False honey ants and aphids

Ants tending aphids in a symbiotic relationship--so cool! I'd heard about this, but was so excited to see it for myself, happening in my own backyard! The ants guard the aphids from predators and in exchange, get the aphids' sugary wastes called honeydew to drink. In the bottom right of the first photo, I think it is actually a drop of honeydew you can see in the ant's jaws! These ants are a tiny species, each individual hardly as big as a rice grain. I had to crop the photos pretty severely to have the ants looking so large. If anyone can identify the species, please let me know! The fourth photo shows the weedy-looking host plant, which I could also use help identifying, and the unkempt area of our backyard where it was growing.