Probably plum moth - you can get a pheremone trap for these, but if you have more than one plum tree, or there are others around, this alone may not be enough to prevent the trouble next year. Its other purpose is to tell you when the best time is to spray the tree, usually in early summer and a second application three weeks later. Chemicals that work are bifenthrin, permethrin, pirimiphos-methyl or fenitrothion, but I am not certain these are all still available, since so much has been withdrawn from sale in the last few years.

Adding that you should also remove and bin any rotten fruit so as not to spread the rot to the other plums or leave spores for next year.
We find that the traps are enough to keep all but the most determined plum moths under control without having to spray with anything.

that's good to know, Owdboggy, I take it you mean the pheremone traps? Have you got more than one plum tree? I've never used one in a plum, only on apple, and that worked pretty well, but the book says pheremone traps might not be effective enough where there are a few plum trees, so I'd be interested in your situation.

We have a Victoria and a Marjorie plus a hedge of wild plums, damson and greengage and of course the Damson Wood itself. We usually place one trap in the Victoria and that is enough, most seasons. Probably ought to put one in the wildings as well, but when you are getting over 100 lbs of Damsons in a good year I am not sure we need to bother.

well that's great news for you Guest, all you need is a pheremone trap next year by the sound of it. Thanks owdboggy.