Farmers in south-east Australia urged to hold fire on Russian wheat aphid treatments

By Gregor Heard

Russian wheat aphids (RWAs) are being observed in cropping areas over much of south-east Australia, but while farmers may be uneasy at the presence of the novel pest, first detected in Australia in 2016, entomologists suggest control may not be necessary.

Jessica Lye, researcher with pest monitoring service Cesar said that studies showed that while the aphid was consistently present in winter sown cereals they were not present in economically damaging numbers.

Therefore Dr Lye cautioned against the use of a prophylactic seed treatment to control RWA, saying it added to the risk of insecticide resistance in other pest species to the insecticide and did not deliver an economic return on the cost.

"Instead, a combination of autumn monitoring when crops are emerging and the use of economic thresholds to determine cost effectiveness of intervention is the recommended management strategy," Dr Lye said in Cesar's PestFacts newsletter.

She said the areas where control may be required were where there were large grass burdens over the summer, such as irrigated areas.

So far this year Dr Lye said there had only been one report of RWA infestation approaching the threshold levels where treatment was needed, in East Gippsland, not a major grain growing region.

Most of the sightings have been found on paddocks where there were large numbers of volunteers over the summer or where crops were early sown.

However, Dr Lye said Cesar was still looking to fully understand how RWA behaved in Australian conditions and urged any farmers that detected the pest to report it to help researchers get a better understanding of the aphids.

Grains Research and Development (GRDC) funded research found that a green bridge over summer was critical to allowing RWA numbers to build up in the autumn.

In most grain growing regions where this aphid has been observed, populations grow within cultivated crops over the winter, then disperse during a spring migration into refuges to 'over-summer', and then re-disperse back into emerging crops during an autumn migration.

The research found RWA persisted more when there were moderate temperatures and low to moderate moisture levels in the top 10cm of the soil.

Dr Lye said a greater understanding of the factors that supported the proliferation of RWA could help develop a forecasting tool that could predict where RWA may build up based on climatic conditions over summer.

By doing this, farmers could predict where RWA would be most likely to persist and be able to more readily colonise winter sown crops.

The models predicted refuge risk to be low across Australia's south-east over the past two seasons, which has been backed up by RWA numbers observed in crop.