Maggots in plums

Wasps love plums so it could be wasp larvae.  Not sure though

These are the culprits, http://ukmoths.org.uk/show.php?bf=1247

Pheromone traps are put in the trees early in the year to collect the males so there are none to breed with the females.

I have found that maggoty fruit tend to ripen before the others so a good shake of the branches will remove affected fruit which can be left in a bucket of water to drown and then composted.
After the first flush of fruit with lodgers the quality should improve.