Cereal aphids still patchy

Some early reports of cereal aphids in NSW (but not Victoria) 

Where have they been reported?

Corn aphids have been found in large numbers in a 1-2 leaf wheat crop near Forbes in the NSW Central West Slopes and Plains. The seed had been treated with imidacloprid but despite this the aphids were in relatively high densities (>8 aphides per leaf) and were widespread across the paddock. Neighbouring cereal paddocks had very few aphids. The same paddock was sown to wheat in 2015 and had a summer crop of sorghum, providing an excellent green bridge from which aphids could persist and aggregate.

Very low numbers of cereal aphids have been observed in the Victorian Wimmera and Mallee.

About cereal aphids

The two aphid species commonly found in cereal crops in south-eastern Australia are the oat aphid (Rhopalosiphum padi) and the corn aphid (Rhopalosiphum maidis). Although oat aphids are mostly found on oats and wheat, and corn aphids are more common on barley, both species may attack all cereals. Rose-grain aphids (Metopolophium dirhodum) also occasionally appear in wheat.

Unlike Russian wheat aphid, both oat aphids and aphids have a prominent pair of siphuncles (‘exhaust pipes’), which are tube like projections from the base of the body. Both aphids 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 siphuncles (see photo above). Oat aphids have a pear-shaped body and are olive-green to almost black in colour with a rusty red patch at the end of the abdomen. The antennae of corn aphids extend about a third of the length of their body, while the antennae of oat aphids extend about half the length of their body.

Corn aphid (left) and oat aphid (right); notice the oblong shape of the corn aphid and the red rusty patch on the abdomen of the oat aphid (Source: cesar)

 

Cereal aphids are important vectors of Barley Yellow Dwarf Virus (BYDV), which can be detrimental to crops, particularly when infection occurs early in the season. The risk of virus is typically increased in higher rainfall regions (>500 mm p.a.), following good pre-season rainfall, or when virus was an issue the previous spring. In those circumstances, taking steps to reduce early aphid colonisation and the risk of virus spread, such as seed treatments, may be justified. Note that the presence of aphids in crops does not necessarily mean crops will become infected with virus.

For comprehensive information on cereal aphids, including their occurrence, lifecycle, behaviour and management strategies, go to the corn aphid and oat aphid PestNotes.

Our advice

The risk of aphids building up and causing feeding damage in cereal crops from now on over winter is low. We do recommend monitoring cereal crops for the presence of aphids, especially when the weather becomes warmer.

The decision on the need for and timing of a spray is complex and involves trade-offs. Spraying early will manage populations before they cause too much feeding damage. But the use of broad-spectrum insecticides, particularly so early in the season, will destroy beneficial insects and may result in a secondary, potentially more destructive infestation later in winter or spring.

If chemical control is warranted, selective insecticides (e.g. pirimicarb) are recommended as they are less harmful to beneficial insects and therefore unlikely to induce a secondary pest outbreak. Beneficials invariably play an active role in keeping aphid populations in check, even though their presence is often overlooked.

Identifying aphids using their colour alone is often misleading because aphid species often have a diverse range in colour. A Back Pocket Guide containing descriptions of 11 key crop aphid species found in Australian broadacre systems has been produced by cesar and GRDC. Keep the guide on your phone or tablet by downloading it from: http://www.grdc.com.au/CropAphidsBackPocketGuide

 

Sources of field reports of cereal aphids

Matt Bissett – Agronomist, AGRIvision Consultants (Victorian Mallee)

Jim Cronin – Agronomist, Landmark (NSW Central West Slopes & Plains)

Simon Mock – Agronomist, Clovercrest Consulting (Victorian Wimmera)

John Stuchbery – Agronomist, AGRIvision Consultants (Victorian Wimmera)