The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is an important pest of brassicaceous oilseed crops, especially canola (Brassica napus L. and Brassica rapa L.) in North America and Europe. Application of foliar insecticide is the only method currently employed to control C. obstrictus populations; because this approach is environmentally unsustainable, alternatives including host plant resistance have been explored. White mustard, Sinapis alba L., is resistant to C. obstrictus and was chosen as a potential source of resistance for B. napus oilseed. Interspecific crosses of S. alba x B. napus have produced several lines that are resistant to C. obstrictus feeding and oviposition and yield fewer, lighter-weight weevil larvae that take longer to develop. I investigated potential mechanisms of this resistance, including assessing differences in visual and olfactory cues among resistant and susceptible genotypes, and antixenosis and antibiosis. Determining effects of visual cues associated with host plant resistance required investigation of weevil vision. Deployment strategies for resistant germplasm were assessed to evaluate incorporation of susceptible refugia to promote long-term durability of resistance traits. Results reported in Chapter 2 indicate that the C. obstrictus visual system is apparently trichromatic and incorporates receptors with response maxima near 350, 450, and 550 nm. Modelling indicated that UV light alone reduced weevil responses but the interaction of yellow and UV light increased responses at a threshold reflectance level of UV. Results reported in Chapter 3 indicated that differences in yellow and UV reflectance among host plant flowers influence host selection in C. obstrictus. Results described in Chapter 4 determine differential attraction to the odours of S. alba and B. napus and among resistant and susceptible accessions. Inferences of the identities of glucosinolates found in varying amounts among susceptible and resistant genotypes suggested that 2-phenylethyl glucosinolate influenced attractiveness. Results described in Chapter 5 indicate differences in adult feeding and oviposition preferences among resistant and susceptible genotypes. Oocyte development, larval biomass and larval development time varied among weevils feeding on resistant and susceptible genotypes. Based on results of Chapter 4, 1-methoxy-3-indolylmethyl glucosinolate was implicated as contributing to antixenosis and antibiosis resistance. Results reported in Chapter 6 describe effects of mixed plots of resistant and susceptible genotypes on weevil spatial distribution and oviposition. These results are consistent with associational resistance and attributed to reduced apparency of susceptible plants in mixtures and antixenosis resistance associated with resistant germplasm. / Plant Science

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The effects of nitrogen and sulfur fertilization on the oviposition, feeding preferences, and larval performance of Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae) on Brassica napus L. were examined in a series of laboratory experiments. The associations between C. obstrictus adults, larvae and parasitoids, and environmental factors including plant vigor indicators and soil and plant nutrients were evaluated within two commercial fields of B. napus in southern Alberta. Nitrogen fertilization, and sulfur fertilization at low levels of nitrogen fertilization had positive effects on oviposition preference. Nitrogen had a positive effect on larval development times and no effect on larval weights. Within one field, gravid C. obstrictus females were dissociated with high levels of plant nutrients including nitrogen. The synthesis of the lab and field experiments seems to support the plant stress and the preference-performance hypotheses. Differences in olfactory and visual cues are identified as a possible mechanism for the observed differences. / Ecology