The single and comprehensive factors including the free amino acids,sucrose,content of water in wheat,temperature and light that influence the population density on Rhopalosiphum padi were studied according to theory of variance and path net.The relationship between some single factors and population densities of R.padi was y=a+b 1x+b 2x 2 or y=a+b 1x+b 2x 2+b 3x 3 . The synthetic influence on five factors to the densities of R.padi was analyzed by the model =a +∑∑( b ...