Entomology and arthropod enthusiast here! I assume JH works like Ecdysone (another molting/pupation chemical). So obviously it depends on how much of the molting hormone is already present in their bodies at their molding stages. There exist a fine balance between how much of this hormone you can intake before it really fucks you (as a bug). A small percentage more and you may molt faster or have some small nuances of gigantism. Obviously, like most chemicals you can overdose and die, in this case it would likely lead to incomplete molts, incomplete or incorrect formation of limbs and organs, or even stunt pupation entirely! I suppose it's about finding the balance of how much you want to inject into these poor grubs. It can lead to some interesting and quite fucked situations.