Graingrowers cautioned to consider RWA movements

Graingrowers cautioned to consider RWA movements

Cropping
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Graingrowers should keep an eye on their crops in the coming weeks as warmer spring conditions may promote flights of Russian wheat aphid, the GRDC warns.

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Graingrowers should keep an eye on their crops in the coming weeks as warmer spring conditions may promote flights of Russian wheat aphid, the GRDC warns.

RWA numbers have been minimal so far this season, which has been attributed to low survival rates during the hot and dry 2018-19 summer.

Higher temperatures in spring may lead to an increase in aphid migration, but scientists say crops older than growth stage 40 do not appear to be attractive to migrating RWA, therefore colonisation of such advanced crops during spring is unlikely.

Rare cases of RWA presence and symptoms, below intervention thresholds, have been reported this year by growers and advisers in areas such as Vic's southern Mallee and East Gippsland, as well as the NSW Riverina, Central West Slopes and Plains, and the Central Tablelands.

Entomologists involved in GRDC research investments say economically significant yield impacts are more likely from infestations that occur before stem elongation, but only if these persist during the later (heading and flowering) stages.

SARDI entomologist Maarten van Helden says detecting RWA in crops is not difficult as indications of infestation are usually quite obvious.

"A tell-tale sign is white or purple leaf streaking in cereal crops," he said.

"And at late tillering and during stem elongation, leaf rolling may occur."

Growers should search for the presence of aphids by peeling back rolled leaves, since symptomatic tillers do not always contain aphids and therefore treatment may not be required if the aphids have either moved on or died.

First identified in Australia in 2016, RWA is now present in many cropping areas of SA, Victoria, Tasmania and NSW.

The aphid has not been detected in WA, the NT and Qld.

RWA distribution is expected to move northwards again this year to northern NSW and possibly southern Queensland.

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Since RWA has only been confirmed in Australia in recent years, limited research under local agro-climatic conditions and farming systems has been conducted.

As such, a GRDC investment, Russian wheat aphid risk assessment and regional thresholds, has been established to investigate regional risk and management tactics for RWA.

The collaborative investment is being led by SARDI, which is conducting research in partnership with sustainable agriculture research organisation cesar and farming groups.

The GRDC investment is investigating how RWA survives between winter cropping seasons.

This knowledge is considered pivotal in determining the risk of infestation and potential damage ahead of each new cropping season, as well as aiding RWA management planning and development of cultural controls.

It is also seeking to determine the regional production risk posed by RWA and the economic thresholds that will guide growers in effective management of RWA, taking into account growing regions, crop varieties and climatic conditions.

Dr van Helden will feature in a new GRDC podcast, outlining research efforts to provide Australian growers with guidance about if and when to action chemical control measures in order to avoid significant yield loss, and to avoid time and money being wasted on unnecessary treatments, especially if sufficient predatory insect populations are present to act as a biological control.

"When the aphid was introduced into the United States in the 1980s, the Americans developed economic thresholds and so we are determining whether those same thresholds are valid in Australia," Dr van Helden says.

"Current threshold recommendations for chemical control, based on US research, are more than 20 percent of seedlings infested with aphids up to the start of tillering and 10pc of tillers infested thereafter."

Trial sites have been set up in SA, Vic, Tas and NSW to determine scientifically robust thresholds under varying Australian conditions.

Dr van Helden says trials so far have shown that a considerable amount of RWA population pressure is required before yield loss is incurred.

"Overall, yield loss in our trials has not been as high as expected when aphid numbers have largely been above the overseas threshold," he said.

"It seems that the overseas thresholds are, at this stage, acceptable for affected Australian grain growing regions.

"Be aware that RWA seems to develop better on stressed plants so in very dry conditions the risks may be somewhat higher."

Research will culminate with an update of the GRDC RWA Tips & Tactics guide.

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