Unfortunately there is no guarantee that Japanese beetles will not eat plum trees.  Japanese beetles will feed on the leaves, flowers or fruit of more than 300 species of plants. Some of their preferred plants to eat include: Virginia Creeper, Grape, Rose, Crabapple, Apple, American linden, American elm, American mountain ash, Norway maple, Horsechestnut, Green beans, and raspberry.  If their preferred foods are in the vicinity of your yard, it is likely that the beetles will go to those and not bother your plum too much.  Plum trees are in the same botanical family as roses, so there is always the possibility that they will chew on a plum tree. Here is a link to the most comprehensive and updated information on Japanese beetles.  

https://extension.colostate.edu/topic-areas/insects/japanese-beetle-5-601/

Thank you for your inquiry and best of luck this gardening season.