I just let those aphids enjoy the milkweed too..so they will not bother the other plants
We do not get a lot of monarch's around, more of the swallowtail and also for my hummingbirds..they enjoy it too
Then I get mean and hose down those aphids or spray alcohol mixture on them. And I noticed, the aphids do not like my geraniums beside the mikweed woohoo! More protection!