Oat aphids & cabbage aphids

In the last edition of PestFacts, there were many reports of oat aphids (Rhopalosiphum padi) and cabbage aphids (Brevicoryne brassicae) building up in numbers across the Mallee, Victoria. Agronomist, Warwick Nightingale (Landmark), says oat aphids have also recently been found attacking cereal crops around Wagga Wagga, in the south west slopes of NSW. Warwick says some isolated paddocks have required chemical application to suppress aphid numbers and limit the amount of feeding damage. Organophosphates have been applied due to concerns about the development of resistance to synthetic pyrethroids. Consultant, John Robertson (AgWise Services), has also reported oat aphids attacking cereal crops in the Wimmera and southern Mallee regions of Victoria.

Oat aphids (sometimes called ‘cereal aphids’ or ‘wheat aphids’) can be found on all cereals including wheat, barley and oats. They vary in colour from olive-green to black and are characterised by a dark reddish patch on the tip of the abdomen. Adults are pear-shaped and have antennae which extend half the body length. Importantly, oat aphids can spread barley yellow dwarf virus.

Oilseeds Development Officer, Felicity Pritchard, has reported cabbage aphids on a canola crop, near Horsham, in the Wimmera, Victoria. The numbers are relatively low at this stage, and Felicity says mummified aphids and hoverflies have also been observed in the crop. This suggests natural enemies, which increase in abundance at this time of year, could act to suppress aphid numbers to below economic thresholds. Bronwyn Hunt (Merriwa Pastoral Company), also reports cabbage aphids in a canola crop near Kerang, in the south-east corner of the Mallee, Victoria.

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. They gather to form 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 which reduces 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.

For further information on oat aphids, barley yellow dwarf virus and cabbage aphids, refer to PestFacts Issue No. 9.