Colour - Greyish to dull mid green.
Body Length - 1.6 to 2.8 mm long.
Legs - 6.
Abdomen - always heavily coated with white mealy wax.
Usually forms dense clusters on the plant.
Cabbage aphids occur mainly as asexually reproducing females. They are capable of multiplying rapidly in mild conditions from late winter to spring. Favourable weather for the build up of aphids usually coincides with budding and flowering times of winter grown Brassica crops. The aphid over-summers on summer grown cruciferous crops or more commonly cruciferous weeds like mustard.
Outbreaks are favoured by mild winters and cool, mild springs. Late flowering canola crops are usually affected more than early flowering crops.
Habitats:Origin and History:
The cabbage aphid is a pest of Brassica crops in particular canola. The aphid attacks canola before and during flowering. They form characteristically dense clusters on the growing tips. Substantial infestations can result in the deformation or loss of reproductive parts. Direct feeding can cause a reduction in seed quality and yield. Yield losses may also result from a number of viruses transmitted by the cabbage aphid.
Management and Control:Culturally:
Sowing of resistant or tolerant cultivars of lucerne, medic or clovers can play a major role in overcoming both problems.
Parasitic wasps, hover flies, lady birds, lace wings and fungal disease are biocontrol agents. Biocontrol agents are only effective when aphid numbers are low. Under optimum conditions where aphids have the ability to rapidly multiply the use of insecticides maybe necessary.
There are a number of insecticides available for cabbage aphid control. Use of chemicals that are solely aphicides and 'soft' of beneficial insects is important to encourage biocontrol agents.
Thresholds:On Canola, if aphids cover a 25 mm length of flowering stem on most plants, and biocontrol agents (such as ladybirds, lacewings, hover fly larvae, wasps and fungi) are not active, then it may be worth spraying. In medium to high rainfall zones, Canola usually compensates for aphid damage with little loss of yield or oil content.
Monitor from late winter to the end of flowering.
Others have used 20% of flowering stems with 20 aphids as a threshold at mid flowering or earlier.
Related Species:Similar Species:
See Significance: under the Description for Aphids for a key to distinguish different species that are common on canola.
References:Hart et al 1995.
Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.