If you’re a grain grower in Australia, then chances are that at some stage you’ve used seed treatments to protect your crops from aphid damage.
If so, new research into the sensitivity of the green peach aphid to neonicotinoid seed treatments means that you may want to re-think how and when these treatments are best used.
The growing threat of neonicotinoid resistance
Pest aphids contribute a loss of $200-$400 million per year to the grains industry due to the damage caused by direct feeding and virus transmission. The green peach aphid (Myzus persicae) 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.
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 5 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.
The green peach aphid has evolved high-level resistance to synthetic pyrethroids and carbamates, and 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 the green peach aphid, and are now present in some Australian field populations.
The level of neonicotinoid resistance in Australia is relatively low, and current opinion held that this would not lead to field control difficulties when neonicotinoid seed treatments are used at the registered label rates.
However, new research by researchers at Cesar Australia, with the support of the GRDC, suggests that neonicotinoid resistance in Australian green peach aphid can reduce the efficacy of seed treatments, increasing the risk of early aphid establishment and virus transmission in young canola plants.
Seed treatments versus resistant aphids
Cesar Australia researchers undertook a series of genetic analyses, laboratory bioassays, and a large-scale glass house trial to assess the risk that neonicotinoid resistance in the green peach aphid 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 the green peach aphid.
The aphid populations from Queensland and Western Australia showed an increased copy number of the metabolic gene, CYP6CY3, compared with the susceptible population. Follow-up laboratory bioassays further 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 was quite low.
Research shows a risk of field 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 with either 0.016 mg thiamethoxam + 0.003 mg lambda-cyhalothrin (i.e. Cruiser® Opti) or 0.019 mg imidacloprid (i.e. Gaucho®) per canola seed. The seedlings were exposed to aphid populations at different plant growth stages: 2, 4, 6, 8, and 10 weeks after emergence. The aphids were allowed to establish and then were counted at 7 and 14 days after introduction.
As expected, the two insecticide seed treatments proved 100% effective at controlling the susceptible aphid population through to 10 weeks post emergence. Similar results were expected for the two resistant populations, however quite surprisingly, both Gaucho® and Cruiser® Opti provided only partial control of the Western Australian resistant population, with mortality rates of 70% and 90%, 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.
Whilst partial control of green peach aphid 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.
What are the next steps?
Glass house 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 aphid using neonicotinoid-based seed treatments in canola could become more difficult.
This may require a re-think 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.
Integrating pest management strategies is an important way to keep on top of green peach aphid numbers before, and during, the early crop establishment period.
The Best Management Practice Guide outlines how to adopt such strategies, and the Green Peach Aphid Resistance Management Strategy describes how we to best manage resistance in this pest. Recommendations include:
- Assessing the risk of aphid infestations based on the seasonal history before utilising 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.
- Avoiding the use of broad-spectrum insecticides to encourage beneficial insects.
Have you had any control failures? Does green peach aphid in your crops seem particularly persistent? Do you suspect these aphids might have evolved resistance to neonicotinoids or any other chemicals?
If so, get in contact with Cesar for free insecticide resistance testing thanks to funding from the GRDC. Resistance testing will provide you with results outlining the resistance(s) present in your aphid population, and will help us to better understand the distribution of resistance nationally. Just follow the procedure outlined in the aphid collection guidelines. If you have any queries, please contact Dr Samantha Ward at email@example.com.
This research was carried out by Lisa Kirkland, Evatt Chirgwin, Samantha Ward, Benjamin Congdon, Anthony van Rooyen and Paul Umina, and undertaken as part of a Grains Research and Development Corporation 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.