| Summary: | In this thesis I address the question: does breeding to improve nitrogen use efficiency of winter wheat, Triticum aestivum, increase its susceptibility to infestation by cereal aphids? Laboratory and greenhouse experiments were used to monitor population performance of the aphids Sitobion avenae and Metopolophium dirhodum on double haploid genotypes of a Savannah cross Rialto (SavRia) wheat population. The genotypes varied in nitrogen use efficiency and its two components: nitrogen uptake efficiency and nitrogen utilisation efficiency. A quantitative trait loci analysis, using 94 genotypes grown under greenhouse and laboratory conditions, identified genetic markers associated with individual performance and population measures of S. avenae and M. dirhodum. Twelve markers across chromosomes on the SavRia genetic map were statistically associated with nine of the fourteen aphid performance traits measured. Nine wheat genotypes, representing the range of nitrogen use efficiencies, were used to analyse how aphid development rate, fecundity and intrinsic rate of increase, varied between genotype and whether settling behaviour and preference between genotypes, reflected differences in performance. Significant differences in performance of S. avenae between different genotypes were found but performance was not correlated with plant nitrogen uptake. In a randomised split plot experiment in the field, six SavRia genotypes and both parental varieties were grown under three application rates of nitrogen fertiliser. Significant variation between genotypes and between different nitrogen application rates was observed for eight plant traits, including both nitrogen uptake and utilisation efficiencies. Density of S. avenae differed significantly between genotypes with significant genotype x nitrogen application rate interactions. Aphid densities were not significantly correlated to either components of nitrogen use efficiency. I conclude that this study does not support the hypothesis that increasing nitrogen use efficiency, or either of its primary components, increases the susceptibility of winter wheat to infestation by cereal aphids.
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