Does the disappearance of a 5th instar caterpillar mean it was eaten?

While it is possible that a 5th instar caterpillar could have been eaten, the most likely reason for the disappearance of the caterpillar is that it has left the milkweed to pupate. After a caterpillar pupates, the chrysalis can be very difficult to find, making it appear as if the monarch is no longer at the site. Click here or here to see related questions.