From the June 2006 Idaho Observer:

Aspartame - The World's Best Ant Poison
... contributed by Jan Jensen of WELLthy Choices

We live in the woods and carpenter ants are a huge problem. We
have spent thousands of dollars with Orkin and on ant poisons
trying to keep them under control but nothing has helped.

So when I read somewhere that Aspartame (Nutrasweet) was
actually developed as an ant poison and only changed to being
considered non-poisonous after it was realized that

a lot more money could be made on it as a sweetener than as an
ant poison, I decided to give it a try. I opened two packets of
aspartame sweetener, and dumped one in a corner of each of our
bathrooms. That was about 2 years ago and I have not seen any
carpenter ants for about 9 to
12 months. It works better than the most deadly poisons I have
tried. Any time they show up again, I simply dump another
package of Nutrasweet in a corner, and they will be gone for a
year or so again. Since posting this information I have had many
people tell me of their success solving ant problems with this
substance, when nothing else worked.

We found later that small black ants would not eat the
aspartame. It was determined that if you mixed it with apple
juice, they would quickly take it back to the nest, and all
would be dead within 24 hours, usually. I have found that
sometimes it will kill them, and sometimes it does not. Not sure
why, may be slightly different species of ants or something.

Fire Ants: We got our first fire ant hill about 2 weeks ago.
Poison did not work. We tried Aspartame and the ants ignored it
until we got a light rain. It was just a sprinkle, enough to
moisten the Nutrasweet and ground, but not enough to wash it
away. They went crazy, hundreds of them grabbing it and taking
it back into the mound.
When I checked the mound 2 days later, there was no sign of the
fire ants. I even dug the mound up some, and still saw none of
them.

How it works:
Aspartame is a neuropoison. It most likely kills the ants by
interfering with their nervous system. It could be direct, like
stopping their heart, or something more subtle, like killing
their sense of taste so they can't figure out what is edible, or
smell, so they can't follow their trails, or mis-identify their
colonies members, so they start fighting each other.

Not sure what causes them to end up dying; just know that for
many species of ants, it will kill them quickly and effectively.
As with any poison I recommend wearing gloves and washing any
skin areas that come in contact with this poison, and avoid
getting it in your mouth, despite anything the labeling may
indicate. I suspect it will work for other insects such as
yellow jackets as well, but have not tested that yet.

More information on this fantastic poison can be found
at: http://www.dorway.com