Nilaparvata lugens is a typical phloem feeder. Rice phloem is high in simple sugars and very low in essential amino acids. Nilaparvata lugens harbors an ascomycete Entomomyces delphacidicola that hypothetically biosynthesizes several amino acids to meet the nutrition requirement of the planthopper. Among these amino acids, here, we focused on arginine biosynthesis. A complete cDNA of an E. delphacidicola gene, arginine‐succinate lyase, EdArg4, the last step in arginine biosynthesis, was obtained. RNAi‐mediated suppression of EdArg4 reduced arginine content in the hemolymph, and decreased the expression of several arginine biosynthesis genes. Silencing of EdArg4 delayed nymphal development and led to nymphal lethality. About 20% of the EdArg4 RNAi surviving adults were deformed. The most obvious defect was wider and larger abdomen. The EdArg4 RNAi‐treated planthoppers had thickened wings and enlarged antennae, legs, and anal tubes and a few adults did not normally emerge. Arginine deficiency in the EdArg4 RNAi planthoppers repressed nitric oxide signaling, determined at the transcriptional level. We infer that E. delphacidicola biosynthesizes essential arginine to compensate for nutrition deficiency in N. lugens.