Abstract Reared insects such as black soldier fly larvae ( Hermetia illucens ) are considered a potential alternative feed protein. However, dietary exposure to insecticide residues via the substrate could adversely affect performance indicators (yield/survival) and substance-transfer from substrate to larval biomass could result in non-compliance with low legal limits. Effects of pyrethroid insecticides cypermethrin and deltamethrin were tested at varying concentrations, with or without the synergist piperonyl butoxide (PBO). Concentration/response curves for yield were estimated and samples were analysed to determine concentrations of parent compounds and selected metabolites. Results suggest that deltamethrin is highly toxic to H. illucens larvae: the critical effect dose for 10% yield loss was estimated to be 0.04 mg/kg, compared to a legal limit in wheat of 2.0 mg/kg. Cypermethrin was comparatively less toxic, in line with prior studies, but may also cause significant adverse effects even for exposure levels below the legal limit – especially when combined with PBO. For both substances, transfer from substrate to larvae is a potential issue due to low limits, and transfer as well as toxicity are increased by presence of PBO. Some metabolites could be detected, but more research is needed to determine resistance mechanisms involved.