A good picture would help you gain an accurate identification.

There are a gazillion little yellow eggs all up and down the stems of my butterfly weeds
This sounds like oleander aphids.

Without proper ID it is immpossible to know it this is a beneficial insect or not, so until you have a good ID let them be.
Those "little yellow eggs" on your Butterfly weeds may be milkweed aphids and the best means of control for them is a sharp water stream, from a spray bottle not your garden hose.