You don't need to feed them, they come self-contained in their plastic tub with the substrate that they need, feeding them can increase humidity in the tub and cause the grubs to defecate in their substrate. This will lead to them growing more quickly and becoming flies in less time than usual and can also lead to a bad-smelling substrate that will need cleaning more often. Grubs will last longer if you do not feed them and will be perfectly healthy to feed your favorite pals!