Aphids become a big problem at this time of year, on the swan plants (milkweed), because of our rain and warm temperatures. I don't have the answer to reducing their numbers, despite trying many different ways of removing them, however I do know that Ladybirds are my friend when it comes to keeping the aphids back.
Ladybirds, both mature and immature, eat aphids
In my years of raising Monarchs, aphids have been real nuisance in the early months of the year when humidity is high. I have tried all sorts of spray-on remedies, that will supposedly not damage the plants, yet none seem to work for me.
These days my main method of keeping back aphids is to remove any caterpillars from the plant and hose the aphids off the plant with a strong jet of water. I also squash them while hosing. When I think I have removed the aphids, and the leaves are semi-dry, I put the caterpillars back.
The trouble with not keeping on top of removing aphids from swan plant, is that the dew they secrete will damage the plants significantly. Ants 'farm' aphids for their dewy secretions so, when hosing the aphids off the plant, its important to ensure they've been hosed away from the plants otherwise the ants will just bring them back.
There doesn't seem to be a magic fix for removing aphids, while keeping your plants healthy and your caterpillars (and ladybirds because they eat aphids) alive. You just have to keep working to remove them fairly manually.
It is said that aphids won't attack healthy plants and to feed swan plants plenty of phosphorous to keep them healthy. The jury's out on that one, although I don't fully disagree.
I work hard at feeding my plants, although there still do seem to be aphids on them, albeit not as many aphids when the plants are healthy.