Cereal aphids

Agronomist, Ben Cornes (Tylers Hardware & Rural Supplies), reports finding moderate numbers of aphids in a wheat crop near Rupanyup, in the Wimmera district of Victoria. The crop was sown quite early and is presently at the flag-leaf stage. Ben says over 20 aphids per tiller were found in some parts, however there are no visible signs of feeding damage. The paddock has not received any insecticide applications this season.

There are several aphids that colonise cereals, however the most common species in south-eastern Australia are oat aphids (Rhopalosiphum padi) and corn aphids (Rhopalosiphum maidis). Both species are capable of attacking crops at any stage. They 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. Oat aphids are similar in colour but have a pear shaped body with a rusty red patch at the base of the abdomen. 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.

The impact of aphids on winter cereals (yield and quality) is the result of a complex relationship that involves the timing of infestation, aphid density, where on the plant the aphids feed and crop physiology at the time of infestation. There is no impact on yield after grain has filled and is maturing. Infestations that occur during booting to milky dough, particularly where aphids are colonising the flag leaf, stem and ear, can result in yield loss. Heavy infestations cause the crop to turn yellow, become stunted and generally appear unthrifty. Small or moderate aphid populations have less impact and will often be controlled by natural enemies, although speed of control can be a problem.

Both oat aphids and corn aphids vector barley yellow dwarf virus (BYDV), one of the most important diseases of cereals. BYDV can cause significant losses in cereal crops, particularly when transmission occurs in young plants. Effectively controlling summer weeds is a good way to prevent the build up of aphid numbers as it removes the ‘green-bridge’ between cropping seasons. District agronomist, Paul Parker (I&I NSW), says BYDV has been particularly severe this season across much of the South West Slopes district of New South Wales.

For both species, a commonly used threshold is to consider control when there are 10-20 aphids on 50% of tillers. However, it is important to remember that any economic threshold should be considered to be a flexible guideline only, and is likely to vary with factors such as the crop growth stage, weather conditions and crop vigour.