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Regulation mechanisms in the population dynamics of phytophagous pests

, : Regulation mechanisms in the population dynamics of phytophagous pests. Berichte ueber Landwirtschaft Sonderheft 0(209): 54-67

The processes by which the populations of plant pests in arable land are regulated are demonstrated by the population dynamics or two groups of animals which differ extremely with respect to their ecology: the cyst nematodes as representatives of stationary pests, and cereal aphids as representatives of migratory species which each year recolonize the field. - A well-functioning mechanism of self-limitation presents the populations of cyst nematodes from destroying their own resources by overpopulation. The population level which cannot be surpassed depends essentially on the potential of increase which in turn is determined by the degree of susceptibility of the field crops: under resistent varieties the equilibrium density will be at a rat the potential of increase can be lowered likewise by the administration of large quantities of organic matter. If high nematode densities are permanently present for a long time a community of natural enemies, consisting for a considerable part of nematode-parasitic fungi, will build up in the long run and produce a kind of decline-effect. This effect comes, however, with a consider-able delay and is largely influenced by the external conditions. It is until now not possible, therefore, to exploit it purposefully. - Cereal aphids have an enormous potential of increase and can reach high population densities within very short rime if they are not checked by natural enemies. Among these are the syrphids key factors. It depends on the speed of their aggregation response whether or not an aphid outbreak will occur. In the experimental areas around Stuttgart they stop the aphids population increase usually so early that the crop remains undamaged. In the outbreak areas of northern Germany they arrive in the majority of years so (ate in the cereal crops that they cannot prevent an outbreak any longer. The possible causes are discussed. In spite of the fact that beneficial influences of hibernation and feeding habitates on the efficiency of syrphids cannot be proven in the adjacent fields on account of the great mobility of the hover flies, there is enough circumstantial evidence that the structure of the agroecosystem is of major significance.


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