Aphids are pests of brassica crops throughout NZ, but occur more frequently in the South Island. They cause damage to plants by sucking plant juices and are also a vector for a number of plant viruses.
Of the many aphid species present in NZ, the most significant for brassica crops are the grey cabbage and green peach aphid.
The adult females can be both winged and wingless.
Wingless adults are around 2 mm long and 1 mm wide, tapering at each end. They range from yellow to dark green or grey in colour, and can be covered in a whitish waxy powder.
Winged adult aphids have two pairs of strongly veined wings which extend well beyond the body when the insect is at rest. They are around 2 mm in length, with colourings ranging from yellow through to grey, with more pronounced black markings on the back and virtually no white waxy powder.
The aphid damages plants by sucking plant juices, resulting in yellowing and curling of leaves, and wilting and stunting of growth of plants, especially during hot weather. Aphids are also important vectors of some plant viruses, with the major types transmitting turnip mosaic virus (see page 174), causing stunted growth.
Heavily infested brassica crops may become unpalatable to stock, further increasing losses. The main periods of activity of these pests are October to mid December and mid January to April, with insect populations able to double every few days in warm conditions.
Prevention and management
Aphids can be successfully controlled during establishment using seed treatments such as AGRICOTE Brassica. There are also more aphid tolerant cultivars available, which offer resistance against aphid attack. Removal of alternate hosts such as wild turnip can also limit the damage caused by an aphid outbreak.
The use of systemic insecticides also offers good control in the face of high insect pressure, especially if used according to flight periods and control is carried out early.