Green peach aphids: insecticide resistance, role in the transmission of beet western yellows virus and managing risk in 2015

Paul Umina, Garry McDonald and Siobhan de Little, cesar pty ltd

GRDC project codes: CES00001, CES00002

Keywords: canola, aphid, virus.

Take home messages

  • Beet western yellows virus (syn. Turnip yellows virus) is widespread throughout grain growing areas of Australia.
  • The green peach aphid is the main vector of beet western yellows virus.
  • Green peach aphid has a high prevalence of resistance to insecticides.
  • Integrated management strategies have been devised for green peach aphid. These strategies may vary across regions and industries to be most effective.
  • Growers should implement resistance management strategies for insecticides and consider non-chemical management options.

Background

Widespread infestations of green peach aphids, Myzus persicaeI, during autumn and winter of 2014 contributed to an outbreak of beet western yellows virus (BWYV, syn. Turnip yellows virus) in southern Australia. Canola crops across the lower and mid north regions of SA, the Eyre Peninsula, western Victoria and some parts of NSW were severely affected by the virus.

GPA, the principal vector of BWYV, have an oval shaped body, and may be pale yellow-green, orange or pink in colour. The adults are approximately 3mm long; winged adults have a black patch on the abdomen. Wingless forms are uniform in colour. Their presence on the underside of leaves is also diagnostic.

BWYV infection causes reddening, purpling or yellowing of lower leaves of canola plants. Plants infected early are often pale and stunted and produce few flowers or seeds. Symptoms are milder and stunting is lacking with late infections, which generally result in minimum effect on yield. Leaf symptom type and severity differ depending on plant age at infection, environmental conditions and the canola variety involved.

BWYV is not seed-borne and survives from one growing season to the next in over-summering canola, broad-leafed weed species, and perennial legume pastures. BWYV is spread by several aphid species that colonise canola, including GPA.

The severity of the BWYV outbreak in 2014 is possibly due to a combination of the following factors:

  • summer rainfall which resulted in a ‘green bridge’ of weed hosts of BWYV and aphids;
  • the early start to the season and early sowing;
  • very mild autumn conditions which contributed to early, and extended, levels of aphid activity through till late June;
  • crop management practices;
  • the prevalence of insecticide resistance in GPA, particularly to pyrethroids, organophosphates, carbamates; and
  • the low proportion of canola seed treated with imidacloprid (Gaucho) in some areas.

Methodology

Approximately 50 GPA samples were collected by researchers, advisors and growers in autumn-spring 2014 and subsequently screened for insecticide resistance. These populations spanned NSW, Victoria and SA.

DNA testing was carried out using TaqMan assays to screen: (i) the kdr and/or super-kdr mutations that confer resistance to pyrethroids; (ii) the MACE mutation that confers resistance to carbamates; and (iii) the overproduction of one of two closely related carboxylesterases (E4 and FE4) that confers resistance to organophosphates. In each case, 20 individual aphids from each population were screened.

Results and discussion

Together with CSIRO, cesar have been mapping the extent of insecticide resistance in GPA across Australia for the past few years. In response to the BWYV outbreak in 2014, approximately 50 populations were tested in south-eastern Australia. Resistance testing of populations sampled from the different regions revealed widespread resistance to the three chemical groups tested. Following DNA tests, aphids from all populations were found to be resistant to synthetic pyrethroids, specifically bifenthrin and alpha-cypermethrin, and to carbamates, specifically pirimicarb. The use of these insecticides is not expected to provide control against these GPA populations in most field situations. The mechanism of resistance to pyrethroids is also likely to render the anti-feeding properties of these products ineffective.

The story for organophosphates, such as dimethoate and chlorpyrifos, is more complex. The amplified carboxyl-esterase mechanism leads to organophosphate resistance in GPA. This mechanism is unusual because it is regulated by DNA methylation, and can be ‘switched on’ in response to pesticide exposure. As a result, aphid populations can quickly adapt to survive organophosphates, even though they may have recently been effective. Following DNA tests, all aphid populations tested were found to contain resistance at the esterase genes. Aphids from these populations are expected to have a moderate level of resistance, five to 20 fold, to organophosphates.

The field efficacy of organophosphates is currently unpredictable. Some control may be achieved against populations found to be ‘resistant’, although the use of organophosphate insecticides is risky and may not be effective; particularly if the population has been exposed to insecticides already in the season. When faced with GPA populations with known resistance to organophosphates, growers are advised to spray test strips within paddocks to determine field efficacy.

There is no confirmed resistance to neonicotinoids in GPA, however widespread resistance is known overseas and there is some anecdotal evidence that resistance may be evolving in Australia. Formal testing for neonicotinoid resistance in a number of GPA populations from across Australia will be undertaken in 2015.

What is likely to happen in 2015?

Preparing for 2015, there are a number of approaches that may reduce risk of GPA:

  • Where practical, reducing the green bridge in and around properties is valuable. Ideally, this should be an area-wide management approach. An individual farmer who controls weeds on his property over summer and early autumn will gain some benefit, but if neighbours do not undertake similar strategies, GPA can still fly around and infest these crops in autumn and winter.
  • Paying attention to weather conditions leading into this year’s growing season will provide growers with a good indication of the likely risk of BWYV. This can be achieved through early monitoring for aphids on brassica weeds, volunteer canola and alternate plants before crops are sown. Avoiding early sowing is also likely to reduce the risk of aphid colonisation and build-up in emerging crops.
  • Sowing canola crops into paddocks with standing stubble may also reduce the risk of aphids colonising emerging crops. This practice could help to limit the risk of BWYV infections.
  • The use of seed treatments containing a neonicotinoid insecticide can provide protection of canola seedlings from aphid attack during the early growth stages.
  • Recognising the widespread resistance in GPA to pyrethroids, organophosphates and carbamates, using Transform® when aphid numbers have reached threshold levels should be considered, but this product should be used as part of a broader resistance management strategy.

Conclusion

In many cases in 2014, growers were unable to achieve adequate control of GPA populations due to the prevalence of insecticide resistance. High levels of resistance to carbamates and pyrethroids are now confirmed to be widespread across Australia. Moderate levels of resistance to organophosphates have been observed in many populations, and there is evidence that resistance to neonicotinoids, e.g. imidacloprid, is evolving. Growers and advisors should implement resistance management strategies for GPA. Further information can be found at the following sites:

Acknowledgements

Anthony van Rooyen and Andrew Weeks (cesar); Owain Edwards (CSIRO); Jenny Davidson, Greg Baker, Bill Kimber, Helen de Graaf and Ken Henry (SARDI); DEPI Vic and NSW DPI virologists; growers, advisers and other industry people who submitted aphid samples for testing.

Resources

Contact details

Paul Umina
cesar
293 Royal Parade, Parkville, Victoria
pumina@cesaraustralia.com

GRDC Project code: CES00001, CES00002