"Commercial growers incorporate a juvenile hormone into the feeding process to keep the mealworm in the larval stage and achieve an abnormal length of 2 cm or greater."
Presumably they're given an extra dose before despatch just in case they spend a few weeks stuck on a shelf in a sorting office, unless the commercial growers don't like delivery entities and quite like the idea of them being punished for non-delivery by suffering a beetle infestation.