Insect pest management update: Russian wheat aphid

Take home messages

• Russian wheat aphid (RWA) was found north of Dubbo in October 2018, in the Coonamble, Liverpool Plains and Coonabarabran districts. The detection of small outbreaks means that it is likely outbreaks will be more widespread in coming seasons
• Familiarise yourself with the symptoms RWA produces in barley and wheat, so that early detection will minimise the risk of yield loss
• RWA is a very manageable pest, so there is no need to panic

RWA as a pest in the northern grains region – the current situation

RWA is considered a high priority pest by the grains industry because of its potential to cause significant yield losses in wheat and barley if not detected, or well managed when infestations exceed threshold. Barley is more frequently colonised than wheat, despite the name of the aphid. Triticale and rye are also susceptible to crop loss, but oats are considered relatively tolerant.

Since the first Australian detection in South Australia in 2016, RWA has moved slowly in the northern grains region. Additional discoveries of infestations were found across SA, Victoria and southern NSW in 2016. The extent of spread now includes Tasmania, the Riverina, Coonamble, Liverpool Plains and Coonabarabran. The number of fields infested in central NSW was small, largely because there were relatively few winter cereal crops.

In January 2019, researchers from the South Australian Research Institute (SARDI), Maarten van Helden and Tom Heddle, conducted a survey through NSW and southern Qld looking for signs of RWA. The survey, part of a broader summer surveillance across SA, Vic and southern NSW is part of the GRDC investment in RWA research. The survey passed through the Liverpool Plains, Tamworth, Narrabri, Moree, North Star, Pallamallawa, Garah, Boomi, Goondiwindi, Brookstead, Bongeen, Toowoomba and Dalby. During the survey over 120 samples of grasses were taken, but no RWA found. In contrast, similar surveys in the southern region have made frequent detections of RWA during summer.

It is inevitable that RWA will establish in the northern grains region, but it is impossible to say when this will happen. An outbreak is most likely in the event of favourable conditions for RWA populations to survive over summer (on grass weeds and in non-crop areas) and to build up in winter and move into early crops in autumn. The environmental conditions that favoured the outbreak in the southern region were a wet summer (summer hosts), a warm autumn and early sowing of winter cereals (favour rapid population growth and movement from weed hosts to crops). In addition to crop hosts, non-crop and pasture grass species in the genera Poa, Bromus, Hordeum, Lolium, and Phalaris may also host RWA. It remains to be seen whether RWA over-summer on grass hosts or crops (e.g. sorghum, maize, millet, canary) in the cropping landscape. One significant result from the recent SARDI survey was the detection of RWA on Johnson grass in South Australia. Samples of Johnson grass were taken in northern NSW/southern Qld survey, but no RWA were found on these.

The international experience with RWA has been that in seasons following outbreaks, yield losses are generally lower as growers and their advisors are better equipped to detect and manage infestations, and natural enemies establish and contribute to the suppression of populations. In South Australia and Victoria in 2016, infested crops that were treated to control RWA, recovered to grow and yield normally. Many of the natural enemies, predators and parasitoids, that are associated with other aphid pests in cereals and canola, have been recorded attacking RWA.

The ‘FITE’ strategy has been developed to guide RWA management

• Find - Look for aphids and the characteristic plant symptoms of infection including leaf streaking or leaf rolling on cereal crops and grasses
• Identify - Positively identify RWA by consulting with an industry specialist
• Threshold approach - Before deciding on your plan of attack consider thresholds for control, the presence of natural aphid enemies in the crop, crop growth stage and potential yield losses
• Enact - Take appropriate action: Manage your next steps including encouraging beneficial insects and protecting honeybees before implementing control options.

Additional tips for detecting and managing RWA

Sample for RWA in the same way you would sample for other cereal aphids.

• Concentrate on the field margins and in areas of the paddock that are stressed (dry, wet, root disease). Look for both symptoms (see description below) and then confirm the presence of aphids. RWA may occur in conjunction with other aphid species
• Non-crop grass species (weeds, pasture grasses, native species) do not show leaf symptoms when infested with RWA.

Use the following economic thresholds (provisional, based on US recommendations)

• Emergence to tillering – 20% of plants infested
• Tillering onwards– 10% of tillers infested.

If control of RWA is warranted:

Check the APVMA website for current permits and registrations. Pirimicarb is the softest available option and will kill aphids, but not the beneficial insects in the crop. If possible, use this option first to preserve beneficials which may then suppress further build-up of RWA, or other aphid species.

The use of seed treatments (just in case RWA might occur) is discouraged, particularly in districts where RWA has not yet been confirmed. Regular crop monitoring to ensure timely detection of infestations is encouraged.

Leaf symptoms caused by RWA infestations – what to look for in the field

RWA induce striking symptoms in wheat and barley, unlike the oat and corn aphid which produce no obvious symptoms. Within a week or so of being infested with RWA, plants will start to exhibit symptoms. Plant damage is in response to direct aphid feeding, so only the leaves and/or tillers infested show symptoms.

Figure 1. Symptoms of Russian wheat aphid (Photos: Melina Miles)

Some of these symptoms are similar to those caused by wheat streak mosaic virus (WSMV) and phenoxy damage in cereals –close examination of symptomatic plants to determine the presence of RWA is recommended.

Distinguishing RWA in the field

Use a hand lens to check for key features (Figure 4), specifically for the absence of siphunculi and the double tail (cauda), characteristic of RWA.

Figure 2. Features to look for in a Russian wheat aphid

Resources to assist with identification and understanding of RWA biology

• For information on RWA management go to the Tactics for Future Control publication
• GRDC also has an online RWA resource with key information for growers and advisors
• Growers and advisers are encouraged to report occurrences of RWA to the Exotic Plant Pest Hotline on 1800 084 881. These reports will aid in improving understanding of RWA range and rate of spread.
• In NSW samples can be sent to entomologist Zorica Duric, NSW DPI, Tamworth Agricultural Institute, 4 Marsden Park Road, Calala, New South Wales 2340. For more details about sample collection contact Zorica.Duric@dpi.nsw.gov.au
• For formal identification of RWA, Queensland growers and advisers can send samples to Melina Miles, DAF, PO Box 2282, Toowoomba, Queensland. For more details about sample collection contact Melina.Miles@daf.qld.gov.au

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 author would like to thank them for their continued support. SARDI for sharing results of the northern region survey.

Contact details

Melina Miles
Queensland Department of Agriculture and Fisheries
PO Box 102, Toowoomba. Qld 4350
Mb: 0407 113 306
Email: melina.miles@daf.qld.gov.au