The brown planthopper (BPH) is one of the most destructive insect pests on rice () in Asia. After landing on plants， BPH rapidly accesses plant phloem and sucks the phloem sap through unknown mechanisms. We discovered a salivary endo-β-1，4-glucanase (NlEG1) that has endoglucanase activity with a maximal activity at pH 6 at 37°C and is secreted into rice plants by BPH is highly expressed in the salivary glands and midgut. Silencing decreases the capacity of BPH to reach the phloem and reduces its food intake， mass， survival， and fecundity on rice plants. By contrast， silencing had only a small effect on the survival rate of BPH raised on artificial diet. Moreover， NlEG1 secreted by BPH did not elicit the production of the defense-related signal molecules salicylic acid， jasmonic acid， and jasmonoyl-isoleucine in rice， although wounding plus the application of the recombination protein NlEG1 did slightly enhance the levels of jasmonic acid and jasmonoyl-isoleucine in plants compared with the corresponding controls. These data suggest that NlEG1 enables the BPH's stylet to reach the phloem by degrading celluloses in plant cell walls， thereby functioning as an effector that overcomes the plant cell wall defense in rice.