Cabbage Integrated Pest Management : An Ecological Guide.

Major Cabbage Insect PestsPlant damage and plant compensationThe first signs of attack are small bleached areas on the leaves of infested plants. The leaves then turnyellow and become crumpled. The aphid colonies are protected inside the crumpled leaves. The effectsof infestation are worst on seedlings and young plants. They can be stunted and may die in unfavorableweather. Early damage to the growing point of a cabbage plant distorts the head. Even when youngplants are infested only lightly, the leaves of the plants when they are mature continue to show signs ofthe original attack. Infestations on larger plants may reduce yield and also spoil the plants by contaminatingthem with wax, cast skins and honeydew. Honeydew is the excretion of aphids. It is slightly sweet andis excreted from the syphons. Honeydew can make the leaves sticky and several fungi species grow onthe honeydew producing black marks on the surface of the cabbage. This lowers the quality. Wheninfestation is large, the aphids sometimes penetrate the heart of the cabbage.Aphids tend to be very localized: they usually colonize just a few plants but can be very abundant oneach plant.In addition to the direct crop damage, cabbage aphids also transmit the cauliflower mosaic virus and theturnip mosaic virus to cruciferous crops. Good crop hygiene (uprooting and destroying the virus infectedplants), rather than trying to kill the aphids, is the only way of reducing the impact of these viruses as thetime taken by virus-carrying aphids to infect new crops is often less than one minute. This is too short tokill the aphids by any control practices.Aphids as milk cows for ants...?!Ants are attracted to aphid colonies because the aphids produce honeydew whenthey are disturbed. Ants like to eat the sweet honeydew. The ants keep and sometimeseven protect the aphid colonies. They “milk” the aphids to get the honeydew.Although ants can be natural enemies of some pest insects, in this case, ants cannot beconsidered as natural enemies of aphids because they do not kill aphids but may in fact protectthem.Natural enemiesThe weather is a major natural agent restricting the build-up of cabbage aphid infestation in cold, temperateregions and mountainous areas. In dry, warm seasons the aphids can often produce extremely largeinfestations whereas in wet, cool seasons the aphid population remains small. In a period of frequentrain, aphid populations will be very low if not absent.Predators such as lady beetles and hover flies (Syrphids) and parasitoids like the wasp Diaeretiella rapaeare important natural enemies of the cabbage aphid.In wet seasons, outbreaks of fungi that killaphids may occur. This often coincides withperiod of high humidity and rain. Dead aphidsmay be seen covered with white coloredfungus growth on the body. These fungi canspread quickly to reduce aphid populations.See chapter 6 for details on these naturalenemies.LarvaeLady beetle : an important predator of aphidsAdult83 Cabbage Ecological Guide - 2000