Green peach aphid control in canola
Green peach aphids attack many crops in Australia, with canola being particularly vulnerable in broadacre cropping areas of the WA grain belt. As agronomist Benita Moir explains, resistance is upon us in several popular insecticide groups, making aphid monitoring, identification and management planning critical.
As the name suggests, green peach aphid GPA (Myzus persicae) is an aphid that can be problematic in canola crops, pulse crops and horticultural crops throughout Australia. There are also a range of non-crop hosts, including broadleaf weeds (such as capeweed, marshmallow, radish) and some broadleaf pasture species.
GPAs are soft-bodied, around 3 millimetres long, and can be either winged or wingless. They are typically a pale yellow-green or green colour but can sometimes be pink or orange. Although populations typically peak in autumn and spring, they can be found all year round, with the generation time being less than two weeks in ideal conditions. GPAs typically reproduce asexually and females give birth to live young (imagine a Babushka doll).
Damage and yield effect
GPAs damage crops through direct feeding, depriving the plant of nutrients. Along with sucking the sap out of their victims’ leaves and flowers, GPAs also act as vectors for plant viruses such as Turnip Yellows Virus (TuYV) and Cucumber Mosaic Virus (CMV). Given these viruses cannot be controlled, and only take a few aphids to transmit, the repercussions often prove more threatening than the act of direct feeding itself. Found on the underside of the lower leaves, the GPA will commonly target weak, vulnerable and stressed plants. Interestingly, feeding damage is often lower than that of other aphid species, unless populations are out of control and plants are moisture stressed.
Although GPAs have the potential to knock around canola crops from the seedling stage to podding, they have rarely been found to cause large yield losses at a later crop stage. In fact, it is often late infestations of Diamond Back Moth (DBM) larvae that are more likely to damage canola, particularly if the crop is stressed. Canola can usually ‘outgrow’ or cope with aphid damage once it gets a bit of size to it, so monitoring is often all that is required.
Image showing turnip yellow virus in canola
Integrated management options include:
- Controlling the ‘green bridge’ over summer and autumn, as this often provides a refuge for aphids between cropping seasons.
- Delay sowing to avoid peak aphid flights.
- Choose hybrid varieties for earlier and more robust crop establishment.
- Sow into standing stubble – aphids are more attracted to open rows of plants with bare earth between.
- Seed treatments – to protect seedlings from early infestations. The duration of control will depend on seasonal conditions, resistance and the product used.
- Monitor aphid predators and parasitoids.
If these aren’t viable options, or if infestation levels become severe, a foliar insecticide spray may be necessary. Unfortunately, no official economic spray threshold for direct feeding damage and vectored virus transmission exists for GPAs.
Anecdotally, canola crops are probably worth spraying if 50 per cent of plants have 15 or more aphids, and if crop yield is expected to be greater than 1 tonne a hectare. If there are ‘hotspots’ or bare patches the size of a ute, it is likely worth spraying. So, it really is a case of working with your agronomist to establish a reasonable strategy by weighing up risks, costs and outcomes.
GPA resistance to synthetic pyrethroids and carbamates is widespread, as well as low-level resistance to organophosphates and neonicotinoids. This leaves us with the sulfoximine group, notably sulfoxaflor (Transform).
Sulfoxaflor is translaminar and systemic, moving upward within the plant xylem, providing protection to existing and new growth. Good coverage is still critical in order to reach aphids at the base of the plant. Although two applications per crop per season is allowed, they must not be consecutive (when targeting aphids) and they must not be applied after the full flower stage of canola (to avoid MRL issues).
Introduced to the market in 2012, it wasn’t until 2018 that a concerning shift in tolerance of GPA to sulfoxaflor was first noted in the Esperance region. New insecticide options to the market include flonicamid (Mainman) and afidopyropen (Versys®), which give us the opportunity to control GPAs up to the end of flowering in canola, should the need arise. It also allows us to rotate three chemistries as an effective resistance management strategy.
It is vital to protect these new and existing chemistries by always adhering to label instructions and correctly identifying aphids.
Acknowledgements: GRDC, DPIRD, Corteva.
Elders offers a range of crop protection products.
Need specialist help and advice regarding pests in your crop? Talk to an Elders agronomist today.