RWA as a pest in the northern grains region

Russ­ian wheat aphid (RWA) is con­sid­ered a high pri­or­i­ty pest by the grains indus­try because of its poten­tial to cause sig­nif­i­cant yield loss­es in wheat and bar­ley if not well man­aged. Trit­i­cale and rye are also sus­cep­ti­ble to crop loss, but oats are con­sid­ered rel­a­tive­ly tol­er­ant.

It is inevitable that RWA will estab­lish in the north­ern grains region, but we don’t know when we will start to see it in crops. Giv­en how wide­ly dis­trib­uted RWA is in South Aus­tralia and Vic­to­ria, it seems like­ly that RWA has been in Aus­tralia for some time pri­or to the 2016 out­break. Envi­ron­men­tal con­di­tions that favoured the south­ern region’s out­break were a wet sum­mer (abun­dance of sum­mer hosts), a warm autumn (encour­aged rapid pop­u­la­tion growth), and ear­ly sow­ing of win­ter cere­als (allowed move­ment from weed hosts to crops).

In addi­tion to crop hosts, non-crop and pas­ture grass species in the gen­era Poa, Bro­mus, Hordeum, Loli­um, and Phalaris may also host RWA. It remains to be seen whether RWA are more like­ly to over-sum­mer on grass hosts or crops (e.g. sorghum, maize, mil­let, canary) in the crop­ping land­scape.

The RWA is more dam­ag­ing to cere­als that the aphid species we already have in Aus­tralia. We don’t yet know how sus­cep­ti­ble Aus­tralian vari­eties are to RWA; GRDC is cur­rent­ly invest­ing in the devel­op­ment of resis­tant cul­ti­vars. In the mean­time, inter­na­tion­al expe­ri­ence has been that yield loss­es are gen­er­al­ly low­er in sea­sons fol­low­ing RWA out­breaks, as grow­ers are bet­ter equipped to detect and man­age infes­ta­tions, and nat­ur­al ene­mies estab­lish and con­tribute to the sup­pres­sion of pop­u­la­tions. In South Aus­tralia and Vic­to­ria in 2016, ear­ly infest­ed crops that were treat­ed to con­trol RWA recov­ered to grow and yield nor­mal­ly.

In the 2017 sea­son, it is impor­tant that crops are mon­i­tored more fre­quent­ly than they might usu­al­ly be to ensure ear­ly detec­tion of RWA, should it occur.

Sam­ple for RWA in the same way you would sam­ple for oth­er cere­al aphids. Con­cen­trate on the field mar­gins and in areas of the pad­dock that are stressed (dry, wet, root dis­ease). Look for both plant symp­toms (see descrip­tion below) and the pres­ence of aphids.

The fol­low­ing thresh­olds are rec­om­mend­ed:

  • Emer­gence to tiller­ing: 20 RWA per plant
  • Tiller­ing onwards (Z30-59): 10 aphids per tiller

If con­trol of RWA is war­rant­ed, there is a cur­rent Emer­gency Use Per­mit (APVMA PER82792) for chlor­pyri­fos and pir­im­i­carb. Pir­im­i­carb is selec­tive (will kill aphids, but not the ben­e­fi­cial insects in the crop), so if pos­si­ble, use this option first to pre­serve ben­e­fi­cials that may then sup­press fur­ther out­breaks.

Leaf symptoms caused by RWA infestations – what to look for in the field

RWA induce strik­ing symp­toms in wheat and bar­ley, unlike the oat and corn aphid which pro­duce no obvi­ous symp­toms. Plants will start to exhib­it symp­toms with­in a week or so of being infest­ed, and plant dam­age is in response to direct aphid feed­ing, so only the leaves and/or tillers that are infest­ed will show symp­toms.

1. White streaking of the leaves.  Some varieties show reddening.

RWAsymptom-streaking Reddening_close_IMG_2190

2. Rolled leaves. RWA colonies shelter inside the rolled leaves.

3 leaves rolled_IMG_2201  rwa1

Some of these symp­toms are sim­i­lar to those caused by wheat streak mosa­ic virus (WSMV) and phe­noxy dam­age in cere­als, so close­ly exam­ine plants to con­firm the pres­ence of RWA.

Distinguishing RWA in the field

Use a hand lens to check for key fea­tures (see below), specif­i­cal­ly for the absence of siphun­culi and the dou­ble tail (cau­da), char­ac­ter­is­tic of RWA.

Aphid-ID1 Aphid-ID2

Links to further information on RWA in Australia

RWAbrochure1 RWAbrochure2

A slideshow of this infor­ma­tion is avail­able on our pre­sen­ta­tions page.