Swan Plant (common name that we New Zealanders refer to them by) 'gomphocarpus physocarpus' is our most common milkweed for Monarch caterpillars and butterflies, is also known as hairy balls, balloon plant, balloon cotton-bush, bishop's balls, or nailhead. The plant is native to southeast Africa, but has been widely naturalized.
Along the track leading into the forest, there were eight reasonably-sized Swan Plant bushes. Apart from one, all were vigourously healthy and covered with large 'swans' full of seeds. The plant that was not doing well was covered with Aphids, although none of the other plants had any Aphids on them. In fact it was on the Aphid-ridden plant that I found a 4th instar Monarch caterpillar, so I transferred it to one of the more healthy plants.
And while I was doing all this a beautiful and healthy female Monarch Butterfly was patrolling the area, up and down, up and down, overhead.