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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Factors affecting transmissions of trypanosomes through tsetse flies

Macleop, E. T. January 2005 (has links)
The maintenance of human sleeping sickness and nagana across sub-Saharan Africa depends on cyclical transmission of trypanosomes through tsetse flies. Previous work showed that the symbiotic bacterium <i>Sodalis glossinidius</i> was involved in susceptibility to trypanosome infection. Streptozotocin (a toxic analogue of the bacterium’s main food source) has been recently shown to decrease trypanosome infection rates in the offspring of treated tsetse. In the present work streptozotocin did remove <i>S. glossinidius</i> from the offspring of treated flies but it was not possible to generate a line of tsetse free from <i>S. glossinidius</i> infection. Other potential factors involved in acquisition of trypanosome infection were then examined. A range of antioxidants or cyclic GMP were shown to prevent trypanosomes death in the tsetse midgut. The process was shown to be independent of protein synthesis as D-cysteine (an unphysiological isomer of L-cysteine) also enhanced midgut infection rates. Further experiments showed that cGMP could significantly inhibit trypanosome death when fed up to 96 h post-infection, whereas antioxidants only functioned for 48 h post-infection. Moreover it was found that maturation of established midgut infections could be regulated by environmental stimuli as well as by antioxidants. Cold shock of infected flies as well as addition of L-cysteine but not D-cysteine to the bloodmeal resulted in significant increases in maturation rates, while nitric oxide synthase inhibitors reduced maturation rates. It is concluded that reactive oxygen species play a major role in killing trypanosomes entering the tsetse midgut and that cysteine containing proteins and/or nitric oxide are essential for differentiation of established midgut infections into mammalian infective salivary gland infections.

Diel patterns of behaviour in the western flower thrips

Holmes, Neil David January 2006 (has links)
Frankliniella occidentalis (pergande) is a serious pest of protected crops within the UK because it damages crops and is a vector of tospoviruses. It is resistant to a wide range of insecticides and exhibits thigmokinetic behaviour, making contact between pest and treatment difficult. Applying control procedures to correspond with the diel behaviour patterns of F. occidentalis could enhance control ofthrips. Studies of glasshouse crops showed that the abundance of adults on leaves and flowers increased in ~he afternoon compared to the morning. This could not be accounted for by movements within or between plants, but could be from sites off the plant. This hypothesis deserves further investigation. Infrared videography in the glasshouse generally showed little difference In abundance on plant parts between scotophase and photoppase. In addition, observations made at the time suggested that there was little difference in locomotory activity between the light phases. Contrastingly, laboratory locomotory activity patterns showed an increase in the early morning and late afternoon. A light burst during the scotophase, devised in the lliboratory to enhance locomotory activity, had little effect on thrips in a cucumber glasshouse. These two findings cast doubt on the applicability of laboratory studies to the glasshouse. Flight could be predicted well using ambient light and temperature and showed a clear diel pattern. There was a diel pattern of larval dropping to the ground to pupate, with the majority of larvae falling between late afternoon and midnight. Further sampling within a ,?ucumber glasshouse showed that on any particular day most larvae dropped within a two-hour window. Flight activity, and larvae falling from the crop showed clear' diel patterns. Locomotory activity on the crop djd not show a clear diel pattern. Interpretation and use of these findings would depend upon the type of control procedure applied to a crop.

Characterisation of novel resistance and cross-resistance to Bacillus thuringiensis toxin in the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae)

Johnston, Paul January 2009 (has links)
The mechanisms of resistance to the Bacillus thuringiensis crystal toxin Cry1Ac were explored in the Plutella xylostella SERD4 population, which shows polygenic resistance to Cry1Ac with cross-resistance to the pyrethroid deltamethrin that can be partially overcome by co-administering the esterase inhibitor piperonyl butoxide (PBO). The novelty of SERD4 cross-resistance was confirmed by testing other Cry1Ac-resistant populations for resistance to deltamethrin and for synergism of Cry1Ac toxicity by PBO. Esterase-mediated sequestration of Cry1Ac was tested using electrophoretic mobility shift assays, gut esterase activity inhibition assays and ligand blotting. Additionally, isozyme profiles and gut esterase activity were compared between various populations in an attempt to correlate the pattern and activity of esterases with cross-resistance. No correlation was found between esterase isozymes and Cry1Ac resistance and no evidence was found to indicate a direct interaction between CrylAc and esterases. Previous work showed that a maternal effect contributes to Cry1Ac resistance in SERD4. Therefore factors that are known to be maternally inherited in Lepidoptera and which have been implicated in Cry1Ac resistance were screened to investigate possible correlations with resistance. Various immune parameters were screened including both cell-free and haemocyte-mediated responses. No correlation was observed between immune status and Cry1Ac resistance. The composition of the gut microbiota was compared between SERD4 and a susceptible P. xylostella population and eliminated to assess any contribution to Cry1Ac toxicity. The microbiotas of resistant and susceptible larvae were identical. Furthermore, the gut microbiota of a Cry1Ac-susceptible strain that is adapted to artificial diet was not observed to affect Cry1Ac toxicity. This led to further work concerning a recent novel hypothesis that gut bacteria are obligately required for the toxicity of B. thuringiensis and Cry1Ac towards numerous lepidopterans including the tobacco hornworm, Manduca sexta. Techniques were developed to allow the rearing of aseptic M. sexta larvae. The toxicity of B. thuringiensis was not reduced in the absence of gut bacteria. However, B. thuringiensis was less toxic to larvae that had been recently exposed to antibiotics. Antibiotic exposure did not reduce the toxicity of Cry1Ac.

The toxicity of copper to certain nymphs of the Ephemeroptera

Briffa, Frank January 1976 (has links)
The toxic effects of copper to three species of British Mayfly nymphs were studied. Those species were, Beatin rhodani, Ecdyonurus venosus and Rithrogena semicolorata. The study was divided into the following sections : 1. Toxicity tests. These were carried out so that a basic profile of copper toxicity in respect to concentration could be arrived at for each species. Tests were also carried out to determine the effects of complicating factors such as temperature, pH and water hardness on the basic profiles of toxicity. Threshold of toxicity concentrations were also determined using the toxicity test technique. It was found that B.rhordani is the most susceptible of the three species to copper, with R. semicolorata being the most tolerant. In all cases the extent of toxic effect bore a simple, direct relationship to the concentration of copper used. This toxic effect was made more acute by higher temperatures and low pH values, but was less acute in harder waters. In this part of the study the results were expressed as Tlm survival curves and probit analyses. A series of toxicity tests was also carried out to determine the effect of short exposure times to copper and it was found that complete recovery only occurred or is only possible in the case of low (less than 20mg/L copper sulphate) concentrations and exposures of less than six hours. 2. Serial sections of nymphs exposed to copper solutions were treated with Rubeanic acid which is a copper specific histological stain. This procedure identified areas of copper accumulation within the bodies of the nymphs. The distribution of copper was found to be as follows: a. Within the lumen and cells of the mid-gut. b. In parts of the central nervous system c. In other organs including the Malpighian tubles and the gill filaments. 3. The effect of copper on the rate of respiration as measured by oxygen consumption was studied using a sensitive micromanometer. It was found that in the case of E.venosus there was very little effect but that in the case of the other two species copper repressed the rate of oxygen consumption. These results related to the ecology of the respective species. 4. The rate of uptake of copper by the three species was measured using the radioactive isotope of copper, Cu64. Generally it was found that uptake is rapid up to sixteen hours. The fastest rate was for B.rhodani. An attempt was made to correlate these results with findings from earlier experiments in the study and some relationship was found to exist between the rate at which copper is taken up and the survival curves established by the toxicity tests in the first part of the study.

Bio-accumulation and non-target effects of GM derived Bt endotoxin in the soil

Moore, Sian Pamela January 2007 (has links)
Plants may be genetically modified to express an entomopathogenic protein from the bacterium <italic>Bacillus thuringiensis</italic> (Berliner) (Bt plants). Bt plants are known to affect some above-ground invertebrates, with significant effects on species closely related to target invertebrates and on their natural enemies. Bt proteins may enter the soil through root exudates and decomposition of plant material. This study aimed to analyse the effects of Bt broccoli (<italic>Brassica oleracea</italic> L. var. italica Plenk) on six soil-dwelling invertebrates. No significant differences were detected in nematode (<italic>Panagrellus redivivus</italic> L.) populations living in compost in which Bt and non-5/ broccoli had grown. The other species were introduced to combinations of Bt and non-5/ leaves, and compost in which Bt and non-5/ plants had grown. No differences were detected in Collembola (<italic>Folsomia candida</italic> Willem) populations, but significantly more young woodlice (<italic>Porcellio scaber</italic> Latrielle) survived, and weighed more, in the Bt than the non-5/ treatments. Slugs (<italic>Deroceras reticulatum</italic> Muller) weighed more in the presence of Bt proteins. A higher percentage of earthworm (<italic>Lumbricus terrestris</italic> L.) cocoons hatched in Bt than non-5/ treatments. In contrast, at a third trophic level, fewer predatory beetles (<italic>Nebria brevicollis</italic> Fabricius) survived when feeding on slugs that had fed on Bt leaves than on those fed on non-5/ leaves. Leaves from Bt plants affected more parameters than compost in which Bt plants had grown. Attempts were made to use molecular techniques to analyse the effects of Bt broccoli on soil micro-organisms. Individual Bt broccoli plants expressed different concentrations of Bt protein. The Bt plant's control of three Lepidoptera species was tested and only one species was susceptible. These results show that non-target invertebrates, including pest species, can be affected by Bt broccoli, sometimes beneficially, and underlines the need for prior testing of GM crops on a range of non-target species.

Morphology and epidemiology of some parasitic copepods (Poecilostomatoida: ergasilidae) from British freshwater fish

Abdelhalim, Abuelgasim I. January 1990 (has links)
No description available.

Determinants of variation in the response of the aphid parasitoid Aphidius ervi to aphid sex pheromones

Kutiech, Nisreen January 2008 (has links)
Variation in parasitoid responses to semiochemicals may be influenced by genetics, phenotypic plasticity and the individual's physiological state. Previous experimental work has indicated a high degree of variability among parasitoid individuals in their response to aphid sex pheromones but no work has been done to investigate the factors behind such variation. Some aspects of this variation were investigated in the aphid parasitoid Aphidius ervi (Haliday) (Hymenoptera: Braconidae) in the laboratory. Conducting behavioural experiments, the searching behaviour of females was investigated in the presence and the absence of the aphid sex pheromone (4GS,7S,7a/?)-nepetalactone, observing the same individual female parasitoids during two consecutive foraging attempts on different plants. The first set of observational experiments demonstrated the role of the pheromone as an arrestant in the searching behaviour of A. ervi and its additive effect when it was presented with other foraging cues such as aphid-induced plant volatiles. The second set of behavioural experiments showed significant differences in A. ervi responses to the pheromone depending on their physiological state. Virgin, well-fed and high egg-load females were more active in the presence of the aphid sex pheromone than mated, hungry and low egg-load females although there were no significant differences in their activities in the first foraging attempt when the pheromone was absent. The effect of the pheromone on the searching behaviour of A. ervi within different tritrophic systems was investigated. The results showed variation in this response depending on which host aphid and/or the host plant they had been reared, showing that &quot;conditioning&quot; may have an influence on this response. Using two isofemale lines and an insectary-maintained laboratory population of A.ervi, the genetic basis of this response was investigated. The behavioural experiments showed no significant differences between the three different populations in their response to aphid sex pheromones. A supportive molecular study using DNA microsatellites v/as also conducted, which revealed low genetic variability among the three studied populations. The results are discussed in the context of using the response of A. ervi to aphid sex pheromones in a strategy to manipulate natural populations of the parasitoid for the biological control of aphids and the importance of studying the variation in this response to increase the effectiveness of such strategy.

A review of herring stocks to the west of the British Isles

Özcan, Ali January 1974 (has links)
No description available.

Predation and scavenging by the generalist predator, Pterostichus melanarius

Powell, Adam January 2011 (has links)
The research reported in this thesis investigated the ability of <italic> P. melanarius</italic> to control slug populations, and the impacts that alternative prey, particularly carrion, has on the efficacy of this predator as an agent of slug pest control. A suite of laboratory- and field-based experiments were conducted to achieve those ends. The main findings were: (1) Prey vital status was significant in determining the feeding preference hierarchy of <italic> P. melanarius.</italic> The mucus defence of live slugs (<italic>Deroceras reticulatum)</italic> deterred attacks by beetles, but feeding on dead <italic> D. reticulatum</italic> emphasized a preference for this prey type by <italic> P. melanarius.</italic> (2) The survival rate of <italic>D. reticulatum</italic> bitten by <italic>P. melanarius</italic> was not different to that of non-attacked control slugs. Attacking bites by <italic>P. melanarius,</italic> visited upon live slugs, did not yield slug DNA-positive results during molecular analysis of beetle foregut contents. (3) <italic>Pterostichus melanarius</italic> was not able to detect by olfaction the presence of live or 12 h-decayed dead <italic> D. reticulatum.</italic> (4) The feeding history of <italic>P. melanarius </italic> had a significant influence on subsequent prey selection. However, the effect interacted with an innate, overarching prey preference hierarchy. (5) A large-scale semi-field experiment identified that <italic>P. melanarius </italic> fed upon slugs, but the effect of predation pressure was not sufficient to induce negative growth in slug population density. The presence of alternative prey, and the increasing mass of individual slugs exerted rate-limiting effects on slug-predation by <italic>P. melanarius</italic>.

Insecticidal actions of Citrus aurantium

Siskos, Elias P. January 2004 (has links)
This thesis was aimed at the extraction, isolation and structure elucidation of insecticidally active secondary metabolites of Citrus aurantium plant parts for potential use either as commercial insecticides or as lead compounds. Fruits, leaves and shoots of C. aurantium were extracted in methanol and the chemicals recovered were fractionated by liquid-liquid partitioning using organic solvents of increasing polarity (petroleum ether, dichloromethane, ethyl acetate). Petri dish exposure bioassays revealed that only the petroleum ether fraction of the fruit methanol extract was toxic against Bactrocera oleae adults. The bioactive chemicals present in the fruits were produced and/or accumulated in the peels. Petri dish exposure and topical application bioassays revealed that a petroleum ether peel extract was toxic against B. oleae and Ceratitis capitata adults. In several cases, differences in susceptibility were revealed between the two species and between the two sexes of the same species. First gravity column fractionation of the peel extract revealed that only the F2 fraction was active, while fraction Ft has a synergistic effect. Further purification of the F2 fraction on the second gravity column, followed by HPLC, resulted in the isolation of three major compounds. The chemical structure of these components was elucidated by spectroscopic methods (UV, FTIR, GC-MS and *H NMR) and was assigned as osthol, bergapten and 6',7'- epoxybergamottin. Of the three isolated compounds, only the 6',7'-epoxybergamottin was active and its toxicity was the same to the synthesised 6',7'-epoxybergamottin. However, the synergism revealed between isolated bergapten and 6',7'- epoxybergamottin was not confirmed when isolated bergapten was replaced by synthetic bergapten, indicating that minor components present in the isolated bergapten were responsible for the synergistic effect. Crude or semi-purified peel extracts of C. aurantium may have potential for insect control. Moreover, 6',7'-epoxybergamottin has apparently never been reported to have deleterious effects on insects. At this early stage, the potential of 6',7'- epoxybergamottin as a lead compound for a new class of insecticides remains to be determined.

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