grubby plums

If they are white grubs they are probably Plum Sawfly, get some chickens as they drop off the tree to pupate in the soil, you could try digging around the base of the tree (without going down to the roots) and hope you can bring them to the surface for wild birds to eat, you need a friendly robin who comes when you dig. . I don't think there is a currently approved spray, organic or not.

If they are pinkish then it is Plum Moth which you can trap in pheromone traps.