Once an exotic plant pest arrives in a new country, it can take some time to understand its biology and potential impact on primary production industries.
In 2016, the Russian wheat aphid (RWA) was found for the first time in Australia.
This exotic aphid is a pest of cereals grains and is now found in parts of South Australia, Tasmania, Victoria and New South Wales.
Since that first detection, research efforts, both in the laboratory and on the ground, have sought to better understand how well RWA is surviving in Australian regions, what impact natural enemies may be having on populations, and the best methods of managing this pest.
One research investment by the Grains Research & Development Corporation has included a focus on verifying what environmental conditions support survival of RWA populations over the summer period, as summer survival directly relates to risk of infestation in winter-sown cereal crops.
For 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 summer plant refuges known as the 'green bridge' to 'over-summer', and then re-disperse back into emerging crops during an autumn migration.
Since 2018, researchers from the South Australian Research & Development Institute and independent research group, cesar, have sampled crops and grasses all over the known RWA range.
Throughout these surveys, RWA were found over-summering in a variety of grassy weeds (such as barley grass, brome grasses, rye grass, Phalaris and wild oat) and volunteer cereal crops.
RWA populations were found to be strongly influenced by region and season, with the largest populations present in the spring within the warm dry regions of northern Victoria, southern NSW and South Australia.
This intensive sampling has enabled researchers to better understand the risk posed by the summer green-bridge when it comes to supporting RWA populations in the lead up to autumn migratory behaviour.
Overall, persistence of the RWA in the green bridge was found to be associated with moderate temperatures ( 20°C), low to moderate available soil moisture (5% in top 0-10cm), with migration occurring when daily maximum temperatures exceed 24°C.
The presence of irrigated crops increased the likelihood of RWA detections at all surveillance points throughout the year.
This information has supported development of a forecasting tool by cesar that predicts the locations of the most suitable climatic conditions for supporting RWA.
The model forecast therefore predicts the locations where RWA would be most likely to persist and more readily colonise winter-sown crops.
The model has confirmed that prior to the 2019 growing season, there was a poor green bridge for RWA, while the prediction for the 2020 season shows that some locations in northern Victoria, as well as the southern Mallee region, remained comparatively suitable over the green bridge and should be monitored for early season RWA activity.
More broadly, RWA populations are expected to once again be low across most of the known RWA range this autumn, with a slightly higher risk in areas that received summer rainfall or had persistent summer irrigation allowing for more aphid persistence across the summer.
However, this recent research has shown that even when RWA is consistently present in the environment, they rarely naturally infest winter-sown cereal crops and, when they do, there is still not a strong risk of yield loss.
Therefore, the annual use of insecticide seed treatment against RWA does not appear on the whole to be a cost-effective option and adds to the risk of insecticide resistance emerging in other species.
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.
Exceptions to the expected cost inefficiency of prophylactic seed treatments to manage RWA may be areas where high populations of grasses frequently persist over summer through irrigation: irrigated grass paddocks, irrigated orchards, and maybe some summer cereals.
About the author: Dr Jessica Lye is cesar Australia's extension and communications lead.
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