Amdro can go stale or rancid, and then the ants won't take it back to the nest. It's best to store it indoors in cool conditions so this doesn't happen, and buy fresh supplies when you find a new colony. You can sniff it to tell if it is still fresh.
But a good bait - either a sweet bait or a protein/fat based bait like Amdro - is still the best solution over just killing the nest with something like Orthene. If some ants survive because they got a low dose, or if eggs hatch after you've treated a nest with straight poison, the ants will be increasingly resistant to it. That doesn't happen with bait, as it is ingested by the whole colony eventually and they all die. Even hatchlings find the bait still in the nest and eat it, so it gets them all.
With a poison, they can also move and form two or more nests where there was only one originally. This is ok for something like moving them out of pots so you can bring the pots in, but doesn't really do the job as well long term out in the garden.
Here's the (way too long) article that we distribute to folks asking about fire ant control from our local Master Gardener Plant Clinic. Scroll down about 2/3 of the page to the heading "Management" and note where it says that baits are slower acting but more effective. Every time I read this I think they should also have noted that you use a lot less chemical with a bait solution than a drench or powder application, and most of it is metabolized by the ants, too.
http://entnemdept.ufl.edu/crea...
