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Cabbage aphids

Numbers of cabbage aphids in canola crops are on the rise in some regions as temperatures gradually become warmer. Agronomist, Greg Toomey (Landmark), reports finding cabbage aphid colonies roughly 5-8 cm in length in several canola crops around Elmore, in the Northern Country district of Victoria. At this stage Greg says the colonies are sparsely distributed every few metres across the crops, however they will be closely monitored as numbers can rise rapidly under favourable weather conditions (i.e. warm-hot and dry days). On those plants with significant aphid colonies, Greg says pods above the bottom of the colony are virtually empty and contain no viable seed. Research agronomist, Simon Craig (BCG), has also observed low numbers of cabbage aphids in several canola crops between Birchip and Hopetoun, in the Mallee district of Victoria. Research consultant, Samantha Strano (cesar), has also observed low numbers of cabbage aphids in canola at a trial site near Charlton, in the Northern Country district of Victoria. 

Cabbage aphids (Brevicoryne brassicae) are one of several aphid species that can be problematic in canola crops. They are about 3 mm long and dull grey-green in colour. The characteristic behaviour of cabbage aphids is the formation of dense colonies on canola flowering spikes in spring. These colonies appear bluish-grey and are covered with a fine, whitish powder. Cabbage aphids suck sap and can reduce yields when numbers are high. Typically, cooler weather conditions and/or significant rainfall will slow the rate of aphid development, whereas warm and relatively dry conditions will favour the rapid build up of populations. Click here for images of cabbage aphids.

Other aphid species typically found in canola are turnip aphids (Lipaphis erysimi) and green peach aphids (Myzus persicae). Turnip aphids are superficially similar to cabbage aphids and also form dense colonies on the flowering spikes. Colonies may have a light covering of wax however this is much less obvious than the thicker powdery covering of cabbage aphid colonies. Green peach aphids have an oval shape body, are approximately 3 mm long and may be pale yellow-green, orange or pink in colour. Unlike cabbage aphids and turnip aphids, green peach aphids do not form dense colonies; instead they are usually found sparsely distributed on plants, and in canola they are most commonly seen on the undersides of the lower leaves.  

When checking for aphids in canola, it is important to check representative parts of the entire paddock, and to look for aphids on a minimum of 20 plants at each point. Control measures should be considered if more than 20% of plants are infested, however it is important to consider factors such as crop stage, soil moisture and forecast weather conditions as these can all influence aphid population build up and crop susceptibility to damage. If chemical control of aphids is required, selective insecticides (such as pirimicarb) are available, which are aphid specific and less harmful to other invertebrates.

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