The Plum Sawfly lays its eggs on the flowers. The young maggots which develop
tunnel their way into the developing fruit. The only signs at this stage
are the tiny holes left by the maggots.
the maggots feed on the fruit, often a gooey black liquid exudes from the holes.
variety 'Czar' is one of the worst affected.
Break the pest's lifecycle at two points. First, in late winter and
early Spring, gently loosen and turn the soil around the base of the tree
with a trowel or hoe. This will expose the over-wintering pupae to birds
who love to eat them.
second action is to spray the tree with derris (available at most garden
centres) as soon as possible when roughly half the flower petals have fallen.
the above action is taken, next year it should not be necessary to spray again.