Hi,
I would say silkworms are too rich a diet if they comprise the largest part of it.
The temperature also affects the crickets lifespan but not having any source of water ( or a nice moist food item) will kill em fairly quick.
Best trick is to find out what day the store gets them in so you always pick up a fresh box, feed and treat them well from day 1. I used peices of carrot for water (occasionally tried wet cotton wool but never had much luck with it) and had a small tub of fish food then supplemented with whatever veg I was eating myself.
When I was doing that then they nearly all lasted till they got eaten - you can also select the size down and grow them up yourself.
Oh and don't put more into the tank than your gecko will eat in one sitting.
It should be admited that the temps over here are cooler though.
dr del