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Cabbage aphids

Agronomist, Warwick Nightingale (Landmark), has reported aphids in several canola crops around Wagga Wagga, in the South West Slopes of New South Wales. Warwick says the aphids, which appear bluish-grey in colour, can be seen in dense clusters on the flowering spikes of plants. These are likely to be cabbage aphids (Brevicoryne brassicae), which are a common pest of canola in spring. At this stage the overall numbers within the crops are relatively low and Warwick 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, 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 decreasing plant growth. Greater aphid build-up leading to higher yield losses are more likely when crops suffer moisture 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 infestations. Crops at this vulnerable stage should be checked several times a week for aphids in case numbers escalate enough to cause economic damage. 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% of plants are infested, control measures should be considered to avoid yield losses.

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.

For images of cabbage aphids, click here.

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