AbstractFrass, a byproduct of insect rearing, has become popular for its potential use in sustainable agriculture. The rapid growth of insect production results in an increased frass output. This study examined the effects of frass as soil amendment on plant growth and resistance to insect herbivory. In greenhouse experiments,Brassica rapaL. (Brassicaceae), was grown in unamended soil (NoFrass; control) or soil amended with frass (2 g kg−1) from larvae of black soldier fly (BSFF),Hermetia illucensL. (Diptera: Stratiomyidae), or yellow mealworm (MWF),Tenebrio molitorL. (Coleoptera: Tenebrionidae). Frass was applied as raw, incubated, or composted frass before seed germination. Plant growth and performance were measured of larvae of root‐feedingDelia radicumL. (Diptera: Anthomyiidae) and shoot‐feedingPlutella xylostellaL. (Lepidoptera: Plutellidae). Initially, raw BSFF and MWF reduced the growth ofB. rapaand resulted in a smaller leaf area than NoFrass. However, over time, a notable trend emerged. Whereas the difference in leaf area between MWF and NoFrass disappeared, BSFF consistently resulted in a smaller leaf area than MWF and NoFrass. Raw BSFF reducedD. radicumlarval survival and pupal biomass and larval survival ofP. xylostella. In contrast, raw MWF increased larval survival and biomass ofD. radicumand the survival ofP. xylostellalarvae. Interestingly, incubation of frass in the soil for 16 days before seed germination removed plant growth inhibition and increased plant leaf area, especially for MWF compared to NoFrass. In addition, composting MWF increased leaf growth. Therefore, frass may be used as a sustainable and natural alternative to conventional organic fertilisers, promoting plant growth and enhancing resistance to herbivory. Our results indicate that soil amendment with raw BSFF may negatively impact herbivore performance, whereas raw MWF may enhance herbivore performance.