Umm, ferrum still obviously has the same root as ferro plus it still runs into the problem of sounding like a cross between Genesect and Ferrothorn (plus it's a Bug/Steel whose name is basically "Steel Bug".)
Also, it goes Bug, Steel, Bug/Steel?
Edit: What kind of insect is it meant to be, anyway? Cocoon stage makes me think moth/butterfly but, well, butterflies can fly. If we know what kind of bug it is then it's easier to come up with a less generic sounding name. Speaking of which, how about Ferrapace (ferrous + carapace) for stage two?