Green peach aphids and BWYV

BWYV is widespread in canola crops with confirmed distribution as shown here. This follows widespread infestations of green peach aphid (GPA) (Myzus persicae), the principal vector of BWYV, during autumn and early winter. For more information on GPA refer to PestFacts Issue No. 4 (pest description) and PestFacts Issue No. 6 (overview of GPA/BWYV).

Where have we recently recieved reports?

In the Victorian Wimmera, aphids appear to be present throughout most crops. Near Rupanyup, GPA have been found across >50% of paddocks sampled, but only in patches within each crop and mostly only a few aphids per plant; there’s no immediate intention to spray. In canola crops near Donald, BWYV has been confirmed in a large number of crops sampled, even some that were not showing symptoms. Aphid numbers were also quite low (2 per plant) but distributed relatively uniformly across crops. In a crop east of Horsham, aphids appear to have survived some reasonably cold frosts. In canola crops in the Nhill area, low numbers of GPA are being observed, including a few with wings.

Further to the north in Victoria, in the Swan Hill region of the Mallee, virus symptoms are appearing in vetch, field peas and chickpeas (although there are no confirmed positives). The symptoms are patchily distributed and affected plants have a reduced biomass and are lighter green. GPA and BWYV have been widely reported across the Mallee. At Elmore, in the Victorian Northern Country, very low numbers of GPA (non-winged) are being found in some paddocks of canola. Although advisers and growers are concerned about BWYV, there are no current plans to spray and aphid monitoring will continue.

Canola crops in Victoria’s South West are being closely monitored. In crops near Ararat, there are a few aphids on most plants, but no winged aphids and thus no movement is expected between plants. Further south, near Dunkeld, crops of over-summering canola have been showing some virus symptoms since February, but other forage brassica and winter canola crops in the area appear to remain unaffected. Aphid numbers were high up to April, but are now hard to find.

Reports from NSW suggest a few BWYV infections in southern and central regions. The higher infections appear to be around Henty, between Wagga Wagga and Albury, and some localities to the west and south-east. BWYV has also been confirmed in crops further north around West Wyalong, and samples have been submitted from canola crops near Parkes. GPA can be readily found on lower leaves and numbers appear to have been impacted by a series of recent frosts. In some regions, there have been reports of limited flying of aphids as day temperatures reach or exceed about 14ºC.

An update on virus testing results

Vic DEPI pathology staff and virologists have been testing canola samples with tissue blot immunoassay (TBIA) to assess plants for BWYV (see DEPI Crop Alert Vol 3 July 2014). 180 crop samples have been submitted for virus testing and 940 plants have been tested for BWYV, Turnip mosaic virus (TuMV) and Cauliflower mosaic virus (CaMV). All plants tested were negative for TuMV and CaMV.

The overall result for BWYV testing showed 75% of plant samples were positive for BWYV. The severity or amount of BWYV in crops will be assessed using a transect survey due to start next week. The incidence of BWYV in plant samples from crops tested by state is:

SA = 85% infection,

Vic = 59% infection, and

NSW = 48% infection (limited samples from Mildura, Robinvale and Swan Hill).

In Victoria, samples tested for BWYV from the Mallee had 67% infection, the Wimmera 59% infection, Southern region 0% infection (only 3 samples) and Northern region 50% infection. So far, no pulse crops have tested positive for BWYV, though the number of samples has been limited. The latest detailed information on BWYV can be found on the eXtension website.

Monitoring aphids

We suggest that growers and advisors in high-risk areas should closely monitor for the first signs of aphid flights, and be prepared to immediately apply appropriate insecticide treatments to protect susceptible crops. Aphids (and their potential flights) can be monitored using a combination of yellow sticky traps and directly searching for aphids on plants. Sweep nets are also useful for doing a rapid assessment of the presence of GPA and their winged form, and can also assist with monitoring for diamondback moth larvae.

Sticky traps are simple intercept traps, catching winged aphids that fly towards or into them. Yellow traps are mostly used because they are attractive to aphids. The surfaces of the trap are covered with a non-drying sticky substance. When the insects encounter the trap they become stuck and can then be identified and counted. Traps have a useful life of one or more weeks, depending on the field conditions; winter conditions may allow greater endurance (but dust shortens the useful life considerably). Sticky traps are available through the Australian Entomological Supplies or Entosol. 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.

Although the use of sticky traps is most effective for monitoring aphid flights, aphid presence and numbers can be assessed by direct visual searches. GPA are typically (but not always) found on the undersides of leaves. Also check on stems, amongst buds, and flowering heads. Check regularly - at least 5 points in the crop and visually inspect 20 plants at each point. Populations are often patchy (radiating from hotspots) and densities at crop margins may not be representative of the whole field. Regular monitoring, on a weekly basis from early flowering to pod set, is required to detect rapid increases of aphid populations.

Insecticide resistance testing

Deciding on the most appropriate chemical to use against GPA is challenging due to widespread resistance to synthetic pyrethroids (e.g. alpha-cypermethrin), organophosphates (e.g. dimethoate) and carbamates (e.g. pirimicarb). Further complications are the lack of alternative options and the availability of some chemicals. The mechanism of resistance to pyrethroids in GPA is also likely to render these products ineffective as an anti-feed.

GRDC has recently approved emergency funding to thoroughly investigate BWYV issues in canola. The project is led by SARDI, with Ken Henry (0422 002 292) and Bill Kimber (08 8303 9536) acting as coordinators. As part of this project, cesar will be screening field populations of GPA for resistance. These results will be used to assist with recommendations for spraying in spring for both pulses and canola. Contact one of the project coordinators (above) for further information.

Our advice

We are generally advising against spraying to control aphids while winter conditions persist, however this largely depends on the local movement of GPA. If GPA are flying, or populations begin flights, sprays should be considered as this increases the risk of virus infections into new crops. This is particularly so in regions where BWYV is prevalent, such as the Victorian Mallee and Wimmera, and some localities in southern NSW.

Monitoring aphid movement, preferably using sticky traps, is critical. In areas where temperatures remain low, insecticides may not be necessary until late winter or early spring. Generally, the yield consequences of BWYV decrease with infection at later stages of crop development. However, canola crops remain susceptible to yield losses from BWYV infection until approximately the mid-podding stage.

Pulse crops may also be at risk of BWYV infection. Chickpeas, lentils, faba beans and field peas are all susceptible to this virus, although lupins are not affected. GPA may transfer BWYV into these crops. If pulse crops are in close proximity to virus-infected canola, it is advisable to apply insecticides to the canola crop when GPA flights have begun.

Aphid infestation can be reduced by heavy rain events or sustained frosts. If heavy rain occurs after a decision to spray has been made, but before the spray has been applied, check the crop again to determine if the treatment is still required. Note, frosts will suppress GPA populations, but are very unlikely to kill all aphids.

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. Take advantage of the GPA resistance testing currently being performed.

Finally, the decision to spray should also consider the proximity and impact on local beehives. We recommend providing beekeepers with sufficient advanced notice so that bees can be withdrawn if necessary. Do not spray when bees are foraging. We will compile more information on the care of bees while controlling pests in the next issue of PestFacts.

 

* Sources of field reports and pest information

Brett Atkin – Agronomist, Elders (Victorian Mallee)

Ben Cordes – Agronomist, Tylers Hardware & Rural Supplies (Victorian Wimmera)

Jenny Davidson – Plant Pathologist, SARDI (Adelaide)

Chris Dunn – Agronomist, Landmark (Victorian Northern Country)

Bill Kimber – Entomologist, SARDI (Adelaide)

Frank Henry – Grains Pathology Team, DEPI Horsham (Victorian Wimmera)

Craig Henson – Agronomist, Kelly & Henson (Victorian South West)

Leigh Jenkins – Extension Agronomist, NSW DPI (NSW Central West Slopes & Plains)

Rob Launder – Consultant, AgriTech Rural (Victorian Wimmera)

Don McCaffery – Technical Specialist, NSW DPI (NSW Central Tablelands)

Rik Maatman – Agronomist, Landmark (Victorian Wimmera)

Simon Mock – Agronomist, Clovercrest Consulting (Victorian Wimmera)

Tim Pilkington – Agronomist, Elders (Victorian South West)