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Green peach aphids

Agronomist, Nick Martin (Dodgshun Medlin), has observed green peach aphids (Myzus persicae) in a canola crop south of Mildura, in the Mallee district of Victoria. Very high aphid densities have been observed along the edge of the paddock, mostly on the undersides of leaves. Nick says the aphid numbers elsewhere in the paddock are considerably lower and that chemical control is not warranted at this stage. Other common aphids found in canola crops are cabbage aphids and turnip aphids. These aphids typically colonise the terminal end of the flower spikes.

Cool, wet conditions help keep aphid numbers to a minimum over winter, but with the coming warmer weather canola aphids will become more noticeable in crops. Growers should start to keep an eye out for aphids, as well as the beneficial insects that assist in keeping pest populations in check.

There is still some uncertainty around the economic benefit of controlling canola aphids in spring. Numerous research trials have shown that moderate aphid populations in spring have very little or no effect on canola yield, oil content and protein content, particularly when growing conditions are favourable. High-risk situations arise when: aphid infestations rapidly increase during early flowering to bud formation, conditions are warm and dry, and there is little beneficial activity.

Chemical control should be considered if more than 20% of canola plants are infested. When determining economic thresholds it is critical to consider several other factors before making a decision. Most importantly, the current growing conditions and moisture availability should be assessed. Crops that are not moisture stressed have a greater ability to compensate for aphid damage and will generally be able to tolerate far higher infestations than moisture stressed plants before a yield loss occurs. It is also important to note that aphid infestations are typically patchy, and often heavier on crop edges. Ensure that crop sampling covers a representative area of the crop, with a minimum of 20 plants examined at 2-3 locations within the crop.

If chemical control is required, be aware of the species present and keep in mind that green peach aphid populations have widespread resistance to several insecticides, including synthetic pyrethroids, organophosphates and carbamates (e.g. pirimicarb). See PestFacts Issue No. 6 for further details on emerging insecticide resistance issues in this species.

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