Over the past two years, researchers have raced to build their knowledge of Russian wheat aphid (RWA) in southern Australia’s winter cropping cycle.

GRDC investment has allowed a team from Cesar Australia and the South Australian Research and Development Institute (SARDI) – the research division of the Department of Primary Industries and Regions (PIRSA) – to assess the behaviour and damage potential of RWA in southern Australia.

SARDI entomologist Maarten van Helden says trial plots of wheat and barley in New South Wales, Victoria, Tasmania and South Australia were inoculated with RWA in 2018 and 2019 and the results indicated the pest was unlikely to cause significant yield losses in autumn-sown crops.

“We measured RWA infestation as a percentage of tillers with RWA and found the number doubled every 35 days from growth stage 30 (start of stem elongation), peaking around growth stage 50 (start of head emergence),” Dr van Helden says.

“The average yield impact was found to be 0.28 per cent for every percentage of tillers with RWA, which is significantly less than the yield impact of 0.46 to 0.48 per cent recorded in research conducted in the United States.”

Population growth

RWA populations tend to increase slowly in cultivated crops over winter, then grow rapidly in spring.

As the crops mature, the aphids migrate away for the summer, then re-infest emerging crops the following autumn.

To better understand how RWA survives over summer and migrates back into crops, the researchers surveyed potential green bridge hosts in northern Victoria and southern NSW and conducted continuous sampling of grasses in SA for the 26 months from March 2018 to May 2020.

Dr van Helden says the aphids preferred spring hosts including barley grass and brome grass as well as phalaris, ryegrass and wild oats.

“The aphids only feed on tender, green plants and these species all senesce in early summer,” he says.

“The aphids need alternative hosts to carry them through until autumn. We found small RWA populations surviving the hottest months on native grasses such as bottle washers, which continue to actively grow.”

Barley grass is one of many grass species that have been found to provide spring or summer shelter for Russian wheat aphid. Photo: GRDC

Summer or autumn rain events tended to favour increases in the RWA population by triggering early germination of barley grass and brome grass seeds. The greatest populations were observed where soil moisture remained below five per cent in the top 10 centimetres.

This would imply that a mild, dry winter with an abundant green bridge from late summer rain might carry the greatest risk of crop infestation.

The researchers believe RWA is particularly favoured by significant rainfall in February, which encourages suitable grass hosts to germinate and RWA populations to increase. Additional rain would be needed in March and April to support this build-up.

Without February rain, RWA numbers would not be able to increase in time to affect emerging crops, even if germinating rains fell later in summer.

However, the drivers of migration are complex and still not fully understood. “Aphids are small, soft and weak flyers, so migration is always a risky proposition for them,” Dr van Helden says.

“If there has been plenty of rain their host plants will continue to provide ample food and shelter, so they will not migrate into crop fields.

“We think they will only move if the population pressure is already high, and their current hosts begin to mature and senesce.”

As a result of these factors, the RWA risk for southern region growers this season is patchy.

Lower risk this season

Most of SA and Victoria is considered to be at less risk than last season, although the Lower Eyre Peninsula and southern regions of Western Australia are considered to have a slightly more elevated risk following above-average summer rainfall.

Growers in the coolest, wettest areas of Tasmania, Victoria and WA may not face significant RWA risk because winter hosts will continue to provide sufficient food and shelter until after crops are well-established.

Dr van Helden says these risks are only based on modelling and he is hoping regular monitoring will provide a more complete picture. “We need growers and agronomists to monitor crops and potential green bridge hosts closely for RWA through the growing season,” he says.

“Reports from the field will help validate our modelling and provide more-detailed information about how RWA moves between its host plants and crop paddocks.”

RWA monitoring reports including a clear photo and details of date, cereal or host grass and location can be sent to Cesar Australia (Victoria and NSW) or SARDI PestFacts (SA).

Crop monitoring is also essential for determining when intervention to control RWA will provide an economic benefit. Growers can simply enter their data into the online RWA action threshold calculator, which has been developed by Dr van Helden and his team with GRDC investment.

More information: Maarten van Helden, 0481 544 429