It is for sure the crickets. There is a disease that crickets get that can affect various other insects and reptiles. As Christian stated, it can also affect flies and other insects, although it is far more rare. I have had flies infect my mantids 1 time ever. It can also infect fruitflies occasionally, but they are so small it only seems to kill very young mantids, L1-L2 or so. You will usually notice a red line or trail in the mantids enclosure when it has the bacteria, although they do not always die from it. A friend of mine told me more about it a few months ago, but I forget the name of it. It used to kill my mantids every time, but now I have gotten pretty good at fixing the problem by doing the following things:
1. Change/clean and sterilize the container everyday when you notice the bacteria in the enclosure.
2. Don't feed or give water to the mantid for a couple days. This is usually best when the mantid gets bigger and can go a few days without eating.
3. Stop feeding crickets. Switch to roaches or flies, preferably roaches as they seem to be immune to the bacteria (they live through almost anything!). I have never had any problems with roaches.
I have noticed that it only happened to me now when I used crickets I had just purchased, or crickets that had been feeding on carrots. I still use crickets often with my mantids, but only a week after I get them (I order 1000 at a time) and they have been eating the food I prepare for them. I also only feed light colored healthy looking crix to my mantids.
I have also noticed this affected P. wahlbergii more than any of my other species. And something Christian stated may have solved a problem I have been wondering about for a few weeks now. He stated that those infected are often infertile as adults. I keep track of all my breeder mantids... molting dates, problems, dates mated, etc etc. 4 out of 7 male wahlbergii and 2 females were spewing this bacteria when I got them at L6. Looking back at the records now, NONE of the females that were bred with those males, nor those 2 females laid any fertile ooths (or at least have not hatched yet, 7 weeks and counting). They also often lay crappy and/or partial ooths. I guess I can toss out those ooths now eh? The ooths from the 3 females that were mated with the non-infected males are hatching on time, predictable to the day.
Anyway, sorry so long, but that has been my experience.
DeShawn