Seed treatments an important tool in Russian wheat aphid control
Apr. 16, 2018
Colorado has been dealing with RWA for 30 years now, so Professor Peairs was an obvious choice to bring to Australia for a series of GRDC presentations to information-hungry locals in 2017.
Also sharing his knowledge was Dr Paul Umina from Cesar and The University of Melbourne, who explained the spread of RWA in Australia following its discovery in Tarlee (SA) in May 2016.
“RWA is already distributed across a large part of south eastern Australia, with a few isolated cases in Tasmania,” he explains.
“It’s not known to be present in WA at the current point in time, it’s not known to be present in Queensland, and at this stage there’s been no confirmed detections in northern New South Wales.”
The primary damage caused by the aphid, according to Professor Peairs, is due to its injection of toxins during feeding that prevents the leaves of wheat, barley, and other grasses from unrolling, meaning a reduction in photosynthetic ability and a general stunting of the plant.
RWA will also inject, or transfer, a toxin into the phloem sap while feeding, causing rapid, systemic phytotoxic effects to cereal plants.
“We use the rule of thumb that for every 1% of stems infested, you lose about 0.5% of yield in wheat. Another way of putting that is, if you have a 100% infestation you can expect a 50% loss in yield,” Professor Peairs says.
“That number in barley is quite a bit higher - it may be 0.8 or 0.9% yield loss per 1% infested stems, so barley is really a much better host for the aphid than wheat.”
Over the last 30 years Professor Peairs has investigated most elements of Integrated Pest Management to control RWA, with mixed results.
The work has included adjusting planting dates and diversifying crop production systems, as well as some research into breeding host resistance.
“We developed about 10 varieties that were resistant to RWA, but in 2003 the aphid developed a population that was capable of overcoming that resistance, which really took us out of the plant breeding business,” Professor Peairs says.
“We also found the natural enemies involved in biological control of RWA in North America are the same natural enemies that are involved in the biological control of just about any cereal aphid, so there were no surprises there.
“When the aphid first came in, we really saw very little biological control effect, but now, biological control is probably one of the most important factors in terms of reducing RWA abundance.”
Professor Peairs believes seed treatments also hold good potential for management of RWA in Australia’s spring grains.
“I think seed treatments need to get a pretty close look but I hope growers don’t over rely on them because it’s a fragile chemistry and they need to preserve that for a variety of uses rather than just blanket applications against RWA,” he says.
“Here in Australia I think you need to really pin down what defines a risky RWA situation at planting - is it planting early? Is it planting late? Is it failing to control a green bridge?”
“Based on those risk considerations, I think Australian growers should limit seed treatments to those high-risk situations.”
While acknowledging Australia still has a lot to learn about this recently introduced pest, Dr Umina says observations from 2016 showed paddocks with a green bridge, particularly from late summer and early Autumn, were at risk from RWA, as were those sown early.
“Many of the early infestations in South Australia tended to be those paddocks where there was early sowing, inadequate control of the green bridge, and a lack of insecticidal seed dressings,” he explains.
“We have also seen quite a lot of natural enemies attacking RWA, which is really pleasing given it’s a new pest to Australia,” he says.
Dr Umina reports RWA have been attacked by fungi, parasitised by wasps, as well as predatory insects such as hoverflies and ladybird beetles. Like Professor Peairs, Dr Umina also believes insecticide seed treatments will play an important role in RWA management in Australia.
“Experiences in 2016, and again in 2017, have demonstrated existing cereal seed treatments registered for other cereal aphids are very effective against this pest,” he says.
“This has been supported by controlled glasshouse experiments that were conducted in 2017.”
While there is much to learn in Australia about the best control options for RWA, it seems much of the 30 years of work Professor Peairs and his colleagues have compiled in Colorado will be of great use to Australian growers looking to control this threat to local cropping systems.