Paddock Practices: Ongoing monitoring of RWA important in 2018

The presence or otherwise of a green bridge over the 2017-18 summer appears to be the main factor determining infestation levels of Russian wheat aphid (RWA) across the southern cropping region this growing season.

The lack of a green bridge over a dry summer period in the low rainfall zone appears to be resulting in lower RWA pressure this season.

However, medium and high-rainfall zone growers who experienced a green bridge during summer are reporting RWA activity and are therefore especially encouraged to monitor numbers closely.

The GRDC continues to advocate the FITE approach to RWA. This involves:

• 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 economic thresholds for control, the presence of natural enemies in the crop, crop growth stage and potential yield losses.
• Enact: Take appropriate action and manage next steps including encouraging beneficial insects and protecting honeybees before implementing control options.

Efforts should be made to encourage beneficial invertebrates that act as natural enemies to RWA by using softer chemistries and avoiding insurance sprays which increase the risk of resistance development in all pests in the crop.

Monitoring and control

The economic thresholds adopted following the incursion of RWA in May 2016 still stand – 20 percent of seedlings infested up to the start of tillering and 10 percent of tillers infested thereafter. Most uncolonised and vigorous cultivars appear to be less attractive to RWA beyond growth stage 40 (flag leaf/booting). RWA migration away from these advanced growth stages has been recorded.

Growers in all rainfall zones are encouraged to remain vigilant against RWA in 2018 and to monitor crops closely for symptoms of infestation. A critical time to monitor is as temperatures rise in spring, when RWA populations are expected to reach their peak.

RWA is often difficult to find when numbers are low, so growers are encouraged to check for the characteristic and distinctive leaf streaking and rolling. Stressed areas of the paddock are often the first to be infested and these areas should be inspected initially. It is important to remember that mixed aphid populations (including RWA, corn aphids and oat aphids) are often found in the same crop, and it should not be assumed that RWA is the only aphid present.

Plants affected by RWA show symptoms such as chlorosis, necrosis, wilting, stunting, leaf streaking with whitish, yellow and purple longitudinal leaf markings, trapped awns (which give a hooked appearance), stunted growth, rolled leaves and heads that fail to flower. Symptoms can persist for some time after aphids are no longer present.

Recommendations for control focus on adopting a threshold-based integrated pest management strategy.

Infestations that exceed thresholds of 20 percent of seedlings showing symptoms of RWA up to the start of tillering and 10 percent of tillers infested with aphids through the high-risk period of stem elongation to soft dough (GS31-85) are of concern. It is crucial to protect the top three leaves to prevent yield loss.

If populations exceed the recommended thresholds then insecticide options are Group 3A Karate Zeon® (lambda-cyhalothrin), Group 1A Pirimor® and Aphidex 800® (pirimicarb) and Group 1B chlorpyrifos under permit PER83140 until March 31, 2021.

Growers and advisers are reminded to always follow label instructions and implement an effective resistance management strategy of Mode of Action (MoA) rotation when applying an insecticide.

Spray application should aim to achieve maximum penetration into the crop canopy. Results from efficacy trials conducted in 2016 and reports by agronomists suggest that effective control is achieved from the use of at least 100 litres of water per hectare with nozzle pressure to produce medium-coarse size droplets. Some insecticide options have optimal efficacy when temperatures are greater than 15 degrees during application.

GRDC Manager of Pests Dr Leigh Nelson says growers are encouraged to consider softer options for control (such as pirimicarb), where possible, to preserve natural enemies so they will clean up the RWA and other pests in the crop such as caterpillars.

“Monitoring paddocks prior to insecticide applications is key to identifying the presence of beneficials,” she says.

Beyond season 2018

An important non-chemical measure that growers should consider for next season is green bridge control. Controlling volunteer cereals and grass weeds over summer can help to reduce the risk of RWA colonisation and therefore reduce the need for chemical applications.

Thiamethoxam seed treatments are approved for use in wheat and barley against RWA (Australian Pesticides and Veterinary Medicines Authority permit number PER86231) and imidacloprid seed treatments are approved for use in winter cereals against RWA (APVMA permit number PER82304).

Seed treatments are an effective way to prevent early RWA colonisation. However, entomologists are urging caution when using these, since they will increase selection for resistance, as has been observed for green peach aphids (GPA).

Use of insecticide seed treatments should be targeted at those situations deemed to be of higher risk of RWA infestation. This includes early (March/April) sown cereals; paddocks containing volunteer cereals; and/or where live aphids have been identified prior to sowing.

Reporting RWA occurrences in 2018

Growers and advisers are encouraged to continue reporting occurrences of RWA to PestFacts and to take advantage of pest identification services provided by cesar and the South Australian Research and Development Institute) through the National Pest Information Service.

Reports from Tasmania, Victoria and Southern NSW can be made through:

• the cesar website
• the PestFacts Reporter App available on iTunes and Google Play
• through Twitter on @cesaraustralia
• contacting cesar directly at 03 9349 4723.

Reports from South Australia can be submitted through:

• twitter @PestFactsSARDI
• contacting SARDI directly: Rebecca Hamdorf p: 08 8429 0682 e: rebecca.hamdorf@sa.gov.au or Kym Perry p: 08 8429 0738 m: 0421 788 357 e: kym.perry@sa.gov.au
• reports can be submitted online using the PestFacts Map.

GRDC research codes: CES00004, DAS000170, 9174815

More information

Greg Baker, SARDI science leader – entomology, 08 8429 0933, greg.baker@sa.gov.au

James Maino, Senior Consultant – cesar, 03 9349 4723, jmaino@cesaraustralia.com.au

Useful resources

Russian wheat aphid: Tactics for future control

RWA Tips & Tactics