Seed dressings show promise for mitigating RWA impact

Susan Webster, June 12, 2023

Discolouration and streaking on a wheat leaf caused by Russian wheat aphid.

WESTERN Australian research suggests seed dressings could mitigate damage to crops if Russian wheat aphid (RWA) are present in volunteer cereals and grasses.

Where seed dressings were not applied, RWA were 11 times more likely to be detected on tillers, according to the trials led by Department of Primary Industries and Regional Development entomologist Svetlana Micic.

If seed dressings were applied, thresholds for RWA feeding damage were not reached. Control plots averaged 16 percent more yield and 33pc less screenings than plots affected by the aphid.

Where aphid-damage thresholds exceeded 2pc, RWA can cause losses of about $136 per hectare and averaged a 400kg/ha decrease in yield.

RWA was first found in WA in 2020 and is a pest of wheat and barley.

Other non-crop hosts include many grass weeds such as brome and barley grass.

The adult aphid is lime-green with an elongated body about 1.4-2.3mm in length and short antennae.

They are distinguishable from corn and oat aphids by their lack of siphunculi (exhaust pipes).

The RWA injects toxins into the plant during feeding which retard growth, and with heavy infestations, can kill the plant.

Crops infected with RWA can appear purplish and develop longitudinal yellowish and whitish streaks from feeding damage, even with very low numbers of aphids.

“Yield loss is determined by the length of time RWA are in the crop and the percentage of tillers with RWA, not the appearance of feeding damage symptoms,” Ms Micic said.

“Yield loss depends on the time of colonisation and the length of time aphids are in the crop.”

Her work also trialled the Grains Research and Development Corporation RWA action threshold calculator,  and reported that the yield loss in trials was consistent with the model used in the calculator.

The calculator is a tool to help determine if RWA have reached threshold levels and if control is required.

“As RWA is relatively new to WA, the calculator had yet to be tested under WA conditions.”

Ms Micic’s research, undertaken last year, showed that one-fifth of tillers showing feeding damage did not actually host an aphid colony.

“The RWA action calculator accurately predicts if yield loss will occur under WA growing conditions but it is important to get an accurate assessment of percentage of tillers with RWA and not use plant damage symptoms as a measure of RWA presence.”



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