I wouldn't heat it, it says right on the heating mats that they are for glass only, as glass conducts heat away from the mat, preventing overheating, melting, and possibly fire. Crickets don't live long as adults, there could be lots of other factors like diet, water, mold, or just lifespan. Most crickets sold as feeders can survive being nearly frozen and thawed, so I can all but guarantee temperature is not your issue. If you want feeders that last longer look into roaches, or culture some camel crickets, which can live for a few years... That would be my advice, I'm sure others with more experience could actually critique your cricket setup.