The main cause of maggot damage in Plums, Gages and Damsons is the caterpillar of the Plum Fruit Moth. This pest is very difficult to control with insecticides unless applied shortly after egg-laying.
Hang this trap in the garden from mid May until August to help monitor if Plum Fruit Moths are present.
The Pheromone scent of the female moth will attract male moths to the trap, if you are catching more than 50 moths per week, a suitable spray should be applied.