Cabbage aphids

With canola crops now beginning to flower in some regions, numbers of cabbage aphids (Brevicoryne brassicae) are likely to be on the rise. Moderate numbers of cabbage aphids have been sighted in a mid-flowering canola crop near Wedderburn, in the northern country district of Victoria. Aphids were present on the edges of the crop and colonies up to 10cm long were seen on some flowering spikes. With excellent rain recently falling in this region, plants were not under any moisture stress and were not visibly affected by the aphids.

Cabbage aphids grow up to 3 mm long, with a dull grey-green body. Infestations start when winged aphids fly into the crop from autumn weeds. These give rise to dense colonies, which appear bluish-grey and are covered with a fine, whitish powder. Cabbage aphids suck sap and can reduce yields when numbers are high. A large amount of sugary solution is often secreted. This can sometimes lead to black sooty mould, thereby reducing the plants’ ability to photosynthesize, decreasing plant growth. Canola is particularly susceptible to aphid damage during bud formation through to late flowering.

It is important to monitor crops regularly after aphids first appear as numbers can increase rapidly when environmental conditions are favourable. Typically, cooler weather conditions and/or significant rainfall will slow the rate of aphid development, whereas warm and relatively dry conditions will favour the rapid build up of populations. Check representative parts of the entire paddock and look for aphids on a minimum of 20 plants at each point. Crops can withstand relatively high numbers of aphids if the plants are growing strongly (e.g. not suffering moisture stress).

Click here for images of cabbage aphids.