Cabbage aphids have been increasing in canola
- Northam
Cabbage aphids have become noticeable in crops in the Northam region over recent weeks. Research Scientist Dusty Severtson (DPIRD) reports finding high levels in unsprayed canola at Northam, with over one metre of aphid clusters on individual plants. Although the aphid clusters were heavy on plants, they are currently restricted to within 10 to 20 metres of the crop edge.
Cabbage aphids typically continue feeding and reproducing until environmental or plant conditions change, which causes them to produce winged forms to spread further.
The aphid clusters observed on canola racemes were mostly cabbage aphids. Some turnip aphids were also present. Moderate numbers of green peach aphids (GPA) were present on undersides of leaves. Interestingly, many of the aphids were dying from infection by entomopathogenic fungus (insect fungal disease), and so the crop was not sprayed. This fungus is specific to aphids and does not infect plants. Like other fungal spores, it is always present in the environment and only establishes when conditions are suitable, such as a moist canopy from recent morning dews and moderate to warm daytime temperatures.
It is common for multiple aphid species to colonise canola at the same time, and cabbage aphids can cause significant yield loss through feeding damage if numbers are high. The stress from aphid feeding causes stunting and abortion of buds, flowers and pods. Oftentimes canola can compensate for this stress through pod/seed production from other racemes, or compete with nearby less stressed plants for moisture and nutrients. However, yield loss from cabbage and turnip aphids in spring is most pronounced in moisture stressed conditions when plants cannot compensate in time.
There is currently no evidence to suggest that green peach aphids colonising canola in spring causes yield loss. However, yield loss has the potential to occur in cases of consistently warm, dry and very moisture stressed conditions and where high aphid pressure on the pods may cause shriveled seed.
Monitoring and management
Growers and consultants are advised to re-check canola crops for aphids, even if they have been previously sprayed, especially if the crop has experienced frost.
Growers also need to check canola crops for fungal infections and parasitism in aphids before deciding whether to invest in an insecticide spray. Pale or orange aphids indicate that fungal disease infection has occurred. Natural enemies such as parasitoid wasps, ladybird beetles and hoverfly larvae often help to keep aphid populations below threshold levels, and their numbers will also increase with warming weather. If, on close inspection, the aphids look shiny and bloated, it means they have been parasitised by wasps. For more information see the department’s know what beneficials look like in your crop page.
Cabbage aphids form dense bluish grey colonies covered with fine whitish powder on flowering racemes in spring. They also establish on crowns and under leaves in the vegetative phase. Infestations are usually as ‘edge effects’ or 'hotspots' in the paddock. The adult aphid is dull or greyish-green. Cabbage aphids can feed on canola plants, particularly if warm daytime temperatures are experienced which favour aphid reproduction and feeding damage.
If more than 20% of plants are infested with colonies of cabbage or turnip aphids, control measures should be considered to avoid yield losses. The risk of economic yield losses to aphids is increased if canola crops are already under some degree of moisture stress or experience poor finishing rains.
If spraying is required, a border spray may suffice for cabbage aphids, which are most commonly found within 20 to 30 metres of the crop edge. Insecticides registered for use on aphids can be found in DPIRD’s 2023 Winter Spring Insecticide Guide.
Also consider insecticide options that are soft on predators if spraying. Information on what effect the different insecticides have on natural enemies can be found in Cesar Australia’s recently updated Beneficials Chemical Toxicity Table.
GPA are found on the underside of canola leaves and are evenly distributed in the paddock. Applying insecticides to control feeding damage by GPA in the absence of virus may be of little benefit and could increase the rate of insecticide resistance. GPA are resistant to many insecticides, and will develop resistance very quickly, so consider spraying only if these aphids are retarding crop growth and causing crop death.
For more information about GPA, refer to:
- DPIRD’s canola yield loss to green peach aphid, Geraldton 2016 trial report
- GRDC’s yield loss to canola from green peach aphid page
- GRDC’s Resistance Management Strategy for the Green Peach Aphid in Australia Grains page.
Growers, consultants and other field operators looking to see how close they are to the canola aphid threshold can download DPIRD’s CropScout app. The app is designed to assist with inspection of canola for aphids relative to the spray threshold, and allows users to easily record and visualise results on a map. This field intelligence can then be used to optimise spray timing and, where possible, to target sprays to pest infestations across properties or within individual crops.
Growers and consultants can use the PestFacts WA Reporter app to request or confirm identification of aphids found in crops.
More information
For more information on identifying and managing canola aphids refer to:
- DPIRD’s aphid management in canola crops page
- DPIRD’s diagnosing canola aphids page
- DPIRD’s 2017 Protecting WA Crops Issue 3 newsletter Aphids – WA’s insect problem children
- GRDC’s aphids in your crops YouTube video.
For more information contact research scientists Svetlana Micic in Albany on +61 (0)8 9892 8591 or Dusty Severtson in Northam on +61 (0)8 9690 2160.
Article author: Bec Severtson (DPIRD Northam).
Article input: Dusty Severtson (DPIRD Northam).