In many areas, rainfall produces a green bridge (a between-season host for pests and diseases) that includes volunteer canola, wild radish, wild turnip and marshmallow.
These weeds and volunteers can host large populations of insects, including aphids, as well as viruses, and need to be destroyed prior to sowing.
Widespread infestations of green peach aphids (GPA) (Myzus persicae) contributed to an outbreak of Beet Western Yellows Virus (BWYV, syn. Turnip yellows virus) in southern Australia during autumn and winter 2014. BWYV is not seedborne and is transferred into canola crops by aphids carrying the virus.
Several aphid species transmit BWYV, but GPA is the principal vector and the most common species on juvenile canola plants.
Early management decisions are key to reducing aphid and virus risk in 2015
• Destroy the green bridge over summer and autumn prior to sowing.
• Do not sow canola into desiccating weeds/canola volunteers; otherwise, aphids will move directly from the weeds to the emerging seedlings.
• Monitor green peach aphid (GPA) populations on weeds, volunteers and seedling canola crops.
• Use seed treated with a neonicotinoid insecticide and ensure proper application and coverage of seed dressing for efficacy against GPA.
• Ensure correct identification of GPA before applying insecticides to protect seedling canola. Although further rainfall and mild temperatures during summer and early autumn are necessary for an increase in GPA populations and associated virus risk, early weed control will minimise the risk.
Growers should continue to pay attention to weather conditions (rainfall and mild temperatures) leading into the winter growing season because this will provide a good indication of the risk of GPA and BWYV.
In 2014, canola crops across the Lower North, Mid North and Eyre Peninsula in South Australia, western Victoria and some parts of New South Wales were severely affected by the BWYV, leading to significant yield losses.
The severity of the BWYV outbreak was most likely due to a combination of the following factors:
• summer rainfall, which resulted in a green bridge of weed hosts for aphids and BWYV
• the early start to the season and early sowing times
• mild autumn conditions, which contributed to early (and extended) levels of aphid activity through to late June
• crop management practices, including short intervals between weed control and sowing time
• the prevalence of insecticide resistance in GPA (particularly to pyrethroids, organophosphates and carbamates)
• in some areas, the low proportion of canola seed treated with neonicotinoid insecticide, e.g. Gaucho® (imidacloprid) and Cruiser® Opti (thiamethoxam + lambda-cyhalothrin)
• poor coverage of insecticide on canola seed, which reduced efficacy.