Aphids aplenty!

Aphid populations are thriving in some crops this spring. 

Cabbage aphids on the upper stem of canola (Source: cesar).

 

Canola

Aphids reproduce a lot faster during spring than in the colder months of the year. In early October, large clouds of aphids were observed arriving in the NSW Riverina. Around Lockhart and Wagga Wagga, high numbers of cabbage aphid (Brevicoryne brassicae) and turnip aphid (Lipaphis pseudobrassicae) have been reported colonising the upper stems and terminal spikes of canola. Paddock infestations up to 60 – 70% have been reported. Large numbers of green peach aphid (Myzus persicae, GPA) have also been found colonising the underside of canola leaves near Henty.

But thankfully not all species are equally problematic in flowering canola. During spring, cabbage aphid and turnip aphid are the primary concern as these aphids form dense colonies on floral parts of plants, especially at the maturing, terminal flowering spike. Research conducted at the Department of Primary Industries and Regional Development (DPRID) in WA demonstrated that when present from flowering, cabbage aphids on the flowering spikes of canola can cause flowers to abort, and consequently lower yield. Dense colonies on stems and pods during grain fill can also reduce pod set, seed fill and grain quality. Research has also shown that when 20 – 50% of flower spikes were infested, chemical control is likely to be warranted.

Green peach aphid on the other hand, often causes less direct feeding damaging, and is generally more problematic during crop establishment in autumn. In fact, further research conducted by the DPRID demonstrated that GPA feeding damage to flowering canola did not cause yield loss. This suggests that spraying GPA during spring is not beneficial, and may instead simply exacerbate insecticide resistance issues in this pest and harm naturally occurring populations of beneficial insects.

Cereals

Oat aphid (Rhopalosiphum padi), corn aphid (Rhopalosiphum maidis), and of course, Russian wheat aphid (Diuraphis noxia) are the main aphid culprits affecting cereals in south-eastern Australia. Oat aphid and corn aphid, often collectively called cereal aphids, are usually more conspicuous than RWA as they are dark green in colour and can build up in large numbers. Later infestations of cereal aphids on leaf sheaths and flag leaves between booting and the milky dough stages can result in yield losses. In some cases, aphid colonies infest the seed heads and congregate in large numbers. However, after grain fill, aphid feeding has minimal or no impact on yield, so many cropping regions of south-eastern Australia are out of the woods. 

The threat of damage from RWA is also waning as cereal crops approach maturity in many regions, with some exceptions. In the northwest region of the NSW Riverina, RWA infesting a wheat crop during booting resulted in some trapped and bleached heads, however, now that the crop has flowered the extent of damage has stabilised. Bleached, sterile heads have also been reported in irrigated wheat in this region. Fortnightly monitoring of RWA populations by SARDI has shown a significant decline in population densities on cereals beyond the booting stage (>GS40) in regions of SA.

Trapped wheat head from RWA feeding (Source: cesar).

 

Reports from growers and agronomists suggest that neonicotinoid seeding dressings have played a pivotal role in keeping RWA at bay this season. In the Victorian Mallee, one grower who treated their seed with imidacloprid observed relatively little RWA activity this season, while a neighbouring cereal crop grown from untreated seed experienced severe RWA damage earlier in the season. The observed efficacy of neonicotinoids against RWA is a welcome relief to growers after such uncertainty around how this pest will behave in Australia, but also reinforces the necessity to use these chemicals judiciously to extend the life of these valuable tools.

Have you applied chemicals to control oat aphid this year? Are you happy with the results? Although insecticide resistance in oat aphids has not been detected, some field populations in Australia have shown variability in their responses to insecticides, so it is a pest on our radar. We are currently on the hunt for samples of oat aphid from NSW and SA for baseline insecticide sensitivity screening as part of a GRDC funded project. Please get in touch with us if you have oat aphid in your paddocks and are willing to post a sample.

Beneficial insects attacking aphids

Many groups of beneficial insects can be prevalent during spring and play a key role in control when low to moderate numbers of aphids are present in crops. This season is no exception, with numerous reports of beneficials, particularly hoverfly larvae and parasitic wasps, feasting on aphids.

Parasitic wasps produce aphid mummies on canola (Source: Phil Bowden).

 

Beneficial insects can be encouraged by reducing insecticide applications (particularly broad-spectrums), as well as providing alternate food sources and refuge habitats on farms. For more information on beneficial insects likely to be encountered in grain crops, click here.

 

Field reports of aphids

Phil Bowden – Pulse Australia (South West Slopes, NSW)

Rohan Brill – NSW DPI (Riverina, NSW)

David Eksteen – Eksteen Consulting (North East, VIC/Riverina NSW)

David Elwin – Yenda Producers (Riverina, NSW)

Bronwyn Hunt – Merriwa Pastoral Company (Mallee, VIC)

Geoff Minchin – Riverina Local Land Services (Riverina, NSW)

Elizabeth Munn – Yenda Producers (Riverina, NSW)

Julian Minehan – Landmark (Southern Tablelands, NSW)

Warwick Nightingale – Delta Agribusiness (Riverina, NSW)

Rick Rundell-Gordon – Grounded Agronomy (Mallee, VIC)

Glen Smith – Riverina Co-op (Riverina, NSW)

David Wisewould – South West Agronomy (South West, VIC)