Sterile heads appear in Russian wheat aphid-affected crops

Damage is becoming more evident and continuing reports are expected

Field reports

Russian Wheat Aphid (Diuraphis noxia, RWA) populations appear to have stabilised in most parts of Victoria and southern NSW. In South Australia there are some reports that indicate a rise in populations, as would be expected with gradually warming temperatures. The number of winged adults in SA has also increased, in most cases to around 10% of sampled RWA, indicating that some levels of dispersal are likely to have commenced. A student project has found high populations of parasitoid wasps associated with a large RWA population in the SA Mallee, along with brown lacewing, hoverfly and ladybird larvae. With rising temperatures, populations of these beneficial insects will increase and help control RWA populations.

In the NSW Riverina, at the northern edge of the known distribution of RWA, aphids have been located at Barellan east of Griffith. About 100 ha are affected in patches out of 800 ha of wheat. Damage in these areas has become more severe and obvious over the past 6 weeks, highlighted by the presence of many sterile heads. The worst damage appears to be related to stressed plants in patches suffering from water-logging. Notably, these crops were later sown than many others in the district. Further infestations have been found north of Griffith at Tabbita and north-west at Hillston, although only low numbers of RWA have been observed.

In the Victorian Wimmera, Mallee and Northern Country districts, RWA are generally being found in low numbers. The cold and wet conditions in recent weeks are likely to have slowed the rise in populations. Trapped cereal heads and white, sterile heads are becoming more common, but for the most part are only being seen in isolated plants and/or patches. Reports suggest the occasional paddock has been badly affected.

It continues to be reported that, within areas of known infestations, wheat and barley crops sown with insecticide-treated seed remained largely clear of aphids and associated damage symptoms. Outside cereal crops, RWA has been mainly found in barley grass, self-sown barley/wheat, and less commonly, in wild oats.

Bleached, sterile heads in RWA-affected paddocks (Source: Bronwyn Hunt (left), Barry Haskins (right))

 

The latest national surveillance map can be found here. 

Our advice

o As crops enter the high-risk period for yield loss (early booting to soft dough), check crops for percent infestation and continue to monitor throughout spring.

o Although thresholds have not been tested under Australian conditions, research from overseas suggests RWA is likely to warrant chemical control if infestations exceed thresholds of 10% of plants infested through this high-risk period. During this period, protecting the top 3 leaves will be a priority for minimising yield loss.

o If chemical control is warranted, use pirimicarb which is softer (less toxic) on many beneficial species. Follow label directions and consider using lower rates at higher temperatures. Pirimicarb has a 42-day withholding period for both harvesting and grazing.

o In grain growing regions overseas a key management strategy is to preserve populations of beneficials to help control aphid numbers. There is already ample evidence to suggest beneficials are helping to control RWA in Australia. Beneficials are expected to further increase and provide an important biological control role in spring cereal crops.

o Good coverage is essential when spraying for RWA. Use water volumes of at least 80-100 L/ha and include a non-ionic surfactant. It is recommended that nozzle pressure should deliver medium size droplets.

 

Sources of field reports of RWA

John Bennett – Grower (Victorian Wimmera)

Tom Heddle - Honours Student, University of Adelaide

Barry Haskins - Agronomist/Director, Ag Grow Agronomy and Research, (NSW Riverina)

Bronwyn Hunt – Grower, Merriwa Pastoral Company (Victorian Northern Country)

Andrew McMahen – Agronomist, Landmark (Victorian Mallee)  

Greg Toomey – Agronomist, Landmark (Victorian Northern Country)

Dr. Piotr Trebicki – Researcher, Agriculture Victoria (Victorian Wimmera)