They know where you are!
:>)

In other words, aphids happen.

Insect eggs do travel around, on the wind, on the feathers of birds, even on your vehicles from where they are native to where we live. One of the reasons why it is important if you travel someplace to have your vehicle(s) thoroughly washed before you leave. The Gypsy Moth and the Zebra Muscle were both moved from the habitat they had established themselves in to other habitats on vehicles driven by people, so it stands to reason other insects can be transported the same way.

They also have wings in some phases. They come in the greenhouses under their own volition through the air vents.

This aphid species is a bit different than most other aphids. There have never been any male aphids discovered in the wild. This means, obviously, that there are no eggs. All of the adult aphids bear living aphids. Because this species has to migrate, a very large percentage of them have wings.

They are actually considered a warm weather species, and can't over-winter in colder climates.

I grew some Asclepias from seed this year, and have been squishing the darned aphids on a daily basis from the very beginning.

Thanks for the replies and info everyone. So I guess "aphids happen" really does sum it up, lol.
I wish I would have just sprayed that seedling with insecticidal soap instead of ripping it out and throwing it away. I guess I panicked, haha. I must admit the aphids were quite a lovely color. I am guessing they are bright orange like that because of the color of the flowers.. so they blend in.
That's a good suggestion about washing your car before returning home from a road trip. I would have never thought of that, so thanks!! I know all about the Gypsy Moth, but never heard of the Zebra Muscle so I will have to look that one up. Strange name.

Just a thought -- it's Zebra mussel.