You are correct; oleander aphids. The same species that commonly are seen on tropical milkweed plants. And oleanders, although I honestly see them less frequently on oleanders. Surface control requires multiple applications, because they bounce back with a new crop of young quickly. Systemic insecticides give longer-lasting results, but I won't get into an insecticide/organic debate.

Yeah, I felt they were some type of aphid. I don't care so much about insecticides for these plants particularly because of where they're at in my garden. However, i do have an organic veggie garden on other side of yard.

I'm already using Neem oil; do you think a stronger solution/more frequent application would do the trick?

Thanks again gray falcon!

When I wrote repeated applications, I wasn't referring to neem oil specifically. For some pests, control is not necessarily a single application for an entire growing season, or even the following month. Do not use stronger solutions or more frequent applications than is on the instruction label. If one type of control does not seem to be working, you can try a different one. Insecticidal soap would be a safe alternative. But that too will require repeat applications since you noted that aphids are a reoccurring problem on this plant.

Your plant's infestations appears to be rather mild. A squish/wash-off would be a good start, then follow up in a few days with insecticidal soap to try to keep them from reestablishing a colony. Another thing that can help is to control the ant populations. Ants often like to "farm" aphids.

Direct hits of insecticidal soap work well if repeated as needed.

Thanks everyone! Nah, I'm not very squeamish at all (except for slugs, not snails... Slugs. Something about them!), so squishing isn't a problem.

And yes, it's mild now. This is a new plant, haven't put it in the ground yet. I have other Mandeville though and find it fascinating that these little guys always turn up. I have other plants and veggies that get aphids (just black and white variety tho) and am quite familiar - I do same things recommended (squish, rinse with water, regular use of insecticidal soaps/neem, depending on the plant, etc - u know the routine,,lol), but these lil yellow buggers had me stumped. Now I kno how to act.

Thanks again everyone!

If you see this guy in your garden do not kill him. He dines on aphids.

A little bit of Dawn mixed with water and sprayed on the aphids worked for me. They do seem to like milkweed.