Aphids are a priority pest to manage for most Australian producers, and new research from Cesar Australia investigating the implications of neonicotinoid resistance in the green peach aphid (Myzus persicae) could change how and when these chemical treatments are best used in broadacre crops.

Pest aphids contribute a loss of $200 to $400 million per year to the grains industry due to the damage caused by direct feeding and virus transmission. The green peach aphid is a species of particular concern, as it acts as the main vector of a number of economically damaging plant diseases, such as turnip yellows virus (formerly known as beet western yellows virus) in canola and pulse crops.

This risk of damage and disease from aphids is particularly high in the earliest stages of crop development, when the small size of the seedlings and exposed growing tips leave them at risk of significant yield losses.

Growing threat of neonicotinoid resistance

In Australia, the grains industry relies heavily on neonicotinoid-based seed treatments to protect broadacre crops against aphids and other pests. They are seen as a cost-effective, pre-emptive protection measure often referred to as an “insurance policy”.

Seed treatments are designed to provide protection against aphids during the vulnerable establishment stage for a period of five weeks, or even longer. This type of treatment is also favoured as it is seen as an environmentally friendly management option. Seed treatments reduce the impact on non-target species, and when used strategically can reduce the need for foliar insecticide sprays later in the season.

However, the use of seed treatments is not without caveats, and a chemical’s effectiveness will continue to be tempered by a pest’s ability to evolve resistance.

Green peach aphids have evolved high-level resistance to synthetic pyrethroids and carbamates, as well as low-level resistance to organophosphates and neonicotinoids. All four of these resistances are now widespread across Australia. Quite recently, low-level resistance to sulfoxaflor and moderate resistance to spirotetramat have also evolved in green peach aphids, and are now present in some Australian field populations.

Resistance

Only metabolic resistance to neonicotinoids has been identified in Australian green peach aphid populations, and current opinion is that this would not lead to field control difficulties when neonicotinoid seed treatments are used at registered label rates.

However, new research by scientists at Cesar Australia, with the support of GRDC, suggests that neonicotinoid resistance in Australian green peach aphids can reduce the efficacy of seed treatments, increasing the risk of early aphid establishment and virus transmission in young canola plants.

“Our research challenges the assumption that metabolic resistance is not important, and shows that under the right conditions, it can change the way an aphid responds to these seed treatments,” says Lisa Kirkland, the researcher leading the trial.

Laboratory studies

Researchers at Cesar Australia undertook a series of genetic analyses, laboratory bioassays and a large-scale glasshouse trial to assess the risk that neonicotinoid resistance in green peach aphids posed to canola seedlings grown from neonicotinoid-treated seed.

Three aphid populations were used in the research study: two populations known to possess low-level resistance to neonicotinoids (one population originating from Queensland and one from Western Australia); and a laboratory-derived population that does not possess any resistance to chemicals whatsoever.

The resistance status of each population was first confirmed using a series of standard aphicide bioassays as well as genetic testing that is able to identify the presence of the resistance mechanism conferring low-level resistance to neonicotinoids in green peach aphids.

The aphid populations from Queensland and WA showed an increased copy number of the metabolic gene CYP6CY3 compared to the susceptible population. Follow-up laboratory bioassays demonstrated that the increased gene copy allowed these aphids to survive higher doses of neonicotinoids – due to the ability to metabolically detoxify the toxin – albeit the level of resistance in the Queensland and WA populations was quite low.

Research shows risk of control failures

The three aphid populations were then introduced to canola seedlings grown from untreated seeds or seeds treated at the Australian registered label rates of either 0.016 milligrams thiamethoxam plus 0.003mg lambda-cyhalothrin (Cruiser^® Opti) or 0.019mg imidacloprid (Gaucho^®) per canola seed. The seedlings were exposed to aphid populations at different plant growth stages: two, four, six, eight and 10 weeks after emergence. The aphids were allowed to establish and then were counted at seven and 14 days after introduction.

As expected, the two insecticide seed treatments proved 100 per cent effective at controlling the susceptible aphid population through to 10 weeks post-emergence.

Similar results were expected for the two resistant populations, however both Gaucho^® and Cruiser^® Opti provided only partial control of the WA resistant population, with mortality rates of 70 per cent and 90 per cent respectively on average for plants across all growth stages.

The Queensland resistant population proved even hardier, with neither treatment providing sufficient aphid control. This indicates that metabolic resistance is much more important than first thought.

What does this mean for canola management?

Nearly all canola seed in Australia is purchased pre-treated with a treatment containing a neonicotinoid.

This latest research provides new evidence that the ability of aphids to metabolise neonicotinoids through an increased copy number of the CYP6CY3 gene can significantly reduce the effectiveness of neonicotinoid-based seed treatments.

While partial control of green peach aphids might protect young seedlings from feeding damage, it is unlikely to be sufficient to prevent virus transmission. The consequences of this failure could prove to be quite important if the infection occurs early in the crop’s development.

The glasshouse trial experiment set-up. Photo: Cesar Australia

Glasshouse trials, as undertaken in this study, do not reflect the range of variance that occurs in field, however they have been found to be a strong indicator of field performance in the past. As such, attempts to control green peach aphids using neonicotinoid-based seed treatments in canola could become more difficult.

This might require a rethink of how and where seed treatments are best placed in broadacre crop management and also points to the value of closely monitoring crops during the seedling stage, even when neonicotinoid seed treatments have been used.

Mrs Kirkland advises growers: “Don’t take control for granted, and don’t assume that just because you have used a seed treatment that your crop will be protected. The best thing you can do is get out there and monitor for aphids during the early crop establishment phase.”

Integrating pest management strategies is an important way to keep on top of green peach aphid numbers before – and during – the establishment period.

The Green Peach Aphid Best Management Practice Guide outlines how to adopt such strategies, and the Resistance Management Strategy for Green Peach Aphid describes how to best manage resistance in this pest. Recommendations include: assessing the risk of aphid infestations based on the seasonal history before using seed treatments; avoiding the use of neonicotinoid seed treatments in consecutive years; if pests are noticed, conducting monitoring and careful identification of the pest before spraying; rotating insecticides with different modes of action; and avoiding the use of broad-spectrum insecticides to encourage beneficial insects.

If you have experienced chemical control failures or suspect aphids have evolved insecticide resistance, you can contact the team at Cesar Australia.

As part of the investment from GRDC, Cesar Australia is offering resistance testing service for green peach aphids in 2023 at no extra cost to growers and advisers. For further information, contact Dr Samantha Ward at sward@cesaraustralia.com.

Resistance types

There are two primary resistance mechanisms to neonicotinoids in green peach aphids.

The first is known as target site resistance. This occurs due to a genetic mutation that alters the insecticide site of action within the aphid, which stops the neonicotinoid effectively binding there. This mechanism can drastically reduce the efficacy of the chemical and provides a high level of resistance in the aphid.

The second is known as metabolic resistance. This is caused by an overexpression of the gene responsible for metabolising neonicotinoids, referred to as CYP6CY3. This allows the aphids to break down the toxin faster and survive higher doses of neonicotinoids, however it provides a much lower level of resistance in the aphid.

Acknowledgments

This article is based on research carried out by Lisa Kirkland, Evatt Chirgwin, Samantha Ward, Benjamin Congdon, Anthony van Rooyen and Paul Umina, and undertaken as part of a GRDC investment, ‘Insecticide resistance in the green peach aphid: national surveillance, preparedness and implications for virus management (CES2001-001RTX)’. This investment is led by Cesar Australia in collaboration with CSIRO and the WA Department of Primary Industries and Regional Development.