Hey all, I just wanted to ask some of the experienced field herpers if mid to late Sept. is to late for a trip to the big bend area?

No, it's not too late; especially if you're looking for more than just a few target species. I can't remember the latest I've been out there, but it was October if not November. I recall young-of-year snakes like garters and bull/gopher snakes, plus amphibians (more so if it rains).
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Chris McMartin
www.mcmartinville.com
I'm Not a Herpetologist, but I Play One on the Internet