[JTh]

At this time of the year my pears are beginning to ripen, and every year large groups of these gigantic, shiny, metallic green-black beetles gather around some of the pears.  They look a bit like the rose chafer, Cetonia aurata, except that they seem to be even bigger, some of them > 2.5 cm. They don’t attack all the pears, but gather in large numbers on pears which seem to be rotting; there were at least 25 of them on a single pear where I took this photo. I think they are some kind of scarab beetles, but which species?

[Alice]

You might be right, Jorun. But we need an expert to confirm the species.
The ones I am familiar with are about 2.5 cm long and seem to go for ripe fruit. I don't remember seeing them on roses.
They are called "chrysomyges" = golden flies in Greece. Children used to - rather cruelly - tie a thread to them and let them fly around them like pets.

[SusanIbiza]

They look a lot like Christmas beetles which we get in Australia in the summer.  If you squash them, they have the most disgusting smell!  They would sometimes get inside the house and be difficult to catch because they fly.

[JTh]

I believe the Christmas beetle doesn't occur in this part of the world, and I don't feel like squashing them. The description seems to fit quite well  with the rose chafer, except that they are larger (Cetonia aurata is upt to 20 mm long), and the underside is black, not coppery coloured.

Anyhow, there does not seem to be any reason to chase them, since they are supposed to be beneficient, their larvae are the insect equivalent of earth worms and help make very good compost where they are often found in great numbers, according to Wikipedia. And they don't seem to attack fresh fruit, there is nothing left of the rotten pear they feasted on yesterday, today I saw them flying around looking for more rotten fruit, but without any luck. I also saw some escapring  from the covered compost heap when I opened it this morning.