Abstract: Because of the stable ecological environment in the orange garden,the population develop- ments of Citrus red mite and its enemies coincide with Leslie Pattern:H(t+1)=5.019224H(t)/[1+49. 525954P(t)],p(t+1)=2.681411P(t)/[1+3.651781P(t)/H(t)]. Analysis of the populational dynamics shows that:(1)controlling effect of natural enemies(Stethorus punctillum,etc.)on Citrus red mites is usually enlarged with the mite's population rising in the orchard.If the density of the mite's enemies amounts to 0.05-0.08 head per leaf,development of the injurious mite can be controlled.(2)The ratio between beneficial and injurious insects,which is changing with their populatlonal densities,restricts the populational trends of the mite's enemies,so the ratio cannot be simply used as an index to control the injurious mites.(3) Extraordinarily low population of the mite is not beneficial to the development of its enemies.Spraying pesticide too early or too much may lead to great damage from the mite,and unsuitable pesticide using will bring about the same result.

Key words: Taihu Lake;, sediment, microbial biomass.