dejaentendonot9: There are a few different explanations for this, depending on what the circle is made of. Ants will get trapped in chalk and baking soda circles, as well. A big contender is that ants travel largely by a scent trail left behind by other ants. If you watch ants coming for pieces of food left on the floor or the sidewalk, you’ll see that they take the same route. A circle made of something like ink or sodium bicarbonate or chalk can disrupt this scent trail and cause almost a wall of sorts.
WRSaunders: The ink has a solvent in it and the ant doesn’t want to get solvent on itself. It’s feelers detect it is getting close to the solvent, so it turns.
FlyingWolfAngel: Red inked pens and termites is a good example of the ink having a chemical in it that mimics termite follow pheramones. Its really cool to make termites do figure eights…not that i have ever done this…..noooooo (/s)
screennameoutoforder: Ants are not smart. Not even close. Imagine they’re like toy robots.
They follow smells. The first workers to find food bring some back to the colony. They lay down a trail of smell. The next workers follow that smell trail and lay down their own. That’s how mistakes like this happen. https://youtu.be/N0HoqjxfvJ4
A pen’s ink is a mixture of chemicals. Some evaporate very quickly and also can dissolve other chemicals. If you were an ant, that line of pen would smell very strongly. Possibly strong enough to hide or break the smell of the correct trail.
By the way, chalk can have a different effect. Diatomaceous powder can kill insects. Chalks are similar. And many ‘ant’ chalks from China have real insecticides in them. So with chalk, and especially ant chalks, the insects might be avoiding something really noxious.