Losses in seed yield and quality caused by infection with Beet western yellows virus (BWYV) alone or in combination with direct feeding damage by Myzus persicae ( green peach aphid) were quantified in field experiments with Brassica napus ( canola, oilseed rape) in the 'grainbelt' region of south-western Australia. Plants infected with BWYV and infested with M. persicae were introduced into plots early to provide infection sources and spread BWYV to B. napus plants. Insecticides were applied as seed dressings and/or foliar applications to generate a wide range of BWYV incidences in plots. Colonisation by vector aphids and spread of BWYV infection were recorded in the plots of the different treatments. At sites A ( Medina) and B (Badgingarra) in 2001, foliar insecticide applications were applied differentially at first, but, later, 'blanket' insecticide sprays were applied to all plots to exclude any direct feeding damage by aphids. When BWYV infection at sites A and B reached 96% and 100% of plants, it decreased seed yield by up to 46% and 37%, respectively. Also, variation in BWYV incidence explained 95% ( site A) and 96% ( site B) of the variation in yield gaps, where for each 1% increase in virus incidence there was a yield decrease of 12 ( site A) and 6 ( site B) kg/ha. At both sites, this yield decline was entirely because fewer seeds formed on infected plants. At site B, BWYV infection significantly diminished oil content of seeds ( up to 3%), but significantly increased individual seed weight ( up to 11%) and erucic acid content ( up to 44%); significant increases in seed protein content ( up to 6-11%) were recorded at both sites. In field experiments at sites B and C (Avondale) in 2002, insecticides were applied as seed dressings or foliar sprays. At site B, when BWYV incidence reached 98%, the overall yield loss caused by BWYV and direct M. persicae feeding damage combined was 50%. At site C, when BWYV incidence reached 97%, the overall combined yield decline caused by BWYV and direct feeding damage was 46%. This research under Australian conditions shows that, when aphids spread it to B. napus plantings such that many plants become infected at an early growth stage, BWYV has substantial yield-limiting potential in B. napus crops. Although the results represent a worst case scenario, the losses were greater than those reported previously in Europe and are cause for concern for the Australian B. napus industry. When applied at 525 g a. i./100 kg of seed, imidacloprid seed dressing controlled insecticide-resistant M. persicae and effectively suppressed spread of BWYV for 2.5 months and increased seed yield by 84% at site B and 88% at site C. Therefore, provided that mixing the insecticide with seed is suffficiently thorough, dressing seed with imidacloprid before sowing provides good prospects for control of BWYV and M. persicae in B. napus crops.