Cabbage aphids

Advisor, Howard Martin (Logan and Hitchens), has reported cabbage aphids (Brevicoryne brassicae) in an early flowering canola crop near Wagga Wagga, in the South West Slopes district of New South Wales. At this stage, the infestation is light and quite patchy across the paddock. Consultant, Peter Lott (Rural Management Strategies), reports that cabbage aphids have also been observed across several canola crops around Temora and Grenfell, also in the South West Slopes district of New South Wales. The aphids have been found on the terminal flowering spikes of approximately 1 in 20 plants at some locations.

Cabbage aphids grow up to 3 mm in length, 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 yield when numbers are high. A large amount of sugary solution is secreted. This can sometimes lead to black sooty mould, thereby reducing the plants’ ability to photosynthesize and decreasing plant growth. Greater aphid build-up leading to higher yield losses is more likely when crops suffer moisture stress.

Canola is particularly susceptible to aphid damage during bud formation through to late flowering, therefore, it can be important to control aphids to prevent heavy infestations. If chemical control is necessary, use ‘softer’ chemicals (such as pirimicarb), which are aphid-specific and less harmful to other insects. Predators and parasites should be encouraged as a natural way of suppressing aphid numbers. These are a very reliable form of control during the warmer days of spring and when low to moderate numbers of aphids are present.

Typically, cooler weather conditions will slow the rate of aphid development, with many crops able to outgrow the damage caused during winter. These reports are quite early in the season and Peter suggests they could be linked to weather. The mild winter conditions, combined with the lack of consistent rainfall through autumn-early winter, may have favoured the build-up of aphid populations in many regions.

Given these reports, crops should be checked for aphids in case numbers escalate enough to cause economic damage. It is important that representative parts of the entire paddock are sampled. Check at least five points of the paddock, and look for aphids on a minimum of 20 plants at each point. If more than 20% of plants are infested, control measures should be considered to avoid yield losses.

Click here for images of cabbage aphids.