BACKGROUND Insect biomass is a sustainable alternative to traditional animal feeds, particularly when insects are produced on low‐value high‐volume agricultural by‐products. Seven samples of almond by‐product (hulls and shells) were obtained from processors in California and investigated for larvae production. Experiments were completed with and without larvae and spent substrate samples were assessed for their potential as soil amendments based on standard compost quality indicators. RESULTS On average, specific larvae growth and average larval harvest weight were 158% and 109% higher, respectively, when larvae were reared on Monterey and pollinator hulls compared to nonpareil hulls and mixed shells. Larvae methionine and cystine contents were highest when larvae were reared on Monterey hulls and mixed shells, respectively. Available phytonutrients in spent substrate were affected by feedstock sample and larvae rearing. Spent nonpareil substrate without larvae had the highest NH4‐N levels and spent pollinator substrate incubated without larvae had the highest PO4‐P levels. Spent mixed shell substrate had the lowest availability of phytonutrients. CONCLUSION The findings demonstrate that by‐product composition has a significant impact on larvae growth and the properties of the spent substrate, and that spent substrate from larvae rearing requires further stabilization before application as a soil amendment. © 2020 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.