die primˆere doelstellings in hierdie geval is om ’n model tot stand te bring wat in ’n area-wye, ge¨ıntegreerde pesbestrydingsprogram vir E. saccharina gebruik kan word om die doeltreffendheid van verskillende steriele motvrylatingstrategie¨e te bepaal sonder om daadwerklik veldeksperimente uit te voer, en om riglyne daar te stel waarvolgens vrylatingsverhoudings, -frekwensies en -verspreidings bepaal kan word wat na verwagting na ’n onderdrukking van die pes sal lei. Bykomend tot die wiskundige modelle in hierdie proefskrif, word twee praktiese toepassings van die modelle ook beskryf. In die eerste toepassing word ’n gebruikersvriendelike simulasie hulpmiddel ontwikkel om E. saccharina infestasie onder die invloed van steriele insekvrylatings in verskillende ruimtelike gebiede te simuleer. Hierdie toepassing fasiliteer ’n dieper begrip van wat ter sprake is in die toepassing van wiskundige modelle, soos die twee modelle in hierdie tesis, tot werklike scenario’s. In die tweede toepassing word ’n optimale diversifisering van suikerriet habitats as ’n opsie vir die vermindering van die gemiddelde E. saccharina infestasie vlakke beskou, en gevolglik word die verbetering van die koste-doeltreffendheid van steriele insekvrylatings afgeskat. Alhoewel verskeie spesiale gevalle van die bogenoemde twee klasse van modelle reeds in die verlede gebruik is om die doeltreffendheid van die steriele-insektegniek te modelleer, beskryf weinig van hierdie modelle die tegniek vir Lepidopteriese spesies met meer as een lewensfase en waar F1-steriliteit ter sprake is. Verder beskryf geen van hierdie modelle die tegniek waar algeheel steriele wyfies en gedeeltelik steriele mannetjies vrygelaat word nie. Die modelle in hierdie tesis is ook die eerste waar die tegniek spesifiek op E.saccharina toegepas word, en waar die ekonomiese lewensvatbaarheid van die tegniek vir hierdie spesie oorweeg word. Verder bestaan daar min voorbeelde van soortgelyke modelle wat verder gaan as ’n teoretiese beskrywing en wiskundige ontleding na praktiese, werklike toepassings, soos in hierdie proefskrif ge¨ıllustreer.