Notes from the meeting at DAF, Monday 21 January 2019
The South Australian Research Institute (SARDI) leads the major GRDC investment in Russian Wheat Aphid (RWA) research in southern Australia. Dr Maarten van Helden and Tom Heddle (SARDI) were in the northern grains region conducting a survey of RWA in the ‘green bridge’ (roadside grasses) during January 2019 (GRDC project 9174815). Whilst in Toowoomba and Breeza, they took the opportunity to meet with agronomists and growers to answer any questions they had about RWA. This article is a summary of the discussion in Toowoomba.
Results of the RWA survey, so far…
No RWA found in NSW or southern Qld. The survey took a path through Central-Northern NSW and southern Qld, including areas around the Liverpool Plains, Gunnedah, Narrabri, Moree, Crooble, North Star, Garah, Boomi, Goondiwindi, Brookstead, Bongeen, Dalby, Toowoomba.
In contrast, in summer surveys conducted in Victoria, South Australia and Tasmania, RWA have been found on a range of grasses; most frequently on volunteer cereals, barley grass, Bromus species, and couch.
This is a series of questions asked by those present at the Toowoomba meeting, and responses from Maarten.
What is the RWA lifecycle?
Like the other introduced aphid species in Australia (corn aphid, oat aphid, rose-grain aphid, green peach aphid) the RWA population is made up of females which produce live offspring through asexual reproduction. RWA has the ability to tolerate a wide range of temperatures from below freezing to 45˚C. Reproduction can occur from 2-25˚C. Aphids reproduce slowly under cool conditions, faster when it is warm.
RWA are clumsy fliers and initial infestations on a plant tend not to spread, or spread very slowly. Flights of adults occur in the south in October-November as the winter hosts start to dry off.
RWA diagnostic features (how to identify) and symptoms are provided in the resource links below. Symptoms become visible about 2 weeks after initial infestation.
Alternate hosts? Including summer grasses, sorghum and maize. (What constitutes the ‘green bridge’?)
RWA preference for winter cereals is as follows: barley, wheat, oats. Durum wheat seems to show symptoms at relatively low RWA densities compared with wheat and barley.
Internationally, there is no evidence of RWA being a pest in sorghum or maize. The role that these crops might play in maintaining RWA populations over summer is unclear, and an area worthy of research in the northern region.
In the southern region surveys for RWA, there are a large number of grass species on which RWA have been found, but a smaller number of species on which large (reproducing) populations of RWA are found. It is unclear from the limited survey work done to date, what grass species will be key in the northern region. It is likely that the key species common to the north and south will still host RWA. The key species on which larger populations of RWA are found are volunteer winter cereals, barley grass, prairie grass, Bromus species.
The ‘green bridge’ that poses the greatest risk to winter cereals is volunteer and grass hosts in the paddock where cereals are sown. Unless these potential hosts are controlled and dead prior to the cereal emerging, there is a high risk of RWA moving onto the cereals.
Rate of spread through the northern region?
RWA was first detected in South Australia in May 2016. In 2016-17 further detections were made across SA and Victoria and into southern NSW. In late 2018, the first detections were made in north of Dubbo at Coonamble, the Liverpool Plains and Coonabarabran.
No detections have yet been made in Queensland, so it is still a requirement that suspect RWA infestations are reported to a QDAF entomologist (see contacts below).
Northern NSW and Qld have climates suitable for RWA, so it is expected that infestations will continue to be detected in subsequent seasons.
What environmental conditions favour RWA outbreaks?
In the US, where RWA has been established since the mid-1980s, it is a damaging pest in dryland cereals only in regions where annual rainfall is less than 400 mm. In higher rainfall areas, RWA is present, but does not cause crop loss. The risk of RWA outbreaks may be higher in areas with summer irrigation – which can support RWA populations over summer.
How much impact can RWA have on yield?
In recent trials with RWA introduced to plots, very little/no yield loss occurred when RWA were controlled before GS40. The risk of yield loss increases in crops that are stressed (less able to compensate for aphid impact).
What are the economic thresholds for RWA?
SARDI is presently undertaking trial work to establish thresholds suitable for the Australian system. Until this work is concluded, the US thresholds are recommended:
- Before GS32 20% of plants infested
- After GS32 10% of tillers infested.
What control options are available?
Pirimicarb (Aphidex®) and lambda-cyhalothrin (Karate Zeon®) have registrations for RWA. There are current permits in place for the use of foliar chlorpyrifos (PER83140) and thiamethoxam seed treatment (PER86231) and imidacloprid seed treatment (PER82304). These permits do not apply yet in Qld, but will be enacted when RWA is confirmed in the state.
Prophylactic foliar treatments are discouraged as they are largely ineffective, and can be very disruptive to beneficials. Where possible, select the softest option (e.g. pirimicarb).
Seed treatments are very effective at preventing colonisation by RWA. However, selective use of seed treatments is encouraged only for higher risk situations – e.g. early sown crops, especially barley; in high aphid pressure seasons, and if the green bridge cannot be controlled prior to crop emergence). Where the RWA risk is lower, or unknown, regular crop checking and a foliar application, if required, is an effective approach.
What beneficials attack RWA, and how effective are they?
All the beneficials that attack other aphid species have been recorded attacking RWA including lacewings, hoverflies, ladybirds and parasitoids. In southern Australia, the most commonly recorded aphid parasitoid is Diaeretiella rapae, a species commonly found attacking cabbage aphid. This shows how readily the natural enemies will adapt to a new species of prey, and suggests that beneficials will contribute to the suppression of RWA populations – a good reason to consider carefully insecticide control.
In wet seasons, fungal pathogens can be very effective in killing aphids.
How to check for RWA in cereal crops.
1) look for symptoms. Be aware that RWA symptoms can look like manganese deficiency, herbicide damage or Barley yellow dwarf virus symptoms. However, once familiar with the characteristic streaking, it should be easy to distinguish RWA-induced symptoms.
2) check areas in the crop that are stressed – moisture, root disease. Gate ways and wheel tracks are often where the infestations are first detected.
3) Unroll the leaves on tillers with symptoms, to confirm the presence of aphids. Aphids may be under the very edge of the leaf, so unroll the leaf completely.
4) Send photos or specimens of the symptoms and aphids to DAF or NSW DPI Entomologists for confirmation of your identification. Contact details below.
If you think you have found RWA in Queensland
If you think you may be the lucky first person to find RWA in Qld, it is important that you notify DAF entomologists (contacts below). This will allow for insecticide use permits to be enacted for Qld by Biosecurity Queensland.
Agronomist questions – Maarten van Helden responses
Are there any traps that can be used as early warning of RWA infestation?
Suction traps, yellow stick traps and water traps are all used to monitor aphid activity, but are extremely labour intensive to check and process collections. There are none deployed routinely in the northern grains region. On a paddock by paddock basis, the best indication of RWA is early detection of symptoms.
Is there currently breeding for resistance in Australia?
Australian breeders have the genetic material from the US, but there are no commercial varieties available currently. It will take some years to incorporate this material into Australian varieties.
Is barley grass a good place to start looking for RWA?
Yes, in southern Australia, 50% of barley grass samples had RWA on them – this is a high incidence rate.
How much of a risk are grazing oats as a host for populations that can then move into barley and wheat?
Oats are not a good host, and we don’t tend to see population build up in oats. Oats will show symptoms, but at very low aphid densities. In addition, movement from one crop (that is actively growing) to another doesn’t tend to occur. The host has to be senescing or dying before RWA will produce winged forms and move, or it is exceptionally warm.
For these reasons, the risk posed by grazing oats is considered low.
- GRDC tips and tactics _ Russian wheat aphid (PDF)
- Earlier publication on RWA_more comprehensive background (PDF)
Queensland Department of Agriculture and Fisheries
Melina Miles, Entomologist
PO Box 102, Toowoomba. Qld 4350
NSW Department of Primary Industries
Zorica Duric, Entomologist
Maarten van Helden
08 8303 9537