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Cabbage aphids

Cabbage aphids (Brevicoryne brassicae) have been reported by agronomist, Ross Henley (Riverina Co-op), who has observed high numbers in many canola crops north-west of Wagga Wagga, in the South West Slopes district of New South Wales. The crops have finished flowering and Ross says dense colonies can be found on every flowering spike in some paddocks. Earlier sown crops have soaked up more moisture and are now struggling, and these appear to have higher numbers of aphids than later sown crops.

Agronomist, Steve Brabin (AGnVET services), reports finding aphids, which are likely to be cabbage aphids, in several canola crops around Temora, in the South West Slopes district of New South Wales. Steve says the crops have been suffering from moisture stress although aphid numbers are not at high enough levels to warrant spraying. These crops are likely to be harvested around mid-November and will need to be monitored closely in the coming weeks as aphid numbers can escalate very quickly under favourable conditions.

Cabbage aphids have been observed in several crops in the Mallee and Wimmera districts of Victoria. Agronomist, Kate McCormick (John Stuchbery and associates), has reported finding them in canola just north of Warracknabeal and also near Murrayville. Kate says she has only seen low numbers of aphids at this stage and no infestations have warranted spraying. Research agronomist, Simon Craig  (Birchip Cropping Group), has also reported cabbage aphids in fairly high numbers in several canola crops around Birchip.

Cabbage aphids grow up to 3 mm in length and have a dull grey-green coloured 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. This can sometimes lead to black sooty mould, thereby reducing the plants’ ability to photosynthesize and decreasing plant growth.

Chemical control should be considered when 20% of plants are infested. If plants are stressed, the attack may be more substantial and treatment should be considered if plants show signs of wilting. Treatment for many crops may not be warranted or could be delayed to coincide with other pest treatments (e.g. native budworm). If chemical control is necessary, use ‘softer’ chemicals, which are less harmful to beneficial insects. Predators and parasites provide a natural way of suppressing aphid numbers, and can be particularly effective in canola crops during spring.

Click here for images of cabbage aphids and refer to PestFacts Issue No. 7 for more information. For information on managing aphids in canola, click here.

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