In my experience, aphids cause huge damage to the plants once they have taken hold. Trying to remove them from the plants is futile because the damage has already been done. If you do manage to remove the aphids the plant is already weak, the stems covered in black blotches and the leaves inedible to monarch caterpillars. Weak plants are a magnet to aphids so, even after you have removed them, the aphids will flock back. Better to remove the aphid-ridden plants altogether and either start again with new swan plants, or plant something else.
So what next? I'm going to plant something else. Nectar flowers to feed the butterflies, rather than the caterpillar-feeding swan plants that will quickly become aphid ridden. The area we live in, has high humidity in Summer. Aphids just love high humidity so I figure that, no matter how many new swan plants I plant, the aphids will always take over. Better to plant nectar flowers for the butterflies and know that there are many other swan plants out there, that are not aphid ridden, that will feed hungry Monarch caterpillars.
Here's a list of nectar flowers for butterflies. It's from an American website,however we do grow most of these in New Zealand and I have many of them growing now.
I have noticed the flowers the Monarchs flock to the most, in our garden, are the tall zinnias and the marigolds. Yes, I'll be planting plenty of those.
Butterfly Nectar Plants -
Aster (Aster spp.)
Black-Eyed Susan (Rudbeckia hirta)
Blazing Stars (Liatris spp.)
Butterfly Milkweed (Asclepias tuberosa)
Buttonbush (Cephalanthus occidentalis)
Cardinal Flower (Lobelia cardinalis)
Common Milkweed (Asclepias syriaca)
Coneflowers (Echinacea purpurea)
Coreopsis (Coreopsis spp.)
Cosmos (Cosmos spp.)
Dianthus Family (Dianthus spp.)
Lantana (Lantana camara)
Marigold (Tagetes spp.)
Mexican Sunflowers (Tithonia rotundifolia)
Petunia (Petunia x hybrida)
Salvia (Salvia spp.)
Shasta Daisy (Leucanthemum spp.)
Sunflower (Helianthus spp.)
Swamp Milkweed (Asclepias incarnata)
Swamp Verbena (Verbena hastata)
Tall Verbena (Verbena bonariensis)
Thistle (Cirsium discolor)
Violet (Viola spp. )
Woodland Stonecrop (Sedum ternatum)
Yarrow (Achillea spp.)
Zinnia (Zinnia elegans)