Not Bubonic, but Beetle.
Plague Beetles to be exact.
I had these in the garden a couple of years ago. A few days ago I noticed some on my beans :
I remembered what they were, but there didn’t seem to be many and they weren’t doing any harm, so I let them be. A day or two later and a friend and I were sitting in the living room having a cup of coffee when she noticed the Woolly Bush in the garden seemed to be drooping significantly (it’s normally an erect shrub) and there were lots of dark blobs in it :
We went out and had a look. OMG, the bush was full of them. Their weight was dragging the branches down :
We went back inside and grabbed for our iPads and Google. This CSIRO site seems to be the best for information. There’s a video. We watched in horror, the sight of millions of these things crawling on a tree trunk. We said “Oh, yuk!” Amazingly, they don’t seem to do any harm to plants. They spend their time sucking nectar from flowers and (to be a bit crude) bonking. No wonder they’re a plague!