We have a tropical milkweed that is frequently visited by Monarch
butterflies. Wife picks the eggs off and raises the caterpillars in the
house (safe from ants) and we feed them native milkweed (Asclepias
syriaca.) She has released at least 100 butterflies so far this year.
Anyway...

The aphids on the tropical have just exploded in the past week and the
plant is covered with them. What can I use to kill them without harming
the butterflies? Soap? Half-strength dormant oil spray?

Since the plant is in a pot, I suppose I /could/ hide it in into the
garage for a day or two if I need to use something stronger like Sevin
or Rotenone or pyrethrin. But I'd like to find a contact poison that
leaves *no* residue.

Thanks,
Bob