Why do I see caterpillars crawling on the ground instead of on milkweed?

Monarchs are known to crawl from plant to plant for a number of reasons. This can happen when the original milkweed plant does not have many leaves, or if there is a lot of competition for leaves on a single plant, they will also leave in search of a plant they can have to themselves. Additionally, caterpillars will leave to escape the heat; sometimes they seek out shade on the ground during the hottest parts of the day. Late stage (5th instar) caterpillars will also often leave the milkweed plant to find a sheltered place to pupate, and will travel as far as 10 meters away!