UBC Theses and Dissertations
The role of foliar glucosinolates in host plant resistance of oilseed rape and mustard to the Bertha armyworm and the diamondback moth McCloskey, Catherine A.
The relationships between host plant glucosinolates and feeding and growth of a polyphagous insect, the Bertha armyworm, Mamestra configurata Walker and field infestation levels of an oligophagous insect, the diamondback moth, Plutella xylostella L. were investigated. M. configurata reared on eight rape and mustard varieties had significantly different final weights by species order Brassica juncea <Sinapis alba < B. napus < B. rapa. In choice tests, B. juncea was least preferred while S. alba was significantly more attractive than rape to neonate larvae. Relative consumption and growth rates of fourth in star larvae were also reduced on B. juncea foliage. Neonate choice and fourth in star growth rates were negatively correlated to concentrations of isothiocyanate-releasing glucosinolates. In feeding tests with pure compounds in meridic diets, sinigrin and its metabolite, allyl isothiocyanate inhibited growth in a dose-dependent manner. No effects were observed with indole-3-carbinol, the metabolite of 3-indolylmethyl glucosinolate. Foliar isothiocyanate-releasing glucosinolates may provide some protection against polyphagous insects. Plant growth stage differences in the relative resistance of brassicaceous host plants were investigated by feeding B. juncea (relatively resistant) or B. rapa cv. Tobin (Canola) (susceptible) foliage of three plant growth stages (2-4) to M. configurata. Relative consumption rates did not differ significantly between the plant species. However, with B. rapa, relative consumption rate increased with advancing plant growth stage. Relative growth rates on B. juncea foliage were significantly lower than on B. rapa. With B. juncea, relative growth rate decreased with plant growth stage. Concentrations of isothiocyanate- and thiocyanate-releasing glucosinolatescor responded to the trends observed in the nutritional indices, while levels of total phenols and catechols appeared unimportant. Analyses of total nitrogen in field-collected plants showed a serial reduction with advancing growth stage. The influence of glucosinolate profile on infestation by P.xylostella was investigated using field plots of six rape and mustard varieties. S. alba plots supported larger populations than the other plant species in the study. Parasitism rates by Diadegma insularis (Hymenoptera: Ichneumonidae) were not significantly different between plant species on foliar samples. On flower head samples, larvae on B. juncea were parasitised the most, whereas larvae on S. alba were parasitised the least. The glucosinolate profile of the host plant may be more important to D. insularis than to P.xylostella.
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