Take home messages

• Russian wheat aphid (RWA) risk is largely dependent on an early break allowing its main wild host barley grass to complete a growth cycle before crops are sown.
• The RWA threshold calculator assists in deciding if control is required.
• No substantial outbreaks of RWA have been documented over the last 8 years.
• Fall army worm (FAW) has not been detected in South Australia so far.
• FAW migrants will reach SA but are unlikely to establish without suitable summer hosts and cold winters.
• Please keep reporting any suspected cases.

Background

Two potentially devastating worldwide pest insects of grains have established in Australia in the last 10 years.

Russian wheat aphid

The Russian wheat aphid (RWA, Diuraphis noxia) was first detected in 2016 in South Australia but was already present over large parts of the south-eastern grain belt. It has, in the meantime, spread to New South Wales, Tasmania and Western Australia. Though potentially able to do extensive damage to barley and wheat, the actual impact is much lower than feared.

The research done through GRDC investments (2016–2019) has allowed an IPM strategy for RWA to be established. The research showed that damage by RWA is dependent on the percentage of tillers with RWA at growth stage (GS) 40 (flag leaf out), with 0.28% of yield loss per percent of tillers infested. It also established the likely increase of tiller infestation between GS30 (end of tillering) and GS40, allowing development of an intervention threshold calculator, available online.

Growers and agronomists can do a simple, fast observation of the percentage of tillers with aphids (usually a fraction of the tillers with symptoms) around GS30 and enter the results in the calculator to see if an intervention is economically justified. If required, an insecticide application can then be combined with another (fungicide or herbicide) application to reduce costs.

After GS40, plants are much less attractive to RWA, and little new infestation of tillers will occur. However, if the plants are severely stressed, their defence weakens and RWA can increase its population even in the later growth stages.

Figure 1. Screenshot examples of RWA intervention threshold calculator.

Green bridge effect

Studies into the over-summering of RWA showed that, while it can survive on many grasses, especially when these are growing new shoots, the population levels in summer are very low. The only grass species on which substantial populations can develop are barley grass species (Hordeum glaucum, Hordeum leporinum). In a paddock with cereals, it is worth checking the barley grass for RWA presence, as they prefer barley grass over the cereals.

For RWA to be a risk in an autumn sown crop, there needs to be a big enough population of aphids building up on barley grass before the crop is sown. This requires an early break (around February) followed by regular rainfalls, as only then will barley grass germinate and grow, and RWA can build up. If that RWA infested barley grass then matures around crop sowing (May), the aphids will leave the drying barley grass and migrate, so the risk of RWA migrating to the crop will be high. However, these conditions will rarely occur.

High infestation levels of RWA or symptoms have not been observed or reported over the last few years in Australia. We consider that this is a very clear indication that the conditions are rarely combined to allow high levels of RWA to build up on wild hosts before the growing season and then migrating to the crops. This is very similar to what is reported for the North American situation, with almost over 40 years of records.

As insecticide seed treatments have become a routine application in the last decade, this might have contributed to reducing the aphid survival during the first weeks of the crop. Unfortunately, the absence of monitoring to compare plots with and without seed treatment make it difficult to have a clear view on that aspect. Insecticide seed treatments are non-specific and will also affect beneficial populations.

Fall armyworm

The fall armyworm (FAW, Spodoptera frugiperda) is a moth species originating from Africa, where it is a well-known pest. It attacks corn and sorghum crops and, when food sources run out, will migrate as caterpillar ‘armies’ to find new food sources. Like other armyworms, the adults can migrate over large distances.

Since 2016, it has shown a surprising expansion of its range through Africa, India, China and south-east Asia, doing extensive damage. It was first detected in 2020 in northern Queensland (January), in March in northern WA, then in September in NSW and, later that year, in Gippsland, Victoria. FAW has not been reported from South Australia. Damaging established populations are reported regularly from Queensland, NT (Kununurra), northern NSW and Northern WA (north of Broome) on corn and sorghum, and occasionally other crops, such as rice and sugarcane. Impact of FAW is still increasing, with reports of winter cereal crops being attacked by leftover FAW populations from summer crops (M. Miles pers. comm.).

Females lay egg masses of several hundreds of eggs on food plants (corn and sorghum are the preferred hosts) and the caterpillars will ingest large amounts of leaf material. When the initial food plant runs out, they will migrate as caterpillars and attack many other plant species, including pulses, cotton, rice, sugarcane, cereals and many others. Most damage is observed on corn and sorghum, with caterpillars eating leaves, attacking maturing seeds, and tunnelling cobs.

Detections of FAW adults in pheromone traps in the WA grain belt (Northam) and Gippsland, Victoria show that this species can migrate long distances. However, in these more temperate ‘southern’ areas of Victoria, Southern NSW and Mid and Southern WA, it seems that they have not been able to establish, let alone develop damaging populations. Therefore, we can expect that adults have already migrated to South Australia, and this will occur more often, especially if populations elsewhere build up in tropical areas. But we do not expect these migrants to initiate pest populations.

In fact, climate modelling shows that the southern half of Australia is not a suitable climate for FAW in winter (May–September) due to low temperatures. Moreover, in summer, when temperatures are potentially suitable, summer crop hosts are very rare, as rainfall is simply too low, and no crops are grown.

Only if corn or sorghum would be cropped in summer in the higher rainfall areas (south-east of SA) or elsewhere using irrigation, or maybe in a very wet summer that allows some suitable grasses to flourish, could we one day see FAW larvae in SA, but again, we would not expect these to survive the winter.

Conclusion

These two relatively new pests seem to not cause any major risk in our state, mainly due to the absence of suitable summer hosts. This does not mean that agronomists and growers should ignore them as a possible threat. In rare cases, of exceptionally early breaks and wet summers, there might be a higher risk. For RWA, an IPM approach can be used and spraying around GS40 can efficiently reduce yield loss. In the case of FAW, we are not sure if summer populations could occur and might persist into autumn and attack crops, but they are not expected to maintain over winter.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. Special thanks to Melina Miles for slides of the FAW presentation part, and Melina Miles and Dusty Severtson for providing advice and updated information on FAW. Thanks also to Tom Heddle (University of Adelaide) and James Maino (CESAR Australia) for the collaboration on the RWA trials and analysis.

References

Du Plessis H, van den Berg J, Kriticos DJ , Ota N (2017) Spodoptera frugiperda (Fall Armyworm). Pest Geography. CSIRO-InSTePP, Canberra.

Heddle T, Van Helden M, Nash M, Muirhead K (2020) Parasitoid communities and interactions with Diuraphis noxia in Australian cereal production systems. BioControl 65(5), 571-582.

Van Helden M, Heddle T, Umina PA, Maino JL (2022) Economic injury levels and dynamic action thresholds for Diuraphis noxia (Hemiptera: Aphididae) in Australian cereal crops. Journal of Economic Entomology 115(2), 592-601.

Van Helden M, Heddle T, Proctor C, Alhwash L, Wake B, Al-Jawahiri F (2021) Noncrop host plant associations for oversummering of Diuraphis noxia in the state of South Australia. Journal of Economic Entomology 114(6), 2336-2345.

Ward SE, Hoffmann AA, Van Helden M, Slavenko A, Umina PA (2024) The effects of insecticide seed treatments on the parasitism and predation of Myzus persicae (Homoptera: Aphididae) in canola. Journal of Economic Entomology 117(1), 102-117.

Ward S, van Helden M, Heddle T, Ridland PM, Pirtle E, Umina PA (2020) Biology, ecology and management of Diuraphis noxia (Hemiptera: Aphididae) in Australia. Austral Entomology 59(2), 238-252.

Russian wheat aphid action threshold calculator

The Beatsheet – Fall armyworm

Contact details

Maarten van Helden
SARDI Crop Sciences
Entomology, Waite Building E110b
Waite Road, Urrbrae SA 5064
0481 544 429
Maarten.vanHelden@sa.gov.au