Abstract: In order to solve the problem of yield and quality decline of aged 3-5-year-old alfalfa(Medicago sativa), and to clarify the effect of root cutting treatment on its yield and insect population dynamics in the field, the alfalfa variety Zhongmu No.4 planted for 5 years was used for root cutting treatment. Two treatments of root cutting (10-15 cm) and non-root cutting were set up to determine the yield of alfalfa (fresh weight and dry weight). The species, occurrence and community structure of insects in the field were investigated, and the relationship between pests and natural enemies was analyzed by principal component analysis and grey correlation analysis. The results showed that the root cutting treatment significantly increased the fresh weight yield of the first crop of alfalfa, which was 46.1% higher than that of the non-root cutting treatment. The plant height and stem number of alfalfa after root cutting were better than those without root cutting. After root cutting, the total number of insects in the alfalfa field increased as a whole. Hemiptera and Thysanoptera were the dominant pest groups, and the number of natural enemy insects also increased. Principal component analysis showed that Adelphocoris fasciaticollis, Helicoverpa armigera and Odontothrips loti were the core factors affecting the insect community. Grey correlation analysis confirmed that natural enemy insects were correlated with Therioaphis trifolii and Odontothrips loti. Root-cutting treatment was an effective measure to improve the yield of the first crop of aged alfalfa, but it was necessary to strengthen the monitoring and control of dominant pests such as thrips and aphids.

Key words: Medicago sativa, root cutting, yield, insect community