Acta Entomologica Sinica ›› 2016, Vol. 59 ›› Issue (8): 846-853.

• RESEARCH PAPERS •

### Effects of applied potassium levels on alfalfa nutrients and resistance to thrips (Thysanoptera: Thripidae)

ZHANG Xiao-Yan1,2, SHI Shang-Li1,2, LI Xiao-Long 1,2, LI Ya-Juan 1,2, HU Gui-Xin1,2,*

1. (1. College of Pratacultural Science, Gansu Agricultural University, Lanzhou 730070, China; 2. Key Laboratory of Grassland Ecosystem, Ministry of Education/Sino-U.S. Centers for Grazingland Ecosystem Sustainability, Lanzhou 730070, China )
• Online:2016-08-20 Published:2016-08-20

Abstract: 【Aim】 To determine whether potassium application can effectively improve the resistance of alfalfa to thrips and the relationship between potassium, nutrients and resistance to thrips. 【Methods】 The changes in damage index, potassium content, thrips density, carbon to nitrogen ratio and free amino acid content in leaves of alfalfa (Medicago sativa) resistant to thrips (Gannong No. 9) and susceptible to thrips (Gannong No. 3) were evaluated and measured after application with different levels of potassium (0, 6, 9, 12, 15 g/m2 K2O) during the thrips damage peak in fields. 【Results】 The potassium content, soluble sugar content and carbon to nitrogen ratio in old and heart leaves of Gannong No.3 and Gannong No.9 increased while the free amino acid content decreased with the potassium levels increasing, and the thrips density showed no significant change in the 2nd cutting of alfalfa. The damage index of alfalfa decreased significantly after application with potassium (P＜0.05) and was the lowest at the potassium level of 9 g/m2 K2O. The damage index, potassium content, carbon to nitrogen ratio and the free amino acid content in leaves of the 3rd cutting of alfalfa displayed the same change trend with those of the 2nd cutting after application with potassium. However, the thrips density increased as the potassium concentration increased. Correlation analysis indicated that the potassium content in leaves of the 2nd and 3rd cuttings of alfalfa had no correlation with the damage index. The damage index of Gannong No.3 applied with potassium were lower than that of Gannong No.9 without potassium application. 【Conclusion】 The potassium application enhances the resistance of alfalfa to thrips effectively by improving the carbon to nitrogen ratio and decreasing the free amino acid content. Therefore, the potassium application is an effective measure to enhance the tolerance of alfalfa to thrips in fields.