Cabbage aphid

Pests of Field Crops in Southern Africa

CABBAGE APHID

(Hemiptera/Homoptera: Aphididae)

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Text Box: The cabbage aphid, Brevicoryne brassicae, is a major pest of crucifers, and is easily recognised by the fine grey, powdery wax covering which obscures the green body colour. The insects themselves are medium-sized aphids, being 1,5-2,5 mm in length, and have a pair of short dark cornicles protruding from the rear of the body. They occur in dense colonies, usually on the underside of the leaves. Another similar species of aphid that can attack cruciferous crops is the turnip aphid, Lipaphis erysimi, which is less powdery and more yellow-green in colour. TOP

Text Box: Cabbage aphid commonly affects cabbage, cauliflower, Brussels sprouts and several of the leafy crucifers such as rape and kale. TOP

Text Box: In cabbages the first symptom is often distortion of the leaves, as they tend to grow cupped around the aphid colonies. The affected area of the leaf may also become marbled in appearance, with paler patches where the aphids are feeding. Heavily infested plants are also made undesirable by the remains of the powdery covering and associated honeydew on the leaves. In addition, this aphid is known to transmit a number of virus diseases to crucifers. TOP

Text Box: The life cycle is simple, reproduction in southern Africa usually occurring by parthenogenesis. Live young are produced continually, and as the colonies become too dense, winged females are formed which migrate to other plants. This pest is most prevalent during warm, dry weather, when crucifers are produced with irrigation. It is somewhat suppressed during the rainy season, if the rains are frequent and heavy. TOP

Text Box: The waxy coating of cabbage aphid, and its preferred position of being on the underside of the leaf, makes it more difficult to control than other aphids. The situation is further exacerbated by the fact that many of the crucifers have very waxy foliage, which causes water-based pesticides to run off the leaves very quickly. A wetter may be needed to help the pesticide spread over the leaves. Resistance to pesticides is also becoming a factor in choice of pesticides. Systemic chemicals are generally more efficient in this case than contact chemicals, which would have to be directed to the undersides of the leaves. Many organophosphate insecticides of varying toxicity have been registered for use on the different brassicas, amongst them, demeton-s-methyl, diazinon, dichlorvos, malathion, methamidophos and mevinphos. Granular products applied to the soil at planting, such as disulfoton, eliminate the spraying difficulties, although will not last the life of the plant. They have the added advantage, however, of being less disruptive to the various natural enemies that will attack a population of cabbage aphids, the main ones being a number of parasitic wasps. Unfortunately these wasps, and other natural enemies such as predacious Syrphid fly larvae, seldom provide consistent control of cabbage aphids. The parasitic wasps often become parasitized themselves by a number of hyperparasites. TOP

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Colony of Aphids on cabbage.

Colony of Aphids on cabbage.