I have a small number of plum trees in the garden. In one in
particular (Marjorie's seedling) something like a quarter of the fruit
had larvae inside. I didn't stop to ask what they were larvae of.

I would imagine that the eggs were laid either in the flower or
possibly directly into the developing fruit. Which is most likely?

How can I prevent or minimise this next year? I do not want to harm
the useful insect brigade (eg bees) but I do want to discourage the
antisocial behaviour of the egg layers. Any suggestions please?

Thanks

Merlin