Learnings from green peach aphid and diamondback moth in 2014

Date: 22 May 2015

SARDI's Jenny Davidson.

SARDI senior pulse pathologist Jenny Davidson says the seasonal conditions in the first half of 2015 are contrasting to that of 2014, meaning the risk of GPA and BWYV is reduced this season.

Season 2014 seemed to be one of curve balls for grain growers in south-eastern Australia. A near-perfect start with good summer and autumn rain coupled with mild temperatures saw crops jump out of the ground. But, as it turned out, crops were not the only ones thriving under such conditions. Insect pests including green peach aphid (GPA) and the resulting beet western yellows virus (BWYV or synonym, turnip yellows virus) and diamondback moth (DBM) both proved headaches for growers at different stages of the growing season.

These pests, coupled with severe frosts and a spring drought, slashed the yield potential of many canola crops across South Australia, Victoria and southern New South Wales. But what did we learn from these experiences and how can we better manage our crops in future?

GPA and BWYV in 2014

South Australian Research and Development Institute (SARDI) senior pulse pathologist Dr Jenny Davidson was in her office in early-June last year when she received a call about a problem with some canola crops in the lower north of SA.

“I went out and had a look and what I could see was canola seedlings from fence line to fence line turning purple and not growing,” Dr Davidson recalled. “Entire crops had been taken out and it looked like herbicide damage or nutrient deficiency, but there were crops behaving the same way on several farms, and growers and agronomists were talking about aphids and how many they had seen early in the season.”

Damaged canola plant samples were sent to Dr Mohammad Aftab at the Department of Economic Development, Jobs, Transport and Resources (DEDJTR) at Horsham, Victoria, for virus testing. Every sample returned a positive test for BWYV.

The severity of the problem became even more apparent to Dr Davidson when she and colleague, SARDI entomologist Greg Baker, spoke at a field day in the lower north at around the same time. They were expecting a handful of concerned growers to turn up, but ended up addressing a crowd of more than 100.

The damage

“Just about every canola crop in the lower north was impacted by GPA and BWYV in some way or another,” Dr Davidson said.

But the lower north of SA was not alone. Across south-east Australia, from the Eyre Peninsula through to south-east SA, the Victorian Mallee and southern NSW, 618 samples were sent to DEDJTR or SARDI, with about 40 percent of those coming from SA regions (table 1). Of the 618 tested from south-east Australia, 57pc of the crops were infected with BWYV. However, 87pc of the crops that were tested from SA farms tested positive to BWYV.

Table 1: Incidence of BWYV in canola for SA regions in 2014 determined by virus testing of plant material at DEDJTR, Horsham.

 SA district
 Number of canola BWYV in 2014
Positive
Negative
 Lower north
 58  4
 Mid north
 10  0
 Upper north
 17  1
 Lower Eyre Peninsula
 36  18
 Mid & upper Eyre Peninsula
 36  8
 SA Mallee
 22  1
 Yorke Peninsula
 56  2
 South East
 16  5
 Total (290)
 251  39
   86.55pc  13.45pc

While not every canola crop was infected or wiped out, Dr Davidson says it was canola plants that were infected at the rosette stage which received the most severe damage

“The crops that were infected at the rosette stage were the most severely damaged because the virus got into the phloem of the plant and prevented the translocation of nutrients and water throughout the plant,” Dr Davidson said. “Working on estimates, about 10,000 hectares of canola in SA was damaged to that level, as well as some isolated pockets of canola in southern NSW and the Victorian Mallee.”

Dr Davidson says less than about 10-15pc of canola in SA was very severely damaged and only yielded 500-600 kilograms per hectare, or 25pc of normal yield.

“Anything in that area would have been hit at an early stage. Just over half of the canola crops tested in SA yielded about 1t/ha or 50pc of normal yield. The hard thing is trying to figure out how much of that was BWYV, drought, frost and DBM, but there has been some work done in WA previously which estimates half of that yield loss comes from virus infestation – so about 15-20pc of yield loss from BWYV.”

Why last year?

Green peach aphids

Green peach aphid has been found to have widespread resistance to three chemical groups.

 

The weather conditions in 2014 played a big part in the incidence of GPA and BWYV in 2014. While both are quite common aspects of most cropping seasons, the weather in 2014 – good summer rain into autumn, combined with mild, almost subtropical temperatures – created a substantial green bridge for GPA to survive on over summer. This was also aided by the fact that there is a vast range of host broadleaf weeds for GPA.

“In May 2014 we had quite warm conditions,” Dr Davidson added. “Looking at Bureau of Meteorology data from last year, across SA and into Victoria, May was about two degrees Celsius warmer than normal, which was when canola was emerging at the seedling stage. When there are warmer temperatures like that, the GPA population expands at an exponential rate.

“With higher temperatures there is increased replication of the virus in the plant – more than what there would normally be – which leads to stronger symptoms of BWYV in the plant.”

Lessons for this year – 2015

So far, the weather conditions over summer and autumn in 2015 have been much less conducive to GPA than they were in 2014. Dr Davidson says this means the risk of BWYV in crops this year is much lower than last year.

However, one thing that was apparent from 2014 was the level of insecticide resistance among GPA populations. Dr Paul Umina from cesar and the University of Melbourne, is leading a Grains Research and Development Corporation (GRDC) funded project looking at resistance levels in GPA populations across south-east Australia.

During autumn and spring in 2014, about 50 GPA samples were collected from SA, Vic and southern NSW and screened for insecticide resistance. Dr Umina says this revealed widespread resistance (table 2) to the three chemical groups tested.

Table 2: Insecticide resistance in GPA

 

 Insecticide group
Example product names
Known resistance
Implications for GPA control
 Pyrethroids (3A)
Astound® Duo, Dominex® Duo, Fastac® Duo, Fastac® Excel
 Widespread These chemicals are not recommended
 Carbamates (1A)
Pirimor®, Aphidex®
Becoming widespread
Consider small strip field test to assess efficacy
 Organophosphates (1B)
Dimethoate
Common Consider small strip field test to assess efficacy
 Sulfoxaflor (4C)
Transform®
None ‘Best-bet’ option if widespread resistance present

"Following DNA tests, aphids from all populations were found to be resistant to synthetic pyrethroids (SPs), such as bifenthrin and alpha-cypermethrin, and to carbamates, specifically pirimicarb,” he said. “The use of these insecticides is not expected to provide effective control against these GPA populations in the field.

“The field efficacy of organophosphates (OPs) is currently unpredictable. Some control may be achieved against populations found to be resistant, although the use of OP insecticides is risky and may not be effective, particular if the population has been exposed to insecticides early in the season. When faced with GPA populations with known resistance to OPs, growers are advised to spray test strips within paddocks to determine field efficacy.”

A resistance management strategy for GPA is available from the GRDC website (see resources below). Dr Umina says growers should familiarise themselves with this strategy to ensure GPA are controlled in a strategic manner that will ensure the greatest possibility of preserving those insecticide products that are currently effective.

Another lesson from 2014 was that canola sown into paddocks with standing stubble helped to reduce the level of BWYV infection in the early stages of the season. Last year, Dr Davidson says paddocks with standing stubble delayed the arrival of GPA into canola crops.

“In one of our field trials there was a paddock with stubble removed and stubble standing and there was a very clear line where you could see the GPA had got into the section with removed stubble first. If stubble is removed or there is a thin area of the paddock with smaller plants, that’s where GPA will land first.”

DBM in 2014

SARDI's Greg Baker

SARDI entomologist Greg Baker says growers should start monitoring for DBM early in the year and work within the threshold guidelines in planning a control strategy. Photo: Rebecca Jennings

Much like GPA, DBM thrived on the favourable weather conditions early in the 2014 growing season. In particular, the amount of brassica weeds and volunteer canola that germinated over the summer following good rain provided an ideal habitat for DBM populations.

The dry spring later in 2014 provided a further boost to DBM, according to Mr Baker.

“That lack of rain from August and throughout spring resulted in greater DBM survival in-crop,” he said. “In many districts, DBM was reported to be increasing in abundance from late winter and, as spring progressed, the populations continued to climb. This resulted in greater spraying for DBM across the board than what occurs in most seasons.”

DBM damage and control

Mr Baker says it is hard to gauge just how much yield loss DBM caused to canola crops across the southern region in 2014, as many crops had also been affected by GPA and BWYV which had impacting the yield potential of crops earlier in the season.

DBM is notoriously hard to control with insecticides because it has overlapping generations with the ability to rapidly develop resistance to insecticides (table 3).

Table 3: Insecticide resistance in DBM

Insecticide group
Example product names
Known resistance
Implications for DBM control
 Carbamates (1A)

Lannate, Marlin, Methomyl (registration limited to certain states)

Expected, but not recently tested
Consider small strip field test to assess efficacy
 OPs (1B)
 Diazinon, Diazol (registration limited to certain states)
Moderate-high, and widespread
Consider small strip field test to assess efficacy
 SPs (3A)
Astound® Duo, Dominex® Duo, Fastac® Duo, Fastac® Xcel
High level and widespread
NOT recommended
 Spinosyns (5)
Success Neo®
Nil–very low
Recommended, rotate with other effective groups

“Whenever you spray DBM barely half of the population will be in the susceptible larval stage - the rest will be eggs, pupae and adults, so it’s a difficult target in terms of its age structure,” Mr Baker said. “And when you factor in its resistance development it makes it a challenging pest to control.

“We heard anecdotal evidence last year from respected agronomists that where there had been less spraying earlier in the growing season for GPA and other pests, the DBM didn’t build up as rapidly or their populations plateaued below the economic threshold so growers didn’t have to spray for DBM. By contrast crops treated with a heavier early spray program were more likely to require a DBM spray. Those agronomists were attributing that to the effect of the earlier spray treatments on the natural enemy populations.”

The 2015 outlook

Diamondback moth larvae

DBM is notoriously hard to control with insecticides because it is a periodic pest and has overlapping generations with the ability to rapidly develop resistance to insecticides.

Mr Baker says the colonisation of DBM in canola crops this year looks to be at a lower level for much the same reason as the risk of GPA and BWYV is this year – a much drier summer and early autumn than last year and less of a green bridge on which to survive. However, growers need to monitor the situation as the season progresses.

“Cold, wet winters tend to reduce and delay a spring build-up of DBM whereas drier and warmer winter and early springs are conducive for DBM survival and population build-up,” he said. “While it is a favourable scenario for growers as we start the season, we can’t be complacent because if warm and dry winter/spring conditions develop, DBM could still develop into damaging infestations come spring.”

Mr Baker said growers should start sweeping for DBM around early bolting and work within the threshold guidelines (table 4) in planning a control strategy.


 

Table 4: DBM threshold guidelines by crop stage

 Crop stage
Moisture stress
DBM larvae threshold
 Pre-flowering  Yes 30 per 10 sweeps
 No 50 per 10 sweeps
 Early to mid-flowering
 Yes  >50 per 10 sweeps
 Mid to late-flowering
 Yes  >100 per 10 sweeps
 Pod maturation
 Yes 200 per 10 sweeps

GRDC projects

The GRDC is funding a number of DBM projects in the southern region.

  • DBM Control and Insecticide Resistance Management (SARDI): Tracking resistance levels to various insecticides in populations across western and southern Australia and facilitating the registration of new chemistries.
  • Regional movement of DBM and colonisation of canola in southern Australia (University of Adelaide): PhD project to improve the understanding of movement of resistance types within a population to manage resistance, and learn more about timing and sources of colonisation to better predict which seasons will present a risk of DBM damage.
  • Options for Improved Insecticide Use and Canopy Penetration in Canola (University of WA): investigating spray coverage and canopy penetration in canola, including a trial to be run this spring in the southern region.
  • Attract and Kill Technology for Diamondback Moth (University of New England): investigating the potential of a product with a food attractant to draw the moths, with a toxicant which will provide a lethal dose of insecticide.

More information

Jenny Davidson, 08 8303 9389, jenny.davidson@sa.gov.au

Greg Baker, 08 8303 9544, greg.baker@sa.gov.au

Paul Umina, 03 9349 4723, pumina@cesaraustralia.com

Resources

Resistance Management Strategy for the Green Peach Aphid in Australia Grains

GRDC Crop Mites Back Pocket Guide

Integrated Pest Management Fact Sheet - South, West

Diamondback moth GRDC fact sheet

Diamondback Moth in Canola GRDC Hot Topics

Green peach aphid and beet western yellows virus GRDC Hot Topics

GRDC Project Code CES00001, DAN00179, DAW00229, DAS00139, CES00002, DAS00151, DAS00094, UA00146, UWA00165, UNE00016

Region South, North