Looks like aphids. Common garden insect. Start with a strong jet of water to knock them off. If they haven't developed wings yet, that might be enough once a week or so. Next level of control is insecticidal soap — Safer's is a popular brand. It kills soft-bodied insects. Follow the instructions exactly. Don't spray if bees are present (if they land on the wet surface they'll die). Don't spray in direct sun (will burn the leaves). Best time is usually late evening. The aphids must be directly coated with the spray and it will kill them quickly. The dried residue is harmless to insects and can be rinsed off if you want. Reapply according to instructions.

As nasty as they seem, aphids are often transient and will move on in a couple of weeks. Or beneficial insects will arrive and eat them. If you don't have a raging infestation, sometimes you can just wait them out.

You've had quite a time with that wisteria. Two conditions can encourage aphids: excessive application of nitrogen-heavy fertilizer and overall poor plant health.

Thanks, in that case I'll give it a couple weeks and see if they leave, and if they don't then I'll try the insecticide to get rid of them. I noticed a lot of ants are crawling around and acting like they're tending to livestock, so maybe they'll help things out there.

It's been slow going, but it does seem overall greener and fuller this year, and some of the new growth looks promising. I think a lot of the trouble has come from a bad starting location, nearly rotting out the roots and a long recovery due to poor quality dirt. I did give it some Miracle Grow rose fertilizer since I'd read from a couple places that it's supposedly good for these. Hopefully it will keep getting better!

Ha, yes, the ants are "tending to livestock" but not to your advantage. They actually herd and protect the aphids to harvest the sweet sticky substance the aphids excrete, known as honeydew. To complete the picture is black sooty mold, a fungal disease that grows on honeydew. The whole scene can get yucky. Really depends on a number of factors and the extent of infestation, so I do recommend the water spray at the very least if numbers increase.

Just a suggestion, I would remove a lot more of the rocks and the bricks, they make for a heat sink.

Good to know, thanks! As for the bricks and mulch, I added those to protect from the heat of the rocks. This is where I was permitted to plant it, so I can't move it to a grassy area or redo the landscaping around it.

Mulch is good, the bricks do the same as the rocks, they become a sink heat.