I've used Sta-flo for awhile now and haven't noticed it goes bad. Just shake it up some before you use it again. The problem I have is the sprayer gets clogged if I haven't used the starch for a long period (a few months). I put the sprayer in some hot tap water and let the starch melt and it works fine.
So far I haven't had problems with bugs. I get a lot of donated fabrics because I make quilts for charity and sometimes they have an oder that I have to wash out. THen I'll starch and press those pieces before putting them away on the shelf.