Answer: Unfortunately, Japanese beetles are particularly attracted to trees in this genus (Prunus -- includes both plums and cherries) and so will be an ongoing problem for you.
Japanese beetles are difficult to control, in part because they fly long distances. The pheromone traps are not recommended because they seem to attract more than they catch.
You can try to repel the beetles using a spray containing neem, or you can handpick them (they are sluggish early and late in the day), or you can spray with spinosad or with carbaryl (the active ingredient in Sevin) but this is a contact insecticide with no residual action.
You can also use beneficial nematodes or neem sprayed on the lawn in late summer to early fall to kill them at the grub stage. Milky spore disease in granular form is also applied to the lawn and a treatment lasts in the soil for many years, but it takes some time to see results. Be sure to read and carefully follow all of the label directions on any product you use. If you are using these then you would not need to use additional lawn grub control products.
Unfortunately, most gardeners find they have to tolerate a certain amount of damage, and some years are more severe than others depending on the weather and other conditions.
I hope this helps.
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