Abstract: The result of the research which lasted ten years indicated the rule of the growth and decline of Tetranychus viennensis Zacher in apple, pear and peach orchards in Hebei Province. The insect population increases gradually after waking at the end of March, and in June reahces peak quickly and then declines rapidly. There is only one peak all the year. The time of appearing peak and the density of the insect population are related with a lot of factors, such as the winter survival of the insect, temperature and humidity , and tree vigor, etc. Therefore, the insect population density is different at the same phenological period. The research result will provide theoretical basis to study the control index of Tetranychus viennensis Zacher and draw up new control methods.

Key words: Tetranychus viennensis Zacher, Insect population density, Apple tree, Pear tree, Peach tree