Mealworms and superworms live inside their food. Their substrate is a dry, grain-based layer that serves as bedding and diet at once. Ours is whole oats milled to a fine powder.
1Why oats
Because the larvae eat their substrate, it largely decides how fast they grow. Wheat bran is the traditional choice. In a 2025 trial (Polovinski-Horvatović, Food and Feed Research) larvae fed oat flakes grew slightly more than larvae fed wheat bran, with a better feed conversion ratio (2.00 against 2.16). They also digested the oats much more completely: about 78 % of the dry matter against about 50 % for bran.
Both diets worked well; the authors note that the choice in practice comes down to cost.
2Why powder
Milling the oats fine means even newly hatched larvae, only a few millimetres long, can feed straight away. The trade-off is that oat powder and frass are about equally fine, so they cannot be separated by sieving. Instead, when the layer has turned into a uniform fine dust, sieve the larvae out, discard the old material and refill with fresh powder.
Some keepers prefer coarser flakes, which they find easier to sift and less attractive to grain mites. There is little hard data either way; keeping the substrate dry matters far more than its grain size.
3Moisture
Never wet the substrate. Larvae get their water from slices of carrot, potato or apple laid on top; replace these before they mould. Damp substrate grows mould and grain mites quickly.
4For superworms and beetles
The same substrate suits Zophobas morio larvae and the adult beetles of both species. Beetles lay their eggs into it, so in breeding trays the adults are moved to fresh substrate every one to two weeks and the old tray is left to hatch.