Canola virus: winter spraying may not be warranted

Author: Sharon Watt | Date: 16 Jul 2014

Green peach aphids

Key points:

  • Beet western yellows virus, spread by green peach aphids, has severely damaged canola crops in parts of the southern region
  • The effectiveness of spraying to control aphids while winter conditions persist is questionable
  • Remain vigilant – monitor for aphids and be prepared to apply insecticides in late winter or early spring
  • Use only insecticides registered for the crop and situation. Use of unregistered products could result in severe consequences for Australia’s canola industry

Spraying canola crops to control aphids responsible for the spread of beet western yellows virus (BWYV), which has decimated crops in parts of South Australia and Victoria, may not be warranted over the next few weeks – at least in districts currently experiencing cold and wet conditions.

Assuming aphid movement, in particular aphid flights, has ceased as a result of the weather, pest management experts say applying sprays while winter conditions persist will not provide any benefit in preventing further virus spread.

The onset of cold and wet weather has caused a general decline in the population and activity of green peach aphid (GPA), large infestations of which in autumn and early winter have resulted in extensive crop losses and damage from BWYV. GPA is the principal vector of the virus.

However, it is possible that aphid activity is continuing in some areas, so growers are advised to consult with their agronomists to determine the merit in spraying individual crops.

In South Australia, up to 10,000 hectares of canola in the Lower North and Mid North regions has been severely damaged and a number of crops have had to be re-sown. Virus-like symptoms have also been reported in isolated canola crops in the Upper North, on Eyre Peninsula, upper Yorke Peninsula and in the upper South-East.

In Victoria, BWYV has been found in numerous canola crops in the Wimmera and Mallee, where some reports suggest that more than 50 pc of all canola crops are now infected. Unusually high BWYV autumn infections have also been detected in several canola paddocks in New South Wales.

Although aphid numbers have now declined, crops could remain susceptible to yield loss from BWYV infection until the mid-podding stage. Infection after this stage usually results in minimum yield loss but oil quality can still be affected.

The severity of the BWYV issue has brought together entomologists, plant pathologists, agronomists and other specialists across Australia who, with the support of the Grains Research and Development Corporation (GRDC), have been collaborating to analyse and understand the situation and determine the most appropriate response.

GRDC Manager of Plant Health Surveillance and Mitigation, Sharyn Taylor, says the damage caused by BWYV has been unprecedented and it is now important for growers and their agronomists to follow the advice of pest management authorities, particularly in relation to spraying.

“The experts agree that growers should give serious consideration as to whether spraying to control GPA while winter conditions persist in the southern cropping region is warranted,” Dr Taylor said.

“Not only a costly exercise in terms of labour and product expense, but unnecessary spraying also heightens the risk of insecticide resistance which is already an issue in GPA.

“Vigilance is critical and growers should continue to closely monitor for aphids and be prepared to apply insecticides in late winter or early spring to limit the spread of the virus once aphids start to become more active, and commence their spring flights.”

The virus can be transmitted by relatively few aphids feeding on or probing plants, and symptoms may not present themselves for up to five weeks post infection.

In addition to canola, pulse crops may also be at risk in spring, depending on the BWYV strain involved.

Key advice being issued to growers and agronomists via the GRDC-supported PestFacts electronic news services for SA, Victoria and NSW and the SA Crop Watch bulletin also includes:

  • Monitor aphid populations (and their flights) over the coming weeks using a combination of yellow sticky traps and directly searching for aphids on plants. Sticky traps should ideally be placed at several points 5-25 metres in from the crop edge. Check once per week now, but more frequently when warmer weather occurs. GPA are typically (but not always) found on the undersides of leaves
  • It is important to only use insecticides registered for the crop and situation, to comply with the label directions for the application method, to not exceed application or frequency rates, and to follow all withholding periods
  • The use of unregistered products may result in residue detections in a cell or shipment of canola which could have dire consequences for growers and the broader Australian canola industry.

Entomologists believe the rainfall in February-March created a “green bridge” of weed hosts for BWYV and GPA and warm temperatures into early winter provided ideal conditions for aphid population development, flight activity and widespread colonisation of canola crops.

The presence of insecticide resistance in many GPA populations has compounded the situation. Studies funded by the GRDC have found that the extent of insecticide resistance in GPA across Australia has escalated as a result of heavy reliance on insecticides to manage aphid populations, placing strong selection pressure on the insect to develop resistance and leaving growers with limited future control options.

Insect pest management experts say rotating chemical groups and taking advantage of biological control are essential to extend the useful life of available chemistries.

More information on BWYV and GPA is available via:

ENDS

Caption: Green peach aphids. Photo: Andrew Weeks

More information

For Interviews

Sharyn Taylor, GRDC
02 6166 4500

Greg Baker, SARDI
08 8303 9544

Jenny Davidson, SARDI
08 83039389  

Paul Umina, cesar
03 9349 4723

Contact

Sharon Watt, Porter Novelli
0409 675100

GRDC Project Code CES00001, DAN00179, DAW00229, DAS00139

Region South