Based on the fact that the host here is Common Milkweed, and the deformative effect they have on the upper leaves, could this be Aphis rumicis?

Aphis rumicis is fairly restricted in its hostplants.

From Blackman and Eastop's Aphids on the World's Plants

http://www.aphidsonworldsplants.info/d_APHIDS_A.htm#Aphis

Aphis rumicis L. Apterae are dull black or very dark brown (see influentialpoints.com/Gallery); BL 1.4-2.8 mm. Alatae have secondary rhinaria distributed III 5-20, IV 0-4. On Rumex spp., feeding on undersides of leaves in spring and rolling them longitudinally into tubes. Later it occurs on stems and inflorescences. It can also occur on Rheum spp. (Stroyan 1984, Lee et al. 2002c), but records from other plants mostly apply to A. fabae. Ant-attended. Widely recorded in the northern hemisphere, although many North American and at least some Japanese records (e.g. Takahashi 1966) apply to different species. Monoecious holocyclic, with apterous males and oviparae with barely swollen hind tibiae. 2n=8.

But to give an idea of why these cannot be accurately identified with these images, the characters needed to confirm Aphis fabae would be the dorsal sclerotization, if pre and post - siphuncular sclerites are present, the number and arrangement of sensoria on antennal segment three, the shape and number of hairs on the cauda, and the shape of the abdominal tubercles on segment VII.

Thanks for your time. I understand the difficulty of identification by morphology. My guess is based on a reference in Bug Guide on the species page for A. rumicis, where the article "Insect Herbivores of 12 Milkweed (Asclepias) Species" is cited. A paragraph on A. rumicis (p.15) describes it as attacking the upper parts of A. syriaca, and doing damage similar to that in my photo. It may be that the researcher incorrectly identified the species. Since so few aphid species feed on Asclepias, I was hoping that would narrow it down.

There is definitely a lot of misidentification of aphids. I find that even experienced entomologists often don’t know what characters to look at, and it’s why so many people try to stay away from them! Aphis is also just a tough Genus to ID. I spent hours trying to figure out the difference between Aphis asclepiadis and Aphis helianthi. I just could not see it. I contacted another aphid person, who then informed me helianthi is now a synonym of asclepiadis. Ugggggh.

Since Milkweed is common and requires specialist herbivores, it might be useful to determine definitively which black Aphis is the herbivore. Rumex is in the neighborhood, as are plants consumed by A.fabae.

The scientists who published that paper are an aphid specialists that I know of, and the publication states they are in the department of biology, not entomology. Not saying biologists can't possibly know anything about insects, but as I mentioned, I have found even experienced entomologists have a hard time identifying aphids.

After looking at their methods I am not confident in their aphid IDs. I am from southeastern Wisconsin, and have collected and identified a lot of aphids, including many specimens from milkweed. If you look at their methods they state they did not collect insects and may have double-counted herbivores upon visiting the same plant again. This makes me think they were visually identifying the aphids on the plants instead of slide mounting them or even looking at them under a scope, which is generally not reliable.

In the results section, page 15 you mentioned, they state there are five common species associated with milkweed and list A. rumicis, yet A. rumicis is definitely not commonly associated with milkweed. They call it "Black Aphid", and A. fabae is commonly associated with milkweed, has previously been recorded on milkweed muiltiple times and is known as "Black Bean Aphid" so I'm wondering if there is just a mix up there? I am not sure why they decided the aphids were A. rumicis other than they were black. I am also not finding any information indicating A. rumicis have been slide mounted and confirmed as colonizing Asclepias. All the information I can find states A. rumicis only hosts on Rumex and Rheum.

Blackman and Eastop's Aphids on the World's plants is the most comprehensive study of aphid - hostplant interactions, so that is what I trust more than anything, and A. rumicis is not listed as ever being recorded on Asclepias.

http://www.aphidsonworldsplants.info/C_HOSTS_Art_Asp.htm#Asclepias

If you find more black aphids on milkweed this growing season and would like to send them to me, I would be happy to identify them for you.

Thanks for your guidance. Maybe I will take you up on that. For future guidance of Bug Guide amateurs, a little statement on how to go about making successful Aphid IDs on the aphid family page, such as how to ship specimens, how to prepare slides, best sources, etc might be helpful. This has been an education for me.

I did edit the info page for Aphididae a while ago and tried to add more general information on why aphids are tough to identify with pictures, but it is not very specific. There is so much I can say about aphids, so it can be a challenge trying to be specific yet brief! I'll have to work on it again, and I'd love to get more information on each species page.