Doctoral Degrees (Zoology and Entomology)
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Browsing Doctoral Degrees (Zoology and Entomology) by Subject "Aphelinus hordei"
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Item Open Access Parasitoids and aphid resistant plants : prospects for Diuraphis noxia (Kurdjumov) control(University of the Free State, 2007-11-03) Prinsloo, Godfried Jacob; Van Der Linde, T. C. de K.; Van Der Westhuizen, A. J.; Potting, R. P. J.English: Host plant resistance and biological control by means of natural enemies are becoming more favourable as high potential alternatives for chemical control of insect pests. Tritrophic studies (plant–herbivore-natural enemy) indicated that the application of host plant resistance and biological control to a particular pest could give significantly better or worse results than expected from each component respectively. Russian wheat aphid Diuraphis noxia (Kurdjumov) is a serious pest of wheat in South Africa since 1978. Plant resistant cultivars are being used against D. noxia since 1992. The introduction of parasitoids and predators for biological control of this pest automatically led to the development of an integrated pest control programme involving both control strategies. Nothing is known about interactions between resistant cultivars, D. noxia and natural enemies in South Africa. These interactions could have substantial influence on the efficacy of the control programme. The parasitoid Aphelinus hordei (Kurdjumov), introduced from the Ukraine, established in the Lesotho highlands after being released in the wheat production areas of the Free State Province. This parasitoid together with a native parasitoid Diaeretiella rapae (McIntosh), also parasitising D. noxia in South Africa, was included in a study on tritrophic interactions. A. hordei and D. rapae respectively have narrow and wide host ranges. Field studies on the interaction between A. hordei and resistant and susceptible cultivars indicated reduction in aphid population growth on each of the cultivars. Diuraphis noxia was highly parasitised on a susceptible cultivar Betta, while a positive interaction on resistant Gariep occurred, resulting in the enhancement of the resistance. A slightly lower percentage control was found on SST 333 in the presence of A. hordei. Volatile profiles emitted by infested Betta plants and resistant Elands and SST 333 plants, differed qualitatively (different volatiles) and quantitatively (concentration of volatiles). These differences caused behavioural differences between parasitoid species e.g. A. hordei could not distinguish infested from clean Elands, while D. rapae did. Diaeretiella rapae could not distinguish infested from clean SST 333 while A. hordei did. Aphelinus hordei could not distinguish between infested Betta and Elands, while D. rapae significantly preferred Betta to Elands. Parasitoids therefore responded to different volatiles from the same cultivars. This means that both A. hordei and D. rapae, use different volatiles or volatile combinations from the same plant-herbivore combination in host habitat location. Semiochemicals e.g. methyl salicylate, that act as insect behaviour-modifying chemicals, was tested in the laboratory and the field as potential control options against D. noxia. Volatile compounds released by plants could serve as signals attracting beneficial insects and induce a variety of responses in plants. A slow release wax pellet formulation named OX54 releasing methyl salicylate, menthol and 1,8-cineole was tested. Olfactometric studies showed that D. noxia and R. padi was repelled by each of the compounds although not released by their alternate host plants in South Africa. Both parasitoid species were repelled by some of the semiochemicals tested, but differences occurred between the two species. Aphelinus hordei did not respond to 1,8 -cineole, while D. rapae was not responding to menthol indicating that the different parasitoids differ in sensitivity to other semiochemicals. The repellence of the parasitoids by methyl salicylate indicated that these volatiles are not induced by D. noxia when feeding on wheat. Different host range of the parasitoids may be a reason for this reaction. OX54 and methyl salicylate respectively caused a delay in the immigration of D. noxia into resistant cultivar Elands during field trials, but on susceptible Betta an increase in infestation was found. Lower infestation on treated Elands resulted in a slight increase in yield compared to the control. The positive integration of host plant resistance and biological control can have two objectives namely synergistic reduction of pest densities and the protection of durability of resistance. Some resistant wheat cultivars to D. noxia seem to be highly resistant and the efficacy of natural enemies in these wheat fields is of utmost importance. The application of semiochemicals in these cases should also be investigated . Where resistant cultivars are less effective and hence have more durable resistance, the value of biological control is to enhance the effect of plant resistance on the reduction of the pest population in such a way that the effects of plant resistance and biological control are sufficient to prevent damage. The understanding and effective manipulation of agro -ecosystems in the wheat production areas of the Free State Province is therefore essential for the successful establishment of a successful integrated pest control programme.