Grafting is the technique of physically transferring a young worker larva from its brood cell into an artificial queen cup mounted on a bar, then placing that bar into a queenless (or partitioned) cell-builder colony. If the nurse bees accept the graft, they flood the cup with royal jelly and draw the cup out into an elongated, downward-hanging queen cell over the following days.
Commercial queen breeders often graft with a tiny curved Chinese grafting tool that scoops the larva up in its own pool of brood food, minimising the chance of chilling or damaging it — acceptance rates above 90% are achievable with practice on the right age of larva.
A grafting bar of wax queen cups sits inside a cell-builder colony while a curved grafting tool ferries young larvae from the donor brood comb — watch nurse bees decide which cups to accept and draw out into capped queen cells.
Grafted-larva age and cell-builder nurse-bee strength jointly determine acceptance probability; accepted cups are progressively lengthened with royal jelly over several simulated days, while rejected ones are chewed down.
Set the donor larva's age, the nurse-bee density, how many cups are on the bar, and whether the cell builder is queenless or queenright, then run the time-lapse and watch the acceptance rate update live.
Queens can only be reared from larvae under about 3 days old — after that the fed diet and developmental window for a fully-formed queen closes, and workers develop instead.