Corn aphids and oat aphids

Corn aphids (Rhopalosiphum maidis) have been identified for agronomist, John Ronan (Elders), from a barley crop near Coleambally, in the Riverina district of New South Wales. The crop has been irrigated and John says up to 100 aphids per flag leaf could be found across the paddock. It is possible that the aphids moved into the crop earlier in the season from a nearby paddock which had a maize crop over summer. Aphids have also been found in several nearby crops, however the species have not been confirmed.

Agronomist, Sandy Biddulph (Biddulph Rural Consulting), has reported finding oat aphids (Rhopalosiphum padi) in a triticale crop near Cootamundra, in the South West Slopes district of New South Wales. The aphids are most prevalent around the outer edges of the paddock. Oat aphids have also been reported on wheat crops west of Kerang, in the Victorian Mallee, and Shepparton, in the Northern Country, Victoria.

Corn aphids and oat aphids (sometimes called ‘cereal aphids’) are significant pests of cereal crops and grasses in southern Australia. These two species are similar in appearance but can be distinguished in the field relatively easily. Corn aphids have an oblong shaped, light green to olive coloured body with two dark areas on the abdomen near the base of the cornicles. They have antennae, which extend about one-third the body length. Oat aphids are similar in colour but have a pear shaped body with a rusty red patch at the end of the abdomen, and have antennae about half their body length. Corn aphids tend to occur mostly on barley, whereas oat aphids are generally found on oats and wheat; but both species may attack all cereals. Both corn aphids and oat aphids can attack crops at any stage and heavy infestations can result in yield losses. Click here for images of corn aphids and here for images of oat aphids.

Monitoring crops for the presence of aphids should now be underway, particularly as the weather becomes warmer which can have a strong influence on aphid development and population growth. Representative parts of the entire crop should be checked and if chemical control is warranted, selective insecticides are available which are less harmful to beneficial insects. Interestingly, high numbers of aphid ‘mummies’ have been observed in many of the aphid samples received. These are the result of beneficial parasitic wasps, which lay their eggs into the bodies of live aphids. The developing larva feed inside the aphid, eventually killing it. Other beneficial natural enemies likely to be building-up in numbers include lacewings, hover flies and lady beetles, and these can significantly suppress aphid populations when present in high numbers.