Abstract: Researches are made on the interspecies competition between English grain aphid(Macrosiphum avenae), birdcherry aphid(Rhopalosiphum padi), greenbug(Schizaphis graminium)and their mixed populations. Causes for their improbability to coexist are analysed. Under natural conditions, R. padi and S. graminium can't occur simultaneously in general, and if they do, their quantities differ greatly. However, M. avenae can occur simultaneously with any one of them, and when it is dominant, the three species can occur at the same time. This conclusion is agreed with actual dccurence of wheat aphids under natural conditions. Meanwhile, the theoretical significance and practical values of this conclusion are briefly commented.

Key words: forest ecosystem model, forest resources multiple-objective planning, management scheme, simulated annealing, forest simulation optimisation system