My, similar, situation:
Last fall i got one pair of guineas through a friend of my dad's, mainly for stinck bug control (that destroyed all my tomatoes that season). After conditioning with their new home, they were allowed to free range al day. They were free but they had no interest in insect eating. Just convenient grain feeds (from my part) or whatever they could find. Tried starving them: they just continued the same way and laying down all day long, with the male staring obsesively at my neibghbor' s flock of chickens, across the fence. All day long. They had at their disposal 2000 sqmt of yard but just rush to one corner and stare at the chickens.
I aquired one more pair, for the guineas to function as a natural flock. The now four of them started roaming the yard all day (due to the original male developing a passion for the new lady who appearently had the habbit of roaming) and peck on SOME bugs (due to the habits of the newcomers). But still, they are focused mainly on (weed) seeds.
Anticipating (foolishly, I admit) a real "succes" with the newcomers, i hurried and purchased 10, day-old keets. They are now 12 weeks old and where fed with turkey feed to make them viable, strong birds, which they are.
This morning I acknowledged that the four adults don't even touch the colorado beetle (adults or juveniles) even when put under their beaks. Not even taste them, out of curiosity. As you assume, the same situation is with the youngsters.
My conclusion: Guinea fowl aren't natural born insect eaters/bug control agents, as they are advertised. Maybe perhaps in Africa but NOT Everywhere. They are conditioned to be so, by their environement (including humans who take care of them).
Starting today and for the rest of the summer, I decided to stop feeding the four adults (which I did only in the evening, as part of my "free range" scheme) though, I'm afraid that not feeding the two hanes will cripple their bodies with all their daily egg production.
Apart from the four adults (which are manageable as a number), I m stuck with 10 more nearly adult guineas that I don t want to slaughter but need to feed, with money from my pocket.
All in all, don t get your hopes (really) high with guineas making a formidable insect eating army, because it ain t necesarily so.