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Cereal aphids

Agronomist, Phil Stoddart (Landmark), has reported observing aphids in numerous cereal crops around Mudgee, in the Central Tablelands district of New South Wales. Phil says the highest numbers have been found in early-sown crops, particularly oats. Although many paddocks were sown with insecticide-treated seed, several crops have also had an application of a ‘anti-feed’ synthetic pyrethroid. Now that insecticide sprays are wearing-off, aphids are reported to be re-invading many paddocks. Phil says aphid numbers have increased over the last month, with up to 20-30 aphids per plant observed in some areas. The mild winter conditions experienced around Mudgee have favoured aphid population growth.

The most common aphids in New South Wales and Victoria attacking cereals are the oat aphid (Rhopalosiphum padi) and corn aphid (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. A third species, the rose-grain aphid (Metopolophium dirhodum), can be a problem in certain regions in some years.

The impact of aphids on winter cereals 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 very little 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.

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 as a flexible guideline only, and is likely to vary with factors such as the crop growth stage, weather conditions and crop vigour. 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.

Oat aphids and corn aphids vector barley yellow dwarf virus (BYDV), one of the most important diseases of cereals. The spread of BYDV by aphids is damaging when aphids transmit the virus early in the first 8-10 weeks following emergence. Symptoms of BYDV include yellowing, reddening and pale striping of leaves; infected plants are often stunted. These symptoms can be confused with nutrient deficiencies and other plant stresses. Phil says BYDV has been a widespread issue in cereal crops around Mudgee this season.

Although uncommon, other aphid species can occasionally be found on cereal plants. We recently identified green peach aphids (Myzus persicae) collected from wheat tillers in a paddock east of Deniliquin, in the Riverina district of New South Wales. There were no signs of feeding damage. Green peach aphids are a common pest on pulses, canola and other brassica crops; they are not known to cause damage to cereals. In this instance, aphids survived over summer/early autumn on volunteer canola present within the paddock, and then transferred onto emerging wheat seedlings. Green peach aphids are unable to effectively colonise cereal plants, and their numbers will naturally decline because of this.

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