The caterpillars might wander if food is low on the plant or when they want to make a chrysalis they often crawl off the plant.

Are your plants in the ground or in pots? Just curious cause I’ve had aphid problems across all of mine but never ants since I grow mine solely in pots.

There’s a YouTube series about monarch butterflies by a guy called Mr. Lund who experimented with using powdered cinnamon at the base of the plant to discourage ants. It does require regular reapplication. Could be worth a try?

Usually the ants are there because they love the sap that aphids make so if there are ants, there are bound to be aphids. You can get rid of aphids by crushing them with your fingers (gotta check to make sure no first instar caterpillars are hiding nearby) or hosing them off the plant.