First, examine newly transplanted plants for aphids which may start under the young leaves near the growing tip. Once deep enough under the cover of a couple of leaf layers, they are difficult to control and then become a source of re-infestation. Farmers who have problems with winged aphids at this time of year should use a soil-applied, systemic insecticide for protection in the first month. Thereafter, use economical control with a knapsack on the lands and target only the infested plants, identified by white markings.
The cabbage aphid is grey coloured with a waxy appearance and causes white lesions to develop. The green peach aphid also attacks cabbage, but is less conspicuous and will not cause lesions or curl leaves. It tends to colonise the plant mainly between the older leaves. Sooty mould will develop on the honeydew produced by these aphids. This species is a potential problem in summer, but seldom seen as it is also controlled by spray programmes that control other pests. It is more of a threat to organic growers.
Aphids also have the potential to spread very destructive viruses, such as cauliflower mosaic virus and turnip mosaic virus – the former being more widespread. Infected plants develop a pale colour, growth slows down or stops, and the lower leaves become susceptible to downy mildew. One aphid-infested plant carrying this virus can infect thousands of plants.
I have seen lands that have been infected completely, resulting in total crop loss. Look out for these viruses and make sure there are no aphids on plants. Remember, bagrada insects also spread this virus.
There are various small, parasitic wasps that help control aphids. They work well in conjunction with sensible spraying. These minute, black wasps lay an egg into a mature aphid that turns into a bronze coloured “mummy” from which the new wasp hatches, leaving a distinct round hole. These wasps penetrate plant parts to reach aphids that are inaccessible to insecticides.
The insecticide Aphox (pirimicarb), is effective and does little damage to wasps. As wasps breed up, the insecticide controls aphids on leaves, forcing wasps to search for aphids in immature, developing heads. Farmers, it is worthwhile to get on top of aphids in good time. A little effort and good timing will result in savings and less frustration later. Farming is about the right timing, and dealing with this pest illustrates this point.