As hap and chance, I find a statement that says ants won't cross flour. I'm assuming they'll cross other substances that look like flour.
Effect: Two circles are drawn with flour. Spec is asked to choose one. A small number of ants are placed within each circle. Your mental powers prevent the ants from moving outside the selected circle. The others charge out freely.

Use it with my blessings, folks.

Thanks, Blindbo! I have an assortment of powders left over from cleaning all my cards. I'm adding a pyrotechnical touch to your effect with left-over coffee creamer!

Hehe nice idea...I'll try to come up with a suggestion that I draw a circle and the chosen one - well the spec will try to enter into it, but the harder he tries. . .
Nice idea mate.

This idea works for other insects too. There is a substance that is called dimatious earth (sp?) it is ground up fossils I think. It's available in most gardening centers and some pool filters use it. Gardners sprinkle it around plants to prevent slugs and snails from approaching. Even though it feels smooth to us it tears up the slugs. Of course you would have to have a very patient audience to wait for a snail!

Salem