Sounds like you have a bad case of apple maggots. I'm pretty sure that anything you were to spray the blooms with would also kill your pollinators. The eggs are laid on the fruit, itself by the fly stages which can be trapped using jars of fermented molasses and yeast hanging from the limbs. Also, one should pick up all fallen fruit which might harbor overwintering larvae/eggs. If you feel you must spray your fruit, I would look into pyrethrums and/or rotenone; they are naturally derived and decompose into the soil without long term effects.

eek, when I spoke to Isons about plum trees, they told me they could be easily grown organically here!

What kind of plum tree do you have? I am trying to choose one at the moment.

There's some info on Plum Curculio and Oriental Fruit Moth in the link -- i came across several other info sites via Google as well (seems that the same bugs get into peaches and plums). Sounds like you have to spray right after the flower petals fall.

Do you get any plums that you can eat, or are all of them wormy? I hope you can treat it!

Here is a link that might be useful: clemson link

i did a bit of reading and found an organic spray called "Surround", which is based on kaolin clay, which is supposed to help against curculio and some other bugs. it works by coating the fruit so that the eggs/larvae cannot get inside. I wonder how effective this would be by itself, but think that if i buy a tree, i will probably try this.
Its described a bit in here:

Here is a link that might be useful: Surround

Mayland:

If you are looking for a plum tree which can fruit without all of the mess involved in spraying, I have one that is never a problem. A neighbor gave it to us years ago and the only times it has not fruited were during bad droughts (one caused by the roads department grrrr!) and after bad freezes. It has never suffered from bugs!

It is a yellow, smaller plum, but it makes good jelly and tastes pretty good as well. We have lots of seedlings and seed should you want some.

Thank you for all the information. I don't know what kind of plum tree these are because they were planted many years ago by someone else. The fruit is a reddish-purple and about 1 1/2 inches in diameter.

Nippersdad, so I'm gathering from your post that once again, I'm too late to really do anything about the worms this year since I didn't know I needed to pick up and discard all the fallen fruit?

Again, thanks for your answers!
Beverly

Hi, Bevinga:

No, not at all! Cleaning up the fallen fruit ultimately makes for less of the critters but, probably would not eliminate them. Those yeast traps work pretty well at getting most of the egg layers out of the way in much the same way as beer traps for slugs.

If you have anything like the germination rate of fallen fruit that we have here it probably wouldn't be an issue anyway, we do it in self defense!