Interesting! You probably won't be able to ID bacteria just by culturing (you'll likely need a lab and pcr for that), but to see if it's gram positive or gram negative is helpful.
Right now I actually work on the bacteria in the guts of kissing bugs, and let me tell you, there is quite a lot! All insects have symbiotic bacteria in their gut that helps them digest their meals... my guess is that, since the 'black death' seems cricket related, it's a pathogen from the cricket side based on what they've been eating. It might not really be affecting the crickets enough to be noticeable, but the pathogen might be wreaking havoc with a mantid's symbiotic bacteria. Crickets are scavengers and will eat all sorts of delightful, disgusting things, even in controlled environments!
Anyway, that's just a theory. This is all super interesting though. I might look up to see if there are any published findings about mantids in general.