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Cereal aphids gain a foothold before spring

Oat and corn aphids aren’t hard to spot at the moment, but think twice before reaching for the broad-spectrums…

Where have they been reported?

Often collectively referred to as cereal aphids, oat aphids (Rhopalosiphum padi) and corn aphids (Rhopalosiphum maidis) are building up in cereal crops in many regions of south-eastern Australia. High populations of oat aphids are in wheat and barley crops in the Condobolin region of the NSW Central West Slopes and Plains. Reports of oat aphid have also arrived from the NSW Riverina, including barley crops northeast of Albury and near Lockhart. Corn aphids have also been observed in cereal crops in this region.

In Victoria, oat and corn aphid activity has been reported by agronomists and observed by cesar consultants in cereals across many locations in the Mallee, Wimmera and North East. Some populations of oat aphids have been found cohabiting crops with Russian wheat aphid (RWA, Diuraphis noxia). Oat aphid populations greater than 100 aphids per plant have been observed in numerous wheat paddocks around Elmore in Victoria’s Northern Country. 

About cereal aphids

Oat aphids have a pear-shaped body and are olive-green to almost black in colour. They have distinctive rusty red patches around the aphid’s two ‘exhaust pipes’ or cornicles located toward the tail end of its body. The antennae extend about half the length of their body.

Corn aphids have an oblong shaped, light green to olive-coloured body with two dark areas near the base of the ‘exhaust pipes’.

While oat aphid and corn aphid are the two most prevalent cereal aphids in south-eastern Australia, rose-grain aphids (Metopolophium dirhodum) can also infest most cereals and some grasses. They have green-yellow bodies with a darker green stripe down the middle of the back. Rose-grain aphid could at first glance be mistaken for RWA. However the key is to take note of the aphid’s ‘exhaust pipes’. These are visible to the naked eye on rose-grain aphids but not on RWA. 

Oat aphids are globular in shape (left), while corn aphids are oblong-shaped (right). Note the typical red colouration near the ‘exhaust pipes’ of oat aphids (Source: cesar).

 

Visit our PestNotes on oat aphid and corn aphid for more information.

Impact of aphid feeding

Oat and corn aphids can invade crops at any time between seedling stage and grain fill. Early infestations can cause reduced tillering, stunting and early leaf senescence. Later infestations on leaf sheaths and flag leaves between booting and the milky dough stages can also result in yield losses. In some cases, aphid colonies infest the seed heads and congregate in large numbers. After grain fill, aphid feeding has minimal or no impact on yield. Secretion of honeydew can cause secondary fungal growth, which inhibits photosynthesis and can decrease plant growth.

There have been numerous reports received this year that suggest crops treated with insecticide seed coatings (e.g. imidacloprid) have considerably fewer cereal aphids at present.

Our advice

The recent surge in cereal aphid numbers can likely be attributed to milder than average temperatures this winter. Healthy, unstressed cereal crops can sustain relatively large populations of cereal aphids before yield losses from direct feeding occur. For example, an established economic threshold for winter cereal crops is 15 aphids per tiller on 50% of tillers, where yields are expected to surpass 3 t/ha. Given the current forecast conditions are likely to favour the ongoing build up of aphids during tillering and stem elongation, chemical controls may be warranted before populations peak.

However, keep in mind that corn aphid populations naturally decline as the crop starts to boot and approach heading. Similarly, host suitability may decline as cereal crops become reproductive with respect to oat aphid infestations (see The BeatSheet for further information).

If spraying is required, we strongly recommend using selective insecticides (e.g. pirimicarb) instead of broad-spectrums as they are less harmful to beneficial insects and therefore unlikely to induce a secondary pest outbreak. This is particularly relevant at the moment given it is still relatively early in the season. Beneficials invariably play an active role in keeping aphid and caterpillar (e.g. armyworm) populations in check. 

 

Sources of field reports of cereal aphids

Chris Baker – Agronomist, Baker Ag Advantage (Central West Slopes and Plains NSW)

Rebecca Bingley – Agronomist, Landmark (Riverina NSW)

Pat Connell – Consultant, PC Agronomy, (Riverina NSW)

Simon Craig – Consultant, Agronomise, (Victorian Mallee)

Andrew McMahen – Agronomist, Landmark (Victorian Mallee)

Luke Maher – Agronomy consultant, AGRIVision Consultants (Victorian Mallee)

Warwick Nightingale – Agronomist, Delta Ag (Riverina NSW)

Greg Toomey – Senior Agronomist, Landmark (Victorian Northern Country)

Matt Watt – Agronomist, Baker Ag Advantage (Central West Slopes and Plains NSW)

 

Twitter sources

Grassroots Agronomy (@grassrootsag)

Barry Haskins (@agrobaz)

Rick Rundell-Gordon (@rick_rundell)

Matt Witney (@DMAg_consultant)

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