Cereal aphids

Aphid populations have continued to increase across many regions and are currently being found in high densities in a large number of cereal crops, particularly wheat. Agronomist, Warwick Nightingale (DeltaAg), has reported high numbers of aphids in several wheat crops around Lockhart, in the Riverina district of New South Wales. Consultant, Glen Smith (3D-Ag), has reported high numbers of aphids in wheat crops around Wagga Wagga, in the South West Slopes district of New South Wales. Glen says he cannot recall seeing such high populations of aphids this early in spring.

Agronomist, Andrew Reardon (JJS Glass & Co), reports aphids in a wheat crop near Junee, in the South West Slopes district of New South Wales. Andrew says the highest numbers have been found along fence-lines and paddock edges, where up to 70-80 aphids per tiller were observed. Agronomist, Dan Walsh (AGnVET Services), says cereal aphid populations have been building up across many paddocks around West Wyalong, in the Central West Slopes and Plains district of New South Wales.

In Victoria, agronomist, Chris Dunn (Landmark), has reported large numbers of aphids infesting wheat crops around Elmore in the Northern Country district. Up to 100 aphids per tiller were found in some areas. Cereal aphids have also been reported in several wheat crops near Donald, in the Wimmera district. Cereal aphids are likely to be prevalent in many other districts.  

There are several aphids that colonise cereals, however the most common species in Victoria and New South Wales are oat aphids (Rhopalosiphum padi) and corn aphids (Rhopalosiphum maidis). These species 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 rust red patch at the base of the abdomen. A third species, the rose-grain aphid (Metopolophium dirhodum), is found on all cereals but is less common in southern regions. This season, oat aphids are responsible for the majority of issues reported.

During late winter and spring, aphids can have a direct impact on winter cereal yields but virus transmission is no longer a risk. The risk of feeding damage is influenced by a complex relationship that involves the timing of infestation, aphid density, where on the plant the aphids feed and crop stage 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 do not often require chemical intervention.

Beneficial insects, such as hoverflies, ladybirds and parasitic wasps, will naturally control aphid populations in cereals. There can be a lag for the beneficial insects build up, which can take anywhere between 2-6 weeks. Organophosphates and synthetic pyrethroids are highly disruptive to beneficial species. Applying these insecticides at this time may result in later reinfestation of the crop because beneficial insects will no longer control small numbers of surviving aphids.

Growers are encouraged to accurately assess aphid numbers within paddocks, and to repeat inspections at regular intervals. Sampling should occur away from paddock edges to ensure an unbiased estimate is obtained. Inspect 10-20 plants at six sites within a paddock, counting the number of aphids on individual tillers rather than whole plants. Repeated sampling will allow growers to understand the rate of change of aphid populations and beneficial insects in their crop. It is this rate of change that is most informative when determining the need to spray.

For post-tillering aphid infestations an approximate threshold is 15-20 aphids per tiller with 50% of tillers infested. However for earlier infestations (<5 tillers/plant) the threshold is reduced to an average of 10 aphids per tiller on 20% of tillers. It is important to consider these thresholds as flexible guidelines as they vary with factors such as the crop growth stage, weather conditions and crop vigour. Aphids have the greatest impact on moisture-stressed crops, with large populations exacerbating the impact of the lack of moisture.

Click here for further information on aphid management in cereals, and here for information on beneficial insects to look out for this spring.