Unpacking the Russian Wheat aphid story in Victoria

Surprisingly, RWA already appears widely spread through a considerable area of Victoria’s cropping zone, albeit in low numbers.  But what do we know about its rate of spread?

The range of confirmed sightings of Russian Wheat Aphid (RWA, Diuraphis noxia) in Victoria (23) has now extended eastwards in the Mallee and in the Northern Country to Torrumbarry (12km south of the Vic/NSW border), and south-eastwards to Smeaton, west of Daylesford in Central Victoria. Most sightings have been from isolated plants within barley crops, with others from cereal volunteers or in barley grass and ryegrass. As yet there has been no confirmed sightings in NSW.

Over the last few years, Research scientist Dr Piotr Trebicki and his team (Agriculture Victoria, Horsham) have undertaken trial work and surveys for plant viruses, including wheat streak mosaic virus (WSMV) and aphid vectors in the Mallee and Wimmera. WSMV has symptoms that could be confused with those of RWA. Piotr has seen no evidence of RWA or RWA symptoms in 2015 or earlier.

How long have RWA been in Victoria?

From what we understand of field sightings and international research findings, we suggest that the aphid most likely arrived in Victoria in spring 2015 and settled after crop symptoms would become evident. That conclusion was drawn from the following evidence.

Aphid flight.  Aphids move beyond local fields as the alate (or winged form), although without wind assistance they are not strong fliers.  According to South African research, winged RWA are only abundant in spring when population growth is at its peak. We can also assume that there must be a short flight or dispersal phase in autumn. However, the potential for mass movement is quite limited because there are generally relatively few summer/autumn hosts (see below). The South African research indicates there is little dispersal by flight in autumn.

In the US, RWA has been known to travel up to 600 km over 6 months with the assistance of prevailing winds.  In south-eastern Australia, insect movement is predominantly assisted by north-westerly, westerly and south-westerly winds in spring. Assuming the initial Australian incursion commenced in SA, the passage of aphids from into the Victorian Wimmera, Mallee, Central and Northern Country districts may have started in spring 2015.

Plant hosts of colonising aphids.  The US literature suggests that winged RWA are very efficient in locating isolated host plants, and particularly wheat and barley volunteers.  RWA infestations in the ‘initial’ SA Tarlee outbreak appeared to have commenced on wheat volunteers.  Many of the earlier Victorian infestations were also detected on volunteers.  For example, in several oat crops north of Horsham in the Victorian Wimmera, volunteer plants of wheat and barley stood out with their RWA symptoms from the otherwise healthy oat crop. Interestingly, in a similar oat crop, the foliage of barley grass infested with RWA appears to have collapsed leaving healthy oats. The Smeaton infestation also occurred in volunteer wheat, which germinated in March, in a home yard approximately 300 m from the nearest cereal paddock, which appeared free of aphids.

RWA symptoms in wheat and barley volunteers in an oat crop in the Wimmera (Source: J. Robertson)

 

Volunteers and annual grasses, particularly barley grass, germinate with the first autumn rains, well ahead of most cereal crops, and provide excellent hosts for RWA moving off summer grasses. The early March rains this year provided sufficient soil moisture for the germination of volunteers. Cereal crops are typically planted in May-June, and therefore potentially germinate once most RWA flights have ceased. South African research concluded “There does not seem to be any large-scale colonization of newly planted wheat fields by alatae during autumn”.

It is plausible that RWA populations have been present in Victorian pastures and cropping environments since last spring (or earlier) without seriously impacting cereal crops. Instead, alates may have colonised annual grasses (Hordeum, Bromus, Lolium, etc) and strayed into the edges of crops but remained unnoticed.

It remains unclear as to the rate and direction of RWA spread and, importantly, the key over-summering host plants. High rainfall or irrigated grass pastures/crops will be an important consideration.

 

Sources of field reports of Russian Wheat Aphid

Charles Edmonston – Agronomist, IK Caldwell (Central Victoria)

John Robertson – Consultant, AgWise Services (Victorian Wimmera)

Luise Sigel – Plant Pathologist, Agriculture Victoria, Horsham (Victorian Wimmera)

Duncan Thomas – Agronomist, Landmark (Victorian Northern Country)

Piotr Trebicki - Research Scientist, Agriculture Victoria, Horsham (Victorian Wimmera)