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Cabbage aphids

Cabbage aphids (Brevicoryne brassicae) have been found on a canola crop near Ouyen, in the Mallee district of Victoria. Agronomist, Ian Cocking (North West Ag Services), says the aphids can be seen in dense colonies on flowering canola heads. At this stage the overall numbers within the crop are relatively low and Ian says spraying is not warranted, although the numbers will be carefully monitored.

Cabbage aphids grow up to 2.5 mm in length, with a dull grey-green body. Infestations start when winged aphids fly into the crop from autumn weeds. These give rise to dense colonies on the flowering spikes, which appear bluish-grey and are covered with a fine, whitish powder.

Cabbage aphids suck sap and can reduce yield when numbers are high. A large amount of sugary solution is secreted; sometimes leading to black sooty mould, thereby reducing the plants’ ability to photosynthesize and generally decreases plant growth. Greater aphid build-up leading to higher yield losses are more likely when crops suffer drought stress.

Canola is particularly susceptible to aphid damage during bud formation through to late flowering, therefore, it is important to control aphids in spring to prevent heavy infestation. Crops at this vulnerable stage should be checked several times a week for aphids in case numbers escalate enough to cause economic damage. Start regular monitoring from late winter and continue through to early spring. It is important that representative parts of the entire paddock are sampled. Check at least five points of the paddock, and look for aphids on a minimum of 20 plants at each point. If more than 20 per cent of plants are infested, control measures should be considered to avoid yield losses.

Management strategies to reduce aphid populations in canola include: (1) control of alternative hosts, such as broadleaf weeds (especially turnip weed and wild radish); (2) sowing into standing cereal stubble (if possible); and (3) if chemical control is necessary, spraying at the crop perimeter may be adequate as population buildup generally starts at crop edges.

Predators and parasites should be encouraged as a natural way of suppressing aphid numbers. These are a very reliable form of control during the warmer days of spring and when low to moderate numbers of aphids are present. If chemical control is necessary, use ‘softer’ chemicals (such as pirimicarb) which are aphid-specific and less harmful to other insects.

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