The best, cheapest, easiest solution I've found for ant problems anywhere is to lay out a solution of water, sugar, and either borax or boric acid. Boric acid works fastest and is usually available at the pharmacy--you may have to ask the pharmacist to show you where it is. Borax, of course, is a laundry additive available in the laundry section in a large box.
Mix the sugar & chemical (whichever one you choose) in a 50/50 proportion and dissolve in hot water to form a syrup or paste. Add some food coloring (I use red for danger) so you can see where it is after you apply it. Place it on a piece of cardboard or scrap wood in an area the ants are crossing to get where you don't want them. Make sure it is out of reach of children and pets, because it will taste sweet but be highly toxic to them.
The ants will eat it like candy, and carry it back to the colony. It will eventually (in a day if you use boric acid, three to four days if you use borax) kill the entire colony, including the queen.
I don't usually do this, but when they have decided that my house or feed room is their favorite foraging area, it's the quickest and most effective way to put an end to it. Of course, other colonies may come in eventually and replace the pests you've gotten rid of, so it's important to address the root of the problem and try to make sure there are no readily available food sources--the dish in water idea is great.
I have heard that DE works great, and will help with flies too. However, it will also kill all the other insects and worms that the ducks love to feed on, so it's a bit of a trade-off.
Good luck!