Ants in the pen?

Most can be controlled with a sugar/boric acid combination.
following is a recipe

boric acid n. A water-soluble white or colorless crystalline compound, H3BO3, used as an antiseptic and preservative and in fireproofing compounds, cosmetics, cements, and enamels.

I made a Great homemade ant bait which did a really good job for almost nothing
1 teaspoon boric acid (available at any drug store,, $2.99 for 4 oz)
6 tablespoons sugar
2 cups water
Mix together in a jar till all dissolved. Label and store safely.

To use, soak a few cotton balls with it. Put them in a small plastic container (margarine or <?> )with a few small openings in it for them to get in. Be sure to Freshen it 1-2 times a week. I also put a brick on top so other curious creatures could not get in.

This is a slow acting 1 percent solution to get them to take some back to the nest and even feed the queen. After a few weeks changing to a 1/2 percent should keep them gone.