The capacity of black soldier fly larvae (BSFL; Hermetia illucens) to transform organic substrates into body proteins and lipids suitable for animal nutrition is receiving growing attention. Carbohydrate content of BSFL diet is positively related to larval growth, fat body content and adult egg production, but underlying mechanisms of feed conversion remain to be explained. Previous studies reported amylase activity in larval midgut, and it has recently been shown that BSFL were highly efficient at digesting starch. This study focused on the effect of starch content and type on BSFL amylase activity and starch Estimated Digestibility (ED). BSFL were fed on five plant-based diets with different starch contents and types, and larvae and substrates were sampled after 4, 7 and 11 days of feeding, along with initial BSFL and diets. Each sample was ground in phosphate-buffered saline with protease inhibitor and centrifuged to collect water phase. Amylase activity was assessed using the Bernfeld technique and reported to total soluble protein measured according to Bradford. For substrate samples that could contain enzymes from the plant material, microbes or larvae, BSFL amylases were detected by Western-Blot using antibodies specific to insect amylases. This approach offers insight into larval amylase regulation mechanisms and the role of extra-oral digestion of starch in BSFL conversion systems.