Insects are unable to synthesize essential amino acids (EAAs) de novo, thus rely on dietary or symbiotic sources for them. Wood is a poor resource of nitrogen in general, and EAAs in particular. In this study, we investigated whether gut microbiota of the Asian longhorned beetle, Anoplophora glabripennis (Motschulsky), a cerambycid that feeds in the heartwood of healthy host trees, serve as sources of EAAs to their host under different dietary conditions. δ¹³C-stable isotope analyses revealed significant δ¹³C-enrichment (3.4 ± 0.1%; mean ± SEM) across five EAAs in wood-fed larvae relative to their woody diet. δ¹³C values for the consumers greater than 1% indicate significant contributions from non-dietary EAA sources (symbionts in this case). In contrast, δ¹³Cenrichment of artificial diet-fed larvae (controls) relative to their food source was markedly less (1.7 ± 0.1%) than was observed in wood-fed larvae, yet still exceeded the threshold of 1%. A predictive model based on δ¹³C_eaa signatures of five EAAs from representative bacterial, fungal, and plant samples identified symbiotic bacteria and fungi as the likely supplementary sources of EAA in wood-fed larvae. Using the same model, but with an artificial diet as the dietary source, we identified minor supplementary bacterial sources of EAA in artificial diet-fed larvae. This study highlights how microbes associated with A. glabripennis can serve as a source of EAAs when fed on nutrient-limited diets, potentially circumventing the dietary limitations of feeding on woody substrates.