Abstract: Entomopathogenic nematodes have been applied as a kind of biological pesticides in pest control. To escalate their control effect on Grubs, screening pesticides as synergists for entomopathogenic nematodes is an important application research. In this paper, we tested the infectivity and the survival rates of three entomopathogenic nematodes (Heterorhabditis bacteriophora Cangzhou strain, Steinernema longicaudum X-7 and Heterorhabditis indica LN2) after dipping in three kinds of commonly used pesticides (70% imidacloprid wettable granule agent, 40% chlorpyrifos emulsifying concentrate and 2.5% beta-cypermethrin emulsifying concentrate). The mortality of the 2^nd-instar larvae of Holotrichia parallela treated with different combinations of the nematodes and chemical pesticides were determined, and the best combination was screened out. Results showed that each of the three kinds of pesticides had no sub-lethal reaction to any of entomopathogenic nematodes and imidacloprid had no significant effect to the three nematodes. Lethality of imidacloprid to LN2 at low concentration treatments of 1/50 RC (9.33 μg·mL^-1) and 1/100 RC (4.67 μg·mL^-1) was significantly lower than that of the control. Lethality of beta-cypermethrin emulsifying with recommended concentrate to three nematodes at the recommended concentration (16.67 μg·mL^-1) was significantly higher than that of the control, and the mortality of three nematodes treated with different concentrations of chlorpyrifos emulsifying concentrate were significantly different. 70% imidacloprid (46.67 μg·mL^-1 effective components) with Hb improved the lethality to the 2^nd-instar larvae of H. parallela by 87.51%; when combined with X-7 and LN2, the lethality could be improved by 39.99% and 47.37%, respectively. Synergistic interaction was observed on the lethality to the 2^nd-instar larvae of H. parallela by the combination of imidacloprid at 1/10 recommended concentration and Hb. Other combinations only resulted in additive mortality.