Abstract: In 1992- 1995, investigation on apple insect pests and their natural enemies in fourdifferent apple orchard ecosystems was systematically carried out in Beijing, the results showed that the density of predators on the apple trees in cover cropping system was higher than non-tiUage or clean cultivated system. Increasing plant diversity in apple orchard ecosystems plays an important role in stabilizing the beneficial enemies. The number of predators on the apple trees in cover cropping system was higher, and the peak population density of mites or aphids or leaf miners was lower than the non-tillage system and clean cultivated system. During the last two years' the predators and parasitoids had controlled the secondary inducedpests, such as spider - mites, aphids and leaf miners, eliminating any need for pesticide applications. The ratio of parasitized Lithocolletis rignoniella Mats in cover cropped system was 62.7 %, which was 1.34 times as large as in the non-tillaged system and 16.9 times as large as in the light planting clean cultivated system and 17.9 times as large as in the combine planting clean cultivated system. A new design of cover cropping system consisted of two beneficial weeds, the Lagopsis supina and Hemistepta lyrata Bunge under the canopy of tree, instead of the chinese rape and cover crop,the alfalfa, in the row of apple trees. In comparison with the past cover cropping system, the population density of predators on cover crops in the 3rd year after new cover cropping system was 12.5times as large as in the first year. The density of predators on apple trees was evidently increased. The predator/prey ratio was changed from 1:34.9 in the first year after new cover cropping systemto 1: 14.4 in the second year after new cropping system, the result was equal to the ratio in the 5th year after the past cover cropping system.

Key words: plant diversity, sustainable pest management, apple orchard, beneficial weeds, natu- ral enemy