Thanks for writing. If your plum is already blooming, you've missed the window to spray dormant oil on it, which is a good way to help deter a lot of insect problems. (Spray dormant oil in late winter, before your tree's buds start to swell in spring.)
It's quite important, as fruits develop, to pick any fallen fruits off the ground and throw them in a plastic bag in the trash. This keeps any insects in the fallen fruits from reproducing and spreading. You can also try spraying your plum with an insecticide containing phosmet or carbaryl every week or so for a month after the blossoms start to fade.
---Justin, Senior Garden Editor, BHG.com