Aphids in canola

Insects and damage Severe infestation by cabbage aphid may cause death of young plants. Green peach aphid is most common in autumn and rarely causes economic damage to canola, however early colonisation by virus-infected aphids (particularly green peach aphid) can result in canola yield losses due to Beet Western Yellows Virus (BWYV) infection.

Canola is most sensitive to damage at bud formation and flowering. Infestations reduce flowering and reduce/prevent pod-set & pod-fill on the infested racemes, although canola can compensate to some extent through larger seed in pods that are set and/or more flowering branches.

Canola is a higher risk of aphid infection if warm dry spring promotes rapid population growth, or if moisture stress limits crop compensation, pod set and pod fill.  Crops are also at high risk after a wet autumn that promotes an abundance of weedy hosts.

Monitor and record In high-risk seasons, visually check stems, lower leaves and buds during the vegetative stage as early infestations may continue to develop into hotspots.  Heavily infested plants will often show signs of wilting, particularly if the crop is water-stressed. The relative benefit of aphid control at vegetative stage is currently unknown.

Monitor weekly during flowering and pod set. Sample at least 5 sites per field, visually inspecting amongst buds and flowers on about 20 plants per site. Populations are often patchy; aphids commonly  infest plants on crop edges although winged aphids can colonise plants throughout the field. Signs of aphid infestation include plants covered in cast skins, and the presence of sticky honeydew. Compare monitoring data over time to determine whether aphid populations are increasing or decreasing.

Heavy rain can reduce aphid populations.

At each sampling note:

  • The aphid species present in the crop and their relative abundance (some species are resistant to some insecticides).
  •  % infestation (=heads infested ÷ heads counted)*100.
  • The size of the infestation on each head, in centimetres of stem infested. This tells you how established the infestation is and can be used at repeat visits to determine how rapidly the population is increasing
  • Impact of the infestation on the crop
  • Presence of beneficials – number and species; include aphid mummies as an indication of parasitoid wasp activity
Beneficial insects Aphid populations can be impacted by both parasitoid wasps and predators (e.g. ladybirds, hover flies and lacewings), although they may not arrive in a crop early enough to prevent the build-up of aphid numbers to above-threshold levels. Befeficials can suppress low to moderate aphid populations but will not control heavy infestations quickly enough to prevent impact on the crop. The number of beneficials required to control a population of aphids has not been studied in canola.

Consider the impact on beneficial populations if spraying for aphids (or other pests), particularly early in a season. Refer to the impact of insecticides on beneficials table.

Cultural control Keep adjacent fields weed free to minimise alternate hosts (e.g. wild turnip, wild radish and capeweed).  Winged aphids may move large distances, so weed control to minimise aphid outbreaks is most likely to be effective on an area-wide basis.

In a season with warm, wet autumn and spring, the abundance of hosts and aphids will make the management of alternative hosts difficult. Sow crops early, where possible, to enable plants to flower and set pods before aphid numbers increase.

Thresholds Consider control if:

  • If >20% of plants infested with colonies (Western Australia).
  • If >50% of plants with clusters 25 mm long on stems or 4–5 stems per m2 with clusters 50 mm long on stems (NSW).

The number of heads per plant infested within a crop is more important than the size of infestations when considering control. Small populations will prevent the head from flowering and setting pods normally. Early infestation is likely to have a greater impact on yield.

It is likely that compensation plays an important part in moderating yield loss in crops. Crops that are drought stressed are more likely to suffer yield loss. However, the reduction in yield potential of stressed crops can mean that it is uneconomic to control pests, even if they are causing visible damage to the crop.

Chemical control Insecticides with fumigative, systemic or translaminar activity tend to be most effective for the control of pests that shelter in areas of the plant not easily reached by chemicals (e.g. aphids in amongst flowers/buds in canola), however the translocation of chemicals may be reduced under very dry conditions.

  • If chemical control is required, consider early application of aphid-specific products to preserve beneficials, particularly where the potential for reinfestation is high. The use of broadspectrum insecticides (e.g. pyrethroids, methomyl)  may flare aphid and DBM numbers because of the reduction in beneficial populations. Refer to the beneficial impact table to identify products least likely to kill non-target, beneficial species.
  • Seed treatments and border spraying (autumn/early winter) when aphids begin to colonise crop edges may provide sufficient control. Early control of aphid populations can delay build up significantly, but may be of little benefit in seasons with high pest pressure high or or if aphid immigration occurs over a long period. Seed treatment has been found to be effective in minimising the impact of Beet Western Yellows Virus in canola.

Insecticide resistance:

  • Rotate chemical groups to minimise the development of resistance, particularly where a pest is treated more than once in the same crop.
  • High levels of resistance to carbamates and pyrethroids are now widespread in green peach aphid across Australia. Moderate levels of resistance to organophosphates have been observed in many populations, and there is evidence that resistance to neonicotinoids is evolving. See the Insecticide Resistance Management Strategies page for more information.
Communication An Area Wide Management approach to the control of weed and volunteer hosts may be useful.

Industry publications and newsletters provide up-to-date regional information about pest activity in crops.

 Further information

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