Spent substrate from beech mushroom (Hypsizygus marmoreus) — the leftover growing medium after harvest — boosted cordyceps (Cordyceps militaris) fruiting body yield by 35.33% and performed statistically on par with silkworm pupae powder, the costly animal-derived supplement that currently drives industrial production costs. The active ingredient is malic acid, naturally concentrated in the spent substrate, which alone increased yields by 20.97% by triggering 2,071 upregulated genes during the critical primordium stage, a nearly 8x larger genetic response than at the vegetative stage. Swapping silkworm pupae for on-site mushroom waste cuts 11–31% of production costs per kg of fresh cordyceps, with the biggest advantage going to facilities already growing multiple species and generating that waste in-house.