+1 for manual pest control. I just snipped these infected leaves off a butterfly flower plant growing in a bucket in the yard. What are these? (besides now ashes)
They are aphids which are easily controlled with soap and water. Three applications of soap and water at five to seven day intervals should do the job. They are quite variable in colour. I have seen green, black and orange ones.
Adults can have wings and this is how they spread to other plants. Ants also farm them so controlling them may help as well.
This is a common garden pest, see here for more details.
Those are milkweed aphids, Aphis nerii. As kevinsky notes, these can be controlled with soap and water. Below I magnified your image to better view detail, and took a comparison photo from online:
Someone else's aphids:
These look like some kind of aphids to me. It wouldn't hurt to check other nearby plants for these, especially if there are also ants (sometimes ants protect and 'farm' them).
One part rubbing alcohol combined with two parts water in a small spray bottle is nearly instant death to all aphids. Use it all the time.