While aphids are present in cropping regions year-round, mild conditions during autumn and spring are particularly favourable to aphid colonies growing in size.
The ability to reproduce asexually (without a mate) is one reason why aphids can pose a risk to crops, as aphid numbers can increase quickly.
Aphid offspring are clones of the parent aphid, and are born ready to feed on the host plant.
For canola growers and growers of other cruciferous crops there are two aphid species that become prevalent during spring.
These are the cabbage aphid (Brevicoryne brassicae) and the turnip aphid (Lipaphis pseudobrassicae).
Both of these aphid species tend to be found on the growing points of canola plants during the flowering and podding growth stages, rather than on other parts of the plant, such as the leaves.
Heavy infestations of these aphids can pose a risk to development of flowers and pods, in turn impacting oil yield and quality.
These aphids can look quite similar to the casual observer, and they can form mixed colonies on the same plant, making species identification difficult.
However, there are some key morphological characteristics that can be seen with a hand lens or with a macro lens attached to a phone camera.
Generally, when identifying an aphid it is useful to pick out a wingless adult and consider the colour and texture, the length of the antennae, and the length of the siphuncles (exhaust pipe-like features that stick out near the tail area).
Both cabbage and turnip aphid are no more than three millimetres in length as adults.
Cabbage aphid are a dull grey-green and are often covered in a fine coat of white waxy powder.
This makes cabbage aphid colonies appear fuzzy, and blue-grey in colour.
The cabbage aphid 'exhaust pipes' are very short and can hardly be seen.
In comparison, the turnip aphid has exhaust pipes that are slightly longer, which reach the base of the tongue-shaped tail.
The turnip aphid is olive to grey-green in colour and does not have a powdery coating.
Instead, it has a matt texture owing to a covering of wax, which collects along its back to give it the appearance of having dark bars running across its body.
Aphids feed in a piercing and sucking manner, drawing out plant phloem, also known as sap, via a long stylet, which is a tube-like appendage that is inserted into the plant tissue (much like a mosquito feeding on an animal).
Aphid feeding alone can result in reduced plant vigour, tissue necrosis, malformation of developing tissues and wilting, however, transmission of plant viruses can add to the threat that these species pose to crop plants.
Cabbage and turnip aphids carry several plant viruses, including Turnip yellows virus (TuYV).
This virus does not pose a significant risk to canola that has grown past the rosette stage, however, establishing spring sown canola may be at risk.
Stiffening of leaves in young canola, as well as red colouration of leaf tissue and stunting of plants are symptoms of TuYV infection.
At this time of year growers can expect to see beneficial insects out and about in their fields, which provide a valuable service when it comes to aphid control.
This will include species of minute wasps, that are adept at tracking down colonies of aphid and depositing their eggs in them.
As the weather warms, other natural enemy species, such as hoverfly larvae and lacewings, are also likely to be found predating on cabbage and turnip aphid colonies.
For more information on turnip and cabbage aphids visit our PestNote at cesaraustralia.com/sustainable-agriculture/pestnotes/.
- Dr Jessica Lye is an extension lead with cesar.
