I've tried everything suggested, without success, including squashing them manually and spraying the plants with soapy water. This year the plants are less accessible, because I planted them down a bank and there are too many plants to be squashing the aphids manually.
In my experience, soapy water ends up damaging the plants - they get black spots on the stems and the leaves go curly. This happens even if I first spray on the soapy water and come back a little later to hose the plants with clean water. Water is the key word here. Our area hasn't had rain for more than 6 weeks and we are officially in drought conditions. Because we collect our own water from the rain and store it in a big tank, we must use it sparingly when our tank level is low.
This morning I read a review about a spray-on product that was said to work positively to kill aphids, yet wouldn't harm Monarch eggs or caterpillars. Upon inquiry to the manufacturers, I discovered that the particular product is no longer in production and that it would actually smother Monarch eggs and caterpillars if applied to the plants. Clearly the information I read was not entirely correct, so it pays to double check.
My inclination is to cut off the plants and not plant swan plants again, if aphids are going to be such a constant problem on them every Summer. On a positive note, ladybirds eat aphids and we are seeing plenty of ladybirds, and their very active larvae, feasting on the aphids on our swan plants.
Oh - yes. I have read that healthy swan plants don't attract aphids. I do try to keep the swan plants fed. That's not easy when having enough water is an issue, because the 'food' is either in pellet or liquid form, both needing to be activated by watering.
I wonder if anyone can offer other ideas to keep back the aphids on swan plants, that doesn't involve using much water?