Here's the answer. Thanks to bostedo at Texas Gardening. Bifenthrin, which is specifically sold for fire ants, has has a long shelf life - about three years. Ortho Fire Ant Killer, is 0.2% Bifendrin, and there are other products with Bifenthrin as well. Now, if you are broadcasting the stuff over a whole lawn, buying a bunch and using it immediately makes sense. But if you want a long-lasting supply to occasionally treat individual mounds, Amdro isn't an economical way to go.