Cabbage Integrated Pest Management : An Ecological Guide.

Ecology of Insect Pests and Natural Enemies4.1 IntroductionEcology is the study of interrelationships between organisms and their environment. The environment ofan insect consists of physical factors such as temperature, wind, humidity, light and biological factorssuch as other members of the species, food sources, natural enemies and competitors (organismsusing the same space or food sources).temperatureother membersof the specieswindÛhumiditylightcompetitorsfood sourcesnatural enemiesInsect ecology: the interrelationships between insect and environmentThese interrelationships are a reason that insect pest species cannot in all circumstances grow to largepopulations and damage crops. There may be large numbers of predators that eat the pest insects. Theweather conditions may be unfavorable for a quick life cycle because insects usually like warm, dryweather. The plant variety may not be very attractive for the pest insects to eat. And there may be manymore reasons.In Agro-Ecosystem analysis, insects are considered as populations rather than individuals. One singleinsect that eats a cabbage leaf will never cause yield loss in a large field but a population of ten thousandleaf-eating caterpillars may do.Learning to recognize natural enemies and understanding how they work, and how their impact can bequantified, is very important in pest management. Natural enemies do nothing but reduce pest populations,that is why they are called the “Friends of the farmer”! The work of natural enemies can reduce the needfor pesticides. This saves money and time, and possibly the environment and human health. JIn many areas, the use of pesticides is still a common practice for insect and disease control. Mostpesticide sprays are very toxic to natural enemies. The death of natural enemies means that insect pestspecies can increase in number very rapidly. Normally, natural enemies will remove a large number of thepest insects but when there are no natural enemies, the pest insect population can grow rapidly. Especiallywhen there is a lot of food available, like in large fields grown with the same crop, or in areas with manysmaller fields grown with the same crop. When the pest insect population is very large, more insecticideswill be used. Life cycles of natural enemies generally take longer periods of time to complete than thoseof pest insects. Once insecticides are being used in the ecosystem, it is difficult to bring back the naturalenemies within one season. Insecticides should be used only when there are no other options for controland there is a definite and visible need. This is one of the important reasons to monitor fields regularly.(modified from Hoffmann et al, 1993; and Weeden et al, www12)51 Cabbage Ecological Guide - 2000