Cabbage aphids & control issues

Farm manager, Rodney Carter, has reported problems with chemical control of cabbage aphids (Brevicoryne brassicae) in a canola crop near Mellol, in the lower western district of NSW. Both organophosphate and synthetic pyrethroid compounds have failed to adequately control aphid numbers over three separate applications. This is of concern and highlights the importance of considering all available control options (not just chemicals) when targeting aphids. The affected paddock has been sprayed fairly extensively with organophosphates and synthetic pyrethroids for five years or more.

Failure of chemical control often indicates that resistance has developed, although application problems can also be responsible. Insecticide resistance is common in many aphid populations in Australia. If spraying is necessary, it is recommended that growers rotate their use of organophosphates and synthetic pyrethroids with other insecticide groups, such as carbamates and organochlorines, to reduce the speed at which resistance to one chemical group develops and builds up.  'Softer' chemicals, which are more expensive to purchase, but facilitate natural biological control, can decrease the need to apply chemical controls and may therefore be an economically viable alternative.

Rodney says between 50-60% of the canola plants are affected, with damage most noticeable along the paddock edges. The crop presently has about 5-10% of plants at the flowering stage. Cabbage aphids are an ongoing issue in Mellol, although the numbers observed this season are substantially higher than normal. Rodney estimates about 30,000 acres in the Mellol region have been affected by aphids at some stage this year.  

Agronomist, Evan Ryan (Landmark), has also reported high numbers of cabbage aphids on two canola crops near Yarrawonga, in the Northeast of Victoria. Evan reports that hundreds of aphids can be seen on a single plant. The numbers are not confined to the edges of the paddocks, although stressed plants appear to have more aphids than healthy plants. Of concern, the aphids are beginning to affect the canola pods. Although hoverfly larvae (see below for further information) have been identified in the paddocks, Evan says chemical control is needed due to the very high number of aphids present.

Cabbage aphids grow up to 2.5 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 on the flowering spikes, which appear bluish-grey and are covered with a fine, whitish powder.

Canola is particularly susceptible to aphid damage during bud formation through to late flowering, therefore, it is important to control aphids in spring to prevent heavy infestation. Crops at this vulnerable stage should be checked several times a week 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.

For further information on cabbage aphids refer to PestFacts Issue No. 9.