Stop Aphids without Hurting Eggs or Caterpillars?
We don't treat plants with aphids during the season. A better strategy is planting several species of milkweed and spreading them out. Pests like aphids won't discover them all and, over time, you'll attract more aphid predators.
We also plant aphid-repelling plants close to milkweed like allium. More details in this post: