aphids on tropical milkweed

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
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zxcvbob wrote: ...

for one plant, soft brush them off or use a spray of water, see if any remain after that and then hit them with a weak soapy water solution to drown them. keep an eye out for ants that may be "herding" them. repeat as needed...
songbird
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Once upon a time on usenet songbird wrote:

That's exactly what I do. Soapy solution is approximately 1% dish washing liquid (10mls to a litre). Much stronger makes the milkweed grow twisted and stunted.
--
Shaun.

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wrote:

Aphids won't harm milkweed and they won't venture off the plant, I'd leave well enough alone.
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Once upon a time on usenet Brooklyn1 wrote:

They do however stunt its growth considerably.
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