Where do they pupate - in the ground or under leaf litter? Going by the state of the leaves there must have been more, though there was no sign three weeks ago.
Yes they walk up to 20 metres from their foodplant, then burrow under the soil up to 15 centimetres down. Bear in mind also that about 99% perish before reaching adulthood, due to viral, bacterial, fungal disease, or infection by wasp and fly parasitoids, or predation by birds, spiders, ants, mantids, shield bugs, assassin bugs etc.