The extract from leaves of Rhododendron dauricum L. was extracted with 95% alcohol by common method for studying its insecticidal activities. The chemical components of the alcohol extract and relative contents were analyzed by GC-MS. The insecticidal activities of the alcohol extract were tested on the 2nd–3rd instar larvae of Lymantria dispar L. for five days. Five concentrations of the extract samples were designed as 50, 10, 5, 1, and 0.5 g·L−1. The results show that the alcohol extracts from leaves of R. dauricum exhibited insecticidal activities against larvae of L. dispar. The corrected mortality of larvae of L. dispar for was over 50% for both contact toxicity and stomach toxicity at the extract concentration of ≥ 5 g·L−1 after five days of application. The insecticidal activity in contact toxicity is more effect than stomach toxicity for the alcohol extract. Twenty compounds, with total GC relative contents of 93.81% in the alcohol extract from leaves of R. dauricum were identified. The main chemical components in the extract are: (1) 4,5-Dihydro-5-oxo-3-(p-tolyl) isoxazole, with a relative content of 40.03%; (2) 1,3-Benzenediol, 5-methyl-2-(3,7,11-trimethyl-2,6,10-dodecatrienyl)-, (E,E)-, the relative content 18.27%; (3) 3,6-Diphenyl-1,2,3,4,5,6,7,8-octahydro-1,8-acridinedione, the relative content 3.89%; (4) 6H-[1,2,4]Triazolo[1,5-a]indole, 4a,5,7,8,8a,9-hexahydro-9-methylene-, the relative content 2.99%; (5) 7-Amino-4-methyl-1,8-naphthyridin-2-ol, the relative content 2.64%; (6) 4-Methyl-2,6-dihydroxyquinoline, the relative content 2.63%; (7) 2,4,6-Triaminoquinazoline, the relative content 2.27%; (8) 2(1H)-Quinolinone, 4-hydroxy-1-methyl-, the relative content 2.02%.
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