What we know about Russian wheat aphid
Author: Ken Young and Lauren DuFall (GRDC Canberra Office) and Craig Ruch (GRDC Adelaide Office) | Date: 28 Feb 2017
Take home message
- Russian wheat aphid (RWA) is likely to be a significant pest of Australian cereals
- Overseas economic thresholds (20 aphids per plant then 10 aphids per tiller) need confirmation in Australia
- Rain event significantly affects RWA populations particularly if weather allows for entomopathogenic fungal development
- Beneficial insects are a major management mechanism overseas and have shown effects in Australia
- Insecticides will also be a major management tool to be used in conjunction with beneficials
- The biology and ecology of RWA in Australia is still being determined including likely survival in sub-tropical climates of the northern region
- Plant genetics is not the be all and end all in management
Russian wheat aphid occurrence in Australia
Russian wheat aphid (Diuraphis noxia, RWA) was first reported in paddocks in Tarlee SA in May 2016. After initial incursion management under the Exotic Plant Pest Response Deed, determined that RWA had spread over considerable areas of South Australia and Victoria, it was deemed as a management issue and not feasible to eradicate on 8 June 2017.
RWA has since spread across Victoria into southern NSW and has recently been identified in Tasmania. As the occurrence of RWA has been widespread, it is thought that it has been Australia for at least a year if not longer but has gone unnoticed. It is still spreading and its final distribution could be across the whole of the Australian wheat belt.
RWA occurs in all other major grain-growing regions of the world, originating from southern Russia, the Middle East and Central Asia.
Work commissioned by GRDC
Since the declaration that RWA can’t be eradicated, the GRDC has commissioned several projects to enable grower’s better management of this pest this coming season and for the long term. These projects included biology & ecology; economic thresholds, insecticide efficacy and plant resistance options. Also a communications plan has been implemented which includes presentations at Updates and provision of a best bet management guide for growers.
Biology and ecology
Whilst RWA has originated from colder areas, it is associated with warmer drier climates preferring temperatures in the range 18 – 21oC, with low survival when temperatures exceed 25oC. In many areas, only females are present and reproduction is asexual, and this is what has been so far observed in Australia.
The primary hosts are wheat and barley but RWA also lives on triticale, oats and rye and can survive on a range of grass species including pasture grasses and wild genera including Poa, Bromus, Hordeum, Lolium, and Phalaris. The full extent of the role of these other species on the invasion potential and survivability of RWA over summer and between crops is still to be determined under Australian conditions.
Symptoms and effects
Large populations in autumn can severely affect crop establishment. Based on last years’ experience where management practices were taken, crops can recover and yield normally. Currently recommendations are to use the threshold levels of 20 aphids per plant up to tillering, then 10 aphids per tiller after that. It was expected that the populations of RWA would increase in spring last year as this occurs elsewhere overseas. However the wet spring conditions in the south in 2016 caused a large decrease in population levels through displacement of aphids, but also due to beneficial fungal aphid attack. Last season weather hampered testing of the above thresholds under Australian conditions. The estimated yield losses in Colorado are 0.5% per each 1% infested wheat tiller and 0.8% per 1% infested barely tillers (Peairs 2017).
Infected leaves can be seen to have white to purple striping on leaves. Leaves can be rolled, and later on, heads a can be trapped by a rolled flag leaf and can appear bleached.
The aphid releases a toxin during feeding causing the effects only on the infested leaf. So once controlled, new leaves do not show symptoms.
The GRDC commissioned SARDI to conduct population studies across 16 sites in South Australia and Victoria during late winter and spring 2016 and this continued over summer to determine the survival of RWA.
Maintaining a summer fallow clear of grasses will decrease the numbers of aphids surviving the summer months. Due to the wide host range, it is not only volunteer cereals that need to be managed, but grass weeds and possibly also in pasture grasses that are adjacent paddocks intended for cereals.
The current Emergency Use Permit APVMA PER82792 is for chlorpyrifos and pirimicarb and is in place until June 2018. A range of other foliar insecticides registered for use in cereals have also been assessed in Victoria and South Australia. The results of these trials are included in both the best management practices guide and presented at the Adelaide Update. The summary of these trials was that chlorpyrifos was the most efficacious, but that pirimicarb also performed strongly and had the added advantage of being less harmful to beneficials.
Water rate did affect efficacy with high water volumes (100 L/ha) performing better than 60L/ha. Adhere to spray volumes stated on PER82792. A medium coarse spray quality is recommended. The addition of adjuvants had variable results dependent on the insecticide used. There was an advantage in adding Hasten® or BS1000® to pirimicarb and Uptake® and SACOA Biopest® to chlorpyrifos at some sites. Adhere to adjuvant recommendations stated on PER82792 and the insecticide labels.
Information from 2016 indicated that the various seed dressings that include insecticides do have an effect on RWA. Trials are currently underway in controlled environments to determine the length of control seed dressings will provide in RWA management. Permit PER82304 (valid to March 2021) allows for seeds to be treated with seed dressing products containing 600 g/L imidacloprid.
GRDC has previously invested in pre-emptive pre-breeding activities associated with varietal resistance to this pest in a partnership led by Murdoch University and involving national and international collaborators. While no resistance to RWA was identified in a screen of major Australian wheat and barley cultivars conducted several years ago, encouragingly the project did develop some material where resistance genes were introgressed into Australian cereal backgrounds.
Unfortunately, RWA has many biotypes and can develop new biotypes in new environments which make plant breeding more difficult. Currently, the GRDC has commissioned several studies with SARDI and NSW DPI to determine the biotype present in Australia and levels of resistance in current varieties and elite germplasm.
A cautionary tale comes from the USA, where resistant varieties where developed to the original biotypes with >25% uptake by growers until a new biotype was found in 2003. While germplasm was available, no commercial lines were developed (Peairs 2017).
The GRDC in conjunction with Plant Health Australia and state agencies have provided continual updates on the distribution, spread and management advice since the incursion. The GRDC have commissioned a best practice guide to be released around the time of this Update. The information from the various projects commissioned by the GRDC will be included in this best practice guide. For the southern updates, GRDC has hosted Prof. Frank Peairs from Colorado State University to provide information from Colorado’s 30 years’ experience in living with RWA to growers, advisers, entomologist and GRDC staff and panels. Some of Frank’s information is presented here, but is also available from the Adelaide Update website.
Peairs 2017 – Adelaide Update paper
The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC. The authors would like to thank them for their continued support.
GRDC would like to thank our research collaborators in SARDI, cesar, Peracto AgXtra and AgCommunicators for the on ground work.
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