Maggots in plums

We have a lovely crop of plums but sadly most of them have maggots in them. How can we solve this problem for another year or is it particular to this year?

I to lost my crop to a fly pest that lays a white jelly on the fruit.What I failed to do as I normally do on apple and any fruit tree to prevent it is to tie an oily (veg) cloth around its trunk or as my poor gooseberries one year were ruined by saw flies and I now do one spray in spring to prevent attack.

Flowering rose, many thanks for your reply. So if I have understood correctly you tie an oily cloth around the trunk of the fruit tree, Do you mean covered in vegetable oil? Also, what do you spray on the gooseberries, please? I planted two gooseberry plants this year and would be very sad to loose them.