The last antennomere appears to be narrowed apically.

I guess the size should not include the antennae, but I still thought I remembered him being a lot bigger than 1/2". Next time, I will remember to measure. I think your ID is spot on. It is interesting that these guys are mostly specific to one species of milkweed. Some sources say this critter should be found on A. viridiflora, but others acknowledge that in other parts of the range, A. viridis is the host. Asclepias viridiflora is rather scarce where this bug was located, but A. viridis is abundant. The plants they were on were noticeably affected and seemed to be dying.

I think this species will use multiple species of Milkweed, but they seem to favor larger species with broad leaves. In any given area I think they tend to favor one species of Milkweed over the others present, but you may find that at another season or in another year they will favor a different species. In some areas there is only one Asclepias species present, so that is the one they will probably be on. It seems to me that I've occasionally seen them on other genera of the Milkweed family too, and on Dogbane, but I think those may be "hard time" back-up survival choices.

As for the distress on the plants, it could be caused by the beetle's feeding, but it could be that the beetle likes the distressed plants. It's really hard to tell from the photo, but it looks as if the leaves may be affected by a disease, or perhaps may be dehydrated from drought stress (??). Many insects will indeed favor plants that are stressed over plants that are not (however, some insects will favor the healthy plant). Commonly stressed plants do not contain as many defensive chemicals, and other defenses may not work as well either. Also, sometimes they contain different proportions of nutrients as well (which might make them taste better?).

There is an interesting web site here where they claim that most species of this genus of beetles are host specific, but they list more than one host for several of them. The common species around here (T. annulatus - I think) will definitely feed on several different Milkweeds though, and in my yard they favor A. latifolia.

Here is another little blurb about some of them.

I guess it's not possible to be certain without cutting open the stem to look for larvae, but my strong inclination is to think that the sick milkweeds are looking bad because of the milkweed borers. I have many hundreds of milkweed plants--maybe thousands--and only a few of them are showing any signs of distress,scattered around in the middle of a patch of healthy ones. Drought is right out as a cause, since their neighbors look great. Don't worry. I have no plans to "off" the beetles. I thought they were interesting. My main concern was whether or not they would be a threat to some of my other, rarer milkweeds. It appears they have preferences that are compatible with them staying.