There also is a good chance that these could be young nymphs of a leaf-footed bug (family Coreidae), as I can see at least two later instar coreid nymphs in this image. Some nymphal coreids are very similar in appearance to assassin bugs at early stages in their development - see http://farm5.static.flickr.com/4141/4933212356_c5b9e87d75.jpg for an example.
There are leaf footed bugs around, a lot of them, and they ruin sunflowers and tomatoes. I wonder why they would be congregated on this sickly weed though. The weed must look sickly because of the aphids. I did not see these orange nymphs piercing the plant as I have seen adult leaf foots doing sunflowers.I am going to try for a better picture today, if they are still there.