Volnip isn't allowed to eat things that aren't explosive, toxic, corrosive or all three. And I would seriously put a stop to letting your minions talk. They will think the have free will then. Though my succubus, Nazola, talks anyway. That's why I prefer having Haathun, my felguard around. The nature of his enslavement prevents him from answering back.
All in all most warlocks are far too soft on their minions.