Plum tree pest, any ideas?

[starloc]

I have these on 2 of the plum trees in Bulgaria , only 2 of the trees though, the leaf looks swollen on the center vain at first , then swells to a large blob of mutated leaf looking like a cacoon the leaf cacoon is like a pea pod in texture it is very thicker than it should be

This is not something on the leaf this is the leaf, gets a red tinge in areas, chopped open for the first 2 weeks they had nothing inside visible at all..... then there are 1 or 2 tiny about 3mm long and 0.5mm wide very active bright orange almost flourescent caterpillers at one end that arch up and run off quickly

I cant find any description of these searching , they are like a sawfly or something, I have no photos of the orange beasties as they were rapidly squished due to speed, they dont seem to be leaf miners as they stay in the center vain....for now

[fishpond]

As the wildlife in Bulgaria is unknown to me, my suggestion would be to hatch one out in the warmth, then hopefully you could identify it as friend or fo before the rest hatch out.