Hello
It may not help you at the moment. But it is only a question.
I dont feed my mantids with wild catched flys. Bluebottles and the big ones. I buy the larves at the fishmans shop. Where those who go fishing buy their equipment. They sell the little larves in little boxes of about 30 grams.
And to make flys out of the larves just easy. Take a box, put in some earth and some of the larves and keep them warm. The rest keep cold in the fridge. And if you use different boxes and fill them up every week with some larves you will have enough flys.
I have read a lot here in the forum about culturing flys, but I think this is a cheap and easy way to get some.
One box with 30 grams contains hundred of larves and costs here in Germany about 1.50 USD. I am shure you will get it also in the USA.
And then you will never again have a feeding problem.