Aphids cause damage by feeding on the growing shoot tips of canola, causing wilting, flower abortion and reduced pod set.
Svetlana Micic, GRDC-supported entomologist with DAFWA in Albany, said they may also cover plants with sticky honeydew that fosters growth of black sooty mould and reduces plants' ability to photosynthesise.
"It is likely that when aphid colonies are present on more than 20 per cent of flowering heads, in areas where drought stress is likely and with susceptible cultivars, yield loss will occur," she said.
"But, before spraying, look at the aphids. If bloated and shiny they have been parasitized by wasps, or if sluggish with a 'fur' covering their bodies, they have been affected by a fungal pathogen.
"Aphids can increase quickly to reach peak numbers and then 'disappear' - often the disappearance is due to biological control agents such as parasites or fungi," Ms Micic said.
Contact: Svetlana Micic, Tel 08 9892 8444