Phiff,
You should take some time to think about this. You haven't responded with any info on your temps or humidity which leads me to believe you don't know them or aren’t concerned about them.
Now that isn't uncommon, but understanding what they are and what they do for the lizard is.
Let's say the monitor was bitten by insects. The underlying issue could be husbandry related, not insect related.
I leave many crickets and full size roaches in with my Adults and Juvis in many cages (Small juvie ackies get big roaches, lots of them). I have not ever noticed wounds related to such a thing in my enclosures/cages.
Now, that isn't to say they don't happen, but please put this in perspective, 5 “subadult” roaches in with a HEALTHY sav would NOT cause that, especially regularly and multiple times as you say it was .
So for your monitors sake, buy a few gadgets to examine the temperatures and humidity of your enclosure and perhaps rethink some of the husbandry you are implementing.
Cheers,
Mike.